MELT INCLUSIONS AND THEIR IMPLICATIONS
OF AN INTERCONNECTED MAGMA CHAMBER
BENEATH CERRO NEGRO VOLCANO AND LAS
PILAS-EL HOYO COMPLEX, NICARAGUA
by
Swetha Venugopal
B.Sc. Hons, Simon Fraser University, 2014
Thesis Submitted in Partial Fulfillment of the
Requirements for the Degree of
Bachelors of Science, Honours
in the
Department of Earth Sciences
Faculty of Science
© Swetha Venugopal 2014
SIMON FRASER UNIVERSITY
Semester 2014
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APPROVAL
Name:
Swetha Venugopal
Degree:
Bachelors of Science, Honours
Title:
Melt inclusions and their implications of an interconnected
magma chamber beneath Cerro Negro Volcano and Las
Pilas-El Hoyo Complex, Nicaragua.
Examining Committee:
Chair: Dr. Shahin Dashtgard
Associate Professor
Dr. Glyn Williams-Jones
Senior Supervisor
Associate Professor
Dr. Séverine Moune
Supervisor
Visiting Scholar
Kevin Cameron
Internal Examiner
Senior Lecturer
Date Defended/Approved:
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PARTIAL COPYRIGHT
LICENCE
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ABSTRACT
Melt inclusions are tiny parcels of melt entrapped within crystals growing in a magma
chamber. Their composition provides insight into the evolutionary history of the magma from the
initial melt to eruption. Olivine- and pyroxene-hosted melt inclusions were analyzed from Cerro
Negro Volcano and the Las Pilas-El Hoyo Complex in Nicaragua in order to determine their
subsurface linkage. These systems are 1 km apart and have shown historical activity of coupled
eruptions. Previous geochemical and geophysical studies have suggested that a shallow
connection is likely; however, melt inclusion data will provide the deeper geochemical link. Data
from the two volcanoes produce a linear compositional trend with similar magma sources,
indicating an interconnected magma chamber. Cerro Negro data consistently define the
primitive, volatile-rich end member of the trend whereas Las Pilas data are relatively evolved
and represent the late stage, degassed melt. In order to accommodate these two compositions,
four scenarios are proposed, each comprising a magma chamber extending to dyke that
diverges towards the edifice of Cerro Negro and Las Pilas. The primary orientation of the dyke
is towards Las Pilas as it is older than Cerro Negro; the branching of the dyke towards Cerro
Negro follows the mechanism of dyke capture, where ascending magma preferentially follows
the direction of minimum principle stress. Implications of a branching dyke beneath Cerro Negro
include its proper classification as a young stratovolcano and an increase in eruptive intensity.
Keywords:
Melt inclusions; Nicaragua; Cerro Negro; Las Pilas-El Hoyo
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This thesis is dedicated to my hero Venugopal Ganesan, my role
model Sandhya Venugopal and my sidekick Ishaan Ranjith.
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Acknowledgments
I would like to thank my family for their unyielding support. Without your encouragement,
devotion and late night coffee runs I would not know how to function. To my sister, I am grateful
for your uncanny ability to know exactly what to say. A special thank you to my dad for trying to
make me smile by cracking random jokes in typical dad-like fashion. To my supervisors Glyn
Williams-Jones and Séverine Moune, thank you for giving me this wonderful opportunity and
helping me every single step of the way. Your guidance means the world to me. To my friends,
you each have your own amazing way of making me laugh and keeping me grounded. Without
you, I would be a much more normal person leading a boring life; who would want that
anyways? Lastly, I would like to thank Roger Waters, David Gilmour, Syd Barrett, Richard
Wright, Nick Mason, Eric Church and Ella Fitzgerald for keeping me company during endless
nights of writing, thinking, deleting and rewriting.
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Table of Contents
Approval ................................................................................................................................... 2
Partial Copyright Licence ....................................................................................................... 3
Abstract .................................................................................................................................... 4
Acknowledgments.......................................................................................................... 6
Table of Contents .................................................................................................................... 7
List of Tables ........................................................................................................................... 8
List of Figures .......................................................................................................................... 8
Introduction .................................................................................................................... 9
1.1 Background of the Central American Volcanic Belt ...................................................... 9
Methodology ................................................................................................................. 21
2.1 Melt inclusions................................................................................................................. 21
2.2 Sample description ......................................................................................................... 22
2.3 Melt Inclusion description .............................................................................................. 24
2.4 Sample preparation ......................................................................................................... 26
Results .......................................................................................................................... 32
3.1 Cerro Negro...................................................................................................................... 32
3.2 Las Pilas ........................................................................................................................... 40
Discussion .................................................................................................................... 48
4.1 Major oxides and volatiles of Cerro Negro and Las Pilas ........................................... 48
4.2 Relevant prior work ......................................................................................................... 52
4.3 Magma composition during evolution........................................................................... 56
4.4 Mechanisms of an interconnected magma chamber ................................................... 58
4.5 Implications for future eruptions at the Cerro Negro-Las Pilas-El Hoyo Complex ... 62
Conclusion .................................................................................................................... 63
5.1 Further research .............................................................................................................. 63
References .................................................................................................................... 64
Appendix 1 .................................................................................................................... 67
Uncorrected values for olivine-hosted inclusions from Cerro Negro .............................. 67
Appendix 2 .................................................................................................................... 68
Additional data for Cerro Negro ........................................................................................... 68
Additional whole rock data for Las Pilas ............................................................................ 74
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List of Tables
Table 1 ……………………………………………………………………………………...… 27
List of Figures
Figure 1 The Central American Volcanic Belt………..………………………….....…….8
Figure 2 Regional variations along the CAVB.……..……………………………...…….9
Figure 3 Location of Cerro Negro within the Maribios Range in Nicaragua….…...11
Figure 4 Cerro Negro and its associated lava fields. ………..…………………...…...12
Figure 5!The Las Pilas-El Hoyo Complex ………………………………...………...…...14
Figure 6!Major oxides from whole rock analyses. McKnight (1995)…………...…...15
Figure 7 Location of positive density anomalies. MacQueen (2013)…...……...…...17
Figure 8 3D view of the modeled plumbing system beneath Cerro Negro and Las
Pilas. MacQueen (2013).…………………………….……………….………………...…...18
Figure 9 Representative thin section from Cerro Negro, 1999. ………………...….. 21
Figure 10 Representative thin section of the 1528 eruption at Las Pilas….…...… 22
Figure 11 Olivine host with melt inclusions from Cerro Negro. ….……………...… 23
Figure 12 Pyroxene host with melt inclusions from Las Pilas....………………...… 24
Figure 13 Ternary and alkalinity plot of Cerro Negro and Las Pilas whole rocks. 32
Figure 14 Major oxides of melt inclusions versus the Fo content of host olivines
………………………………………………………………………………………………...34-5
Figure 15 Volatiles vs K2O wt % of melt inclusions and groundmass..……………37
Figure 16 Comparison of major oxides at Cerro Negro…………….…………….......39
Figure 17 Volatile variations versus K2O…………...………………………………... 41-2
Figure 18 Comparison of major oxides at Las Pilas…………...………………….......44
Figure 19 Comparing the Mg# against FeOT and MgO…………..………………...… 45
Figure 20 Comparison of the major oxides vs K2O of all melt inclusions,
groundmass and whole rock data from this study…………….…………………...… 48
Figure 21 Comparison of volatile contents of melt inclusions, groundmass and
whole rock between Cerro Negro and Las Pilas…………….……………………...… 49
Figure 22 Major oxide comparison for all Cerro Negro and Las Pilas Data
(including data)…………………………………………………………………………....… 52
Figure 23 Volatile contents of all Cerro Negro and Las Pilas data.……………...… 53
Figure 24 Comparison of MgO and K2O between all data compiled at Cerro Negro
and Las Pilas…………….………………………………………………………………...… 55
Figure 25 The 4 cases proposed by Portnyagin et al. (2012) to explain the
compositional variability between Cerro Negro rocks and melt inclusions…...… 56
Figure 26. Schematic models illustrating the potential plumbing system beneath
Cerro Negro and Las Pilas.……………………………………………………………. 58-60
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Introduction
1.1
Background of the Central American Volcanic Belt
The complex tectonic setting of Central America arises from the interaction between the
Cocos, Caribbean and Nazca Plates and the Panama Block (Figure 1). At present, Central
America is attached to the stable Caribbean plate to the east and bounded by the Middle
American Trench, comprising the Cocos Ridge, to the west. The near-constant and oblique
northeasterly subduction of Cocos Plate beneath the Caribbean plate at 10 cm/year leads to the
Central American Volcanic Belt (CAVB). Extending from Guatemala to northern Panama, the
1100 km long belt contains 39 Quaternary stratovolcanoes, cinder cones, vent complexes and
calderas (Figure 1; Walker and Carr, 1986). Overall, the Central American Volcanic Belt is
atypical of most convergent margins. The volcanic chain is composed of many right stepping
linear segments that are 100-300 km in length (Carr et al, 2003). Each segment trends
approximately north-south and is characterized by a roughly linear chain of closely spaced
volcanoes as well as near consistent regional topography and volcanic structure (Carr et al,
2003). One of the most distinguishing features of the Central American Volcanic Belt is a
geochemical gradient along the volcanic front. This gradient comprises along strike differences
in magmatic composition, density and edifice heights (Carr, 1984). The leading cause of the
geochemical gradient is regional variations in crustal thickness.
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Figure 1. The Central American Volcanic Belt.
The Cocos Plate subducts beneath the Caribbean Plate at a rate of 10 cm/year to the northeast.
Volcanoes are very closely spaced, on average 27 km, which distinguishes the CAVB from other
convergent margin arcs. Adapted from Carr et al. (2003).
The Central American Volcanic Belt erupts andesites and basalts as well as the
infrequent dacite. Andesites along this arc are likely derived from the prolonged crystal
fractionation of basalts (Carr, 1984). In terms of magma composition, the highest silica contents
are found at either end of the arc in Guatemala and Costa Rica with Nicaragua erupting the
most mafic magmas (Carr, 1984). These variations can be explained by the non-uniform crustal
thickness along the arc (Figure 2). Voluminous intrusive and extrusive magmas comprise the
approximately 40-50 km thick crust beneath Guatemala and Costa Rica (Carr, 1984).
Conversely, Tertiary and Cretaceous aged volcanics form the 20-30 km thick crust below
Nicaragua (Carr, 1984). Assuming a common magmatic source, fluids exsolving off the
subducting Cocos slab cause the reduction of the mantle peridotite solidus, which initiates
melting. The initial magnesium-rich melts rise through the mantle wedge and become trapped at
the Moho due to a sharp density difference between the uprising magma and the lower crust.
The high-density magma ponds at the Moho and begins to differentiate and progressively
decreases in density. When the density is sufficiently lowered, the magma rises through the
crust, collects in a chamber and may eventually erupt. The degree of fractionation at the Moho
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is what causes the geochemical gradient along the arc. Beneath Guatemala and Costa Rica,
the thicker crust requires a highly differentiated, lower density, silica rich magma whereas a
thinner crust beneath Nicaragua allows a less differentiated, denser mafic magma to rise;
Nicaraguan magmas are 2.66 g/cm3 while Guatemalan magmas are 2.62 g/cm3 (Carr, 1984).
Magmas beneath Nicaragua are then likely to be trapped by the upper crust and thus form
shallow level intrusions and surficial lava lakes. Therefore, it can be concluded that crustal
thickness leads to varying degrees of fractionation of magma and thus produces the magmatic
diversity along the Central American Volcanic Belt.
Figure 2. Regional variations along the CAVB.
Illustrating the regional variations in magma density, silica content, edifice heights and crustal thickness.
Adapted from Carr (1984).
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Nicaragua
Nicaragua is distinctive along the arc as having the most primitive and mafic magmas,
the lowest degree of mantle melting, low edifice heights and smaller volcano volumes. In fact,
Nicaraguan magmas record the global maximum of slab signals that trace the hemipelagic
sediments of the Cocos slab, such as
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Be/9Be and Ba/La (Bundschuh, et al, 2007). Previous
studies have attributed Nicaraguan magmas as having a distinct enriched mid-ocean ridge
basaltic signature (Carr, 2003). Hydrostatic equilibrium is a major factor that balances
Nicaragua’s thin crust, low edifice heights and dense magma. This equilibrium is maintained by
the expansion of subsurface conduit systems with each explosive eruption (Carr, 1984). Another
major factor that contributes to Nicaragua’s distinct geochemistry is the presence of extensional
faults and fissures that allow for the rapid upwelling of volatile-rich magma through the crust.
The Maribios Range, a volcanic segment in northwestern Nicaragua, hosts 38 volcanic edifices
that appear to be structurally controlled. The oblique and rapid subduction of the Cocos Plate
results in an east-west tensional regime that forms NNW trending normal faults (Bundschuh et
al, 2007). Many volcanoes that lie along these extensional faults record tholeiitic fractionation
patterns. Moving to the northwest and southeast, the olivine field decreases and is correlative
with the increase in crustal thickness and edifice height (Carr, 1984). Nicaragua, based on its
distinct mafic compositions and slab signals, is regarded as the primitive end member of the
Central American Volcanic Belt.
Cerro Negro
Cerro Negro is the youngest volcano in the Central American Volcanic Belt. This
polygenetic basaltic cinder cone first erupted in 1850 and has erupted numerous times since.
Twenty four Strombolian to Subplinian eruptions have constructed a 250 m high black basaltic
cone with an associated lava field that extends to the north-northeast (Figure 3 & 4; Hill et al,
1998). Past eruptions have been classified as having a Volcano Explosivity Index (VEI; Newhall
and Self, 1982) of 0-3 and were frequent, at most occurring every 6 to 7 years. Cerro Negro lies
in close proximity to villages and cities such as Leòn, which is Nicaragua’s second largest city
and is located 20 km to the west (McKnight, 1995). Scattered within a 10 km radius of Cerro
Negro are smaller villages and farmlands totaling to 100,000 people (Connor et al, 2001).
Proximal to Cerro Negro, potential hazards include tephra fall, blocks and bombs, lava flows
and carbon dioxide emissions. Historically, ash fall has been directed westwards towards Leòn
due to dominant easterly trade winds. Ash accumulation can lead to building collapse,
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mechanical damage and adverse health effects. Carbon dioxide levels have recently been
monitored and determined to be passively degassing from the crater, fissures and soil but have
yet to reach toxic levels (e.g., Salazer et al, 2001).
Figure 3. Location of Cerro Negro within the Maribios Range in Nicaragua.
Amongst this tract of volcanoes, Cerro Negro (indicated) is the youngest and one of the most active.
Taken from Portnyagin et al. (2012).
The classification of Cerro Negro as either a polygenetic cinder cone or a young
stratovolcano has been debated in the past. A cinder cone is significantly shorter lived than a
stratovolcano, but a stratovolcano would gradually increase its eruption intensity over time.
Walker and Carr (1986), on the basis of mass eruption rates, regard Cerro Negro as a cinder
cone on the Las Pilas-El Hoyo complex. Conversely, McKnight and Williams (1997) found that,
based on cone morphology and particularly explosive eruptions, Cerro Negro should be
categorized as a young stratovolcano. They stated that Cerro Negro would not achieve the
typical stratovolcano height due to the thin crust beneath Nicaragua. Further lines of evidence
for a stratovolcano lie in the tectonic environment, cone composition and dominant eruptive
products. Cerro Negro has been built by its own scoria deposits and dominantly emits tephra
and carbon dioxide gas as a result of its volatile rich magma. Overall, for long term studies,
Cerro Negro should be classified as a young stratovolcano based on its increasing eruption
intensity. In the short term, however, Cerro Negro should be regarded as a parasitic cinder cone
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and potentially the newest conduit of the Las Pilas-El Hoyo Complex (McKnight and Williams,
1997; Hill et al., 1998).
Figure 4. Cerro Negro and its associated lava fields.
The crater is at an elevation of 390 m.a.s.l. The 1999 flow is not shown but was relatively small scale.
Taken from Hill et al. (1998).
Recent eruptive history
The 1992 eruption at Cerro Negro recorded the highest eruption column to date with a
7.5 km sustained ash column depositing ash as far as 30 km southwest from the main crater
(Salazar et al, 2001). The total tephra fall volume was 2.3 x 107 m3 thus categorizing this as a
VEI 3 Subplinian eruption (Connor et al, 2001). Heavy ashfall in Leòn resulted in building
collapse and the evacuation of 12,000 people from the outskirts as well as neighboring villages
(Connor et al, 2001).
The 79-day long eruption in 1995 comprised two phases of explosive activity, separated
by a 95-day long period of unrest (Hill et al, 1998). Frequent eruptions from a new vent in the
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1992 crater culminated in a phreatomagmatic eruption that produced a small phreatic-driven
pyroclastic surge. The second phase of the eruption produced a 300-m high ash column with
occasional incandescent bombs ejected to an altitude of 200-300 m (McKnight, 1995). A 5 m
high cone formed at the new vent and began to erupt basaltic lava that migrated northward at a
rate of 10-30 m/hour (Hill et al, 1998). The estimated total volume of tephra fall and dense rock
equivalent basalt was 2.16 x 106 m3 and 1.33 x 106 m3, respectively, and thus is classified as a
VEI 2 Strombolian eruption.
The most recent eruption on August 5, 1999 was short-lived, having lower energy than
past eruptions and preceded by large magnitude earthquakes, seismic swarms and noticeable
surface ruptures (La Femina et al, 2004). Activity began with three earthquakes occurring in the
vicinity of Cerro Negro. Seismic swarms then followed these earthquakes and were localized
along the 1992 and 1995 vents. A 200-m long, 1 m wide fissure formed to the south of three
new vents on the southern flank. Two new 40 m tall scoria cones were built by lava fountaining
along the fissure. Small volume lava flows extruded from these cones and resulted in a
combined volume of 6 x 105 m3 of basaltic lava. The total tephra fall volume was 8.4 x 105 m3,
which categorizes this as a VEI 1 Strombolian eruption (Connor et al, 2001).
Geology & Geochemistry
Cerro Negro has the highest values of Ba/La, U/Th and the lowest La/Yb in Central
America, implying a high degree of mantle melting (Portnyagin et al, 2012). The eruptive
products at Cerro Negro have comparatively higher phenocryst content than other regional
basalts (Walker and Carr, 1986). The phenocrysts are large, between 1-7 mm, and dominated
by plagioclase, olivine and pyroxene. The basaltic magma can contain up to 6% water content,
which creates explosive eruptions (Portnyagin et al, 2012). Basalts are effusive and low
viscosity, allowing for the segregation of gases and the settling of phenocrysts from the magma.
This results in gas rich explosions from the main vent and gas poor effusions from fissures
along the flanks of the volcano. Through its evolution, there have been no significant
compositional changes, which could imply a stable magma chamber (Walker and Carr, 1986).
Las Pilas- El Hoyo Complex
Las Pilas is the main edifice of the Las Pilas-El Hoyo Complex, which lies 1 km SE of
Cerro Negro and hosts many adjacent and overlapping eruptive centers (Figure 5; McBirney,
1955). With only 3 confirmed eruptions, between VEI 1-2, the lack of activity at Las Pilas
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juxtaposes its location within one of the most active volcanic arcs in the world. The 1952
eruptions at Las Pilas, the first since its apparent formation in 1528, followed an eruption at
Cerro Negro (McBirney, 1955). Although no unusual activity was documented around the
complex, on October 23, 1952 the summit of Las Pilas was divided by a 1 km long fissure
trending due north-south. From the fissure erupted dense clouds of steam, gas and dust
(McBirney, 1955). Around parts of the fissure were fresh deposits of yellow sulphur as well as
fumaroles emitting low temperature gases. Over time, no vertical or horizontal displacement
was recorded along the fissure. Interestingly, the 1954 eruption at Cerro Negro preceded
another explosion at Las Pilas by a few months. This eruption was mild, sending ash 20 km to
the west in Leon (McBirney, 1955). It has been suggested that the1954 eruption at Cerro Negro
caused a pressure increase within the magma chamber, thus causing Las Pilas to erupt once
more (McBirney, 1955). The pattern of coupled eruptions at Cerro Negro and Las Pilas hint at a
common magma chamber with extending dykes and conduits feeding each system (McKnight,
1995).
Figure 5. The Las Pilas-El Hoyo Complex
Illustration of the Las Pilas-El Hoyo Complex and its numerous volcanic edifices. Cerro Negro can be
seen to the northwest. Adapted from Bice (1980).
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Interconnected plumbing system
McKnight (1995) investigated the potential for a link between Cerro Negro and Las Pilas
using geochemical evidence. He reasoned that since Cerro Negro is located at the intersection
of the regional N70W tensional regime and a shorter volcanic lineament trending N25W,
eruptions must be structurally controlled and the eruptive state of Cerro Negro is dependent
upon changes in the stress regime. Based on whole rock data from 1850 to 1971, a linear trend
of major oxides was noticed between Las Pilas, Cerro Negro and Cerro La Mula, which lies 1
km north of Cerro Negro. Based on major oxide diagrams of SiO2, MgO, K2O and Al2O3, the
general trends of the Las Pilas intersect at high angles to the Cerro Negro trend, with the
intersection occurring near the mafic end member (Figure 6). This intersection, as well as the
similar contents of incompatible elements, suggests that Cerro Negro is part of the Las Pilas
complex and the two share a similar magma source. However, Cerro Negro data consistently
plots within the mafic end member of the trend, implying a unique fractionation process
controlling the final composition of Cerro Negro (McKnight, 1995). This process was proposed
to be phenocryst sorting occurring by relative density within the conduit. During magma ascent,
less dense plagioclase crystals would rise to the top of the conduit whereas denser olivine,
pyroxene and other plagioclase crystals would settle near the bottom. Phenocryst sorting is
plausible within low viscosity basaltic magmas and is also dependent upon eruption rate. A
higher eruption rate, such as the 1992 eruption, would inhibit phenocryst sorting during rapid
ascent. Conversely, a more effusive eruption, such as in 1968, would allow for a greater degree
of phenocryst sorting and lead to a greater compositional difference (McKnight, 1995). Overall,
based on a shared magmatic source, McKnight (1995) concluded that Cerro Negro should be
considered the newest conduit on the Las Pilas complex. He also reasoned that the increase in
eruptive frequency at Cerro Negro after 1947 could be due to the development of a more
efficient conduit that allows larger volumes of melt to ascend.
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Figure 6. Major oxides from whole rock analyses.
Major oxide diagrams from McKnight’s (1995) analyses of whole rocks from Cerro Negro; Cerro Negro
rocks consistently plot towards the mafic end member. Las Pilas rocks (indicated by X’s) appear to
intersect the Cerro Negro trend (large open circles) at high angles.
Recent work by MacQueen (2013) provides geophysical support for the petrogenetic link
proposed by McKnight (1995). Bouguer gravity data from Cerro Negro was collected in order to
image the subsurface structures. Modelling of these data revealed local density anomalies
related to the magmatic plumbing system. Local anomalies include a 2 km3 positive density
anomaly 1 km to the SE of Cerro Negro, connected to a 1 km3 positive density anomaly below
Cerro La Mula and to a 7 km3 positive density anomaly beneath Las Pilas (MacQueen, 2013).
Referring to the progressive appearances of these density anomalies, the base of Cerro Negro
is located at 450 m above sea level (Figure 7). Down to a depth of 1500 m, the density
anomalies of Cerro Negro and Las Pilas are independent and robust (Figure 7d). Below this
depth, however, the two anomalies unite, indicating the presence of an interconnected system
at 2 km (Figure 7 a, b, c). The Cerro Negro anomaly also plots along the regional NNW trend of
fractures, which supports the proposal of structurally controlled dykes.
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Figure 7. Location of positive density anomalies.
Changes in the behavior of the positive density anomalies with depth between Cerro Negro and Las
Pilas. From MacQueen (2013).
Overall, these findings further suggest that Cerro Negro and Las Pilas tap an
interconnected magma chamber beneath Las Pilas and that the density anomalies represent
shallow intrusive complexes feeding each volcanic edifice (Figure 8; MacQueen, 2013). The
depth of the Cerro Negro and Las Pilas anomaly is in agreement with the 1-2 km magma
storage depth of the 1995 magma proposed by Roggensack (1997). This correlation implies
that the chambers imaged by the density anomalies may actually be storage sites for magma
sourced from depth (MacQueen, 2013). Therefore, the shared anomalies, extending to 2 km
depth, could represent a shared pathway for magma before separating along two different paths
towards each edifice. The original magma source must be deeper than 8 km, which is the
maximum depth imaged by the survey.
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Figure 8. 3D view of the modeled plumbing system beneath Cerro Negro and Las Pilas
A model of density anomalies and its implication of a proposed secondary magma site beneath Las Pilas.
Cerro Negro is indicated by a red triangle. From MacQueen (2013).
The focus of this study will be to investigate the proposals by McKnight (1995) and
McQueen (2013) regarding an interconnected magma chamber beneath Las Pilas by using
geochemical evidence to determine the deeper, lower crustal link between Cerro Negro and Las
Pilas.
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Methodology
2.1 Melt inclusions
The imperfect growth of phenocrysts during magma crystallization results in the capture
of small amounts of melt. These inclusions are smaller than the host mineral, between 1 to 300
µm in size, and vary from spherical to elongate in form (Roedder, 1984). Ideally, melt inclusions,
once entrapped, are protected from changes in temperature and pressure by their host mineral
and thereby represent a closed system. Therefore, melt inclusions provide information about the
temperature, pressure and composition of the magma at depth and also during ascent.
Subsequent eruption will cause the inclusions to quench quickly, thus preserving the
composition and volatile content of the inclusion’s silicate melt prior to any fractionation or
degassing processes. Melt inclusions differ from groundmass, which is glass that represents
quenching of the late stage, degassed and modified melt during eruption.
There are two main types of melt inclusions, primary and secondary. Primary inclusions
form during initial crystal growth whereas secondary inclusions are trapped later and within
annealed fractures (Roedder, 1984). Most studies regarding melt inclusions rely on primary
inclusions since their composition is directly related to the melt from which the host minerals
crystallize and thus provide information to considerable depths.
Once entrapped, melt inclusions can undergo modification, which is mainly controlled by
temperature. Due to the thermal contraction of the melt relative to the host mineral, shrinkage
bubbles form within the inclusion. Shrinkage bubbles are smaller than the inclusion itself and
usually account for less than 3 % by volume (Metrich and Wallace, 2008). As the magma
evolves and cools, the internal pressure within the shrinkage bubble may change, causing the
bubble to fill with exsolved H2O or CO2 gas. The condensation of these gases within the melt
during magma ascent may react with the inclusion, causing further modification. Additional
changes may arise due to the fracturing of phenocrysts during explosive eruptions. These
fractures can intercept melt inclusions and facilitate leakage of volatile elements and gases.
During effusive eruptions, however, slower cooling rates can cause crystallization of the host
mineral along the inclusion rim and of daughter minerals within the inclusion itself. Finally, a
diffusive exchange of elements between the host mineral and the melt inclusions can also lead
to a compositional change within the inclusion and the host mineral.
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Overall, melt inclusions can form in most crystals during magma crystallization.
However, as the magma differentiates progressive crystal formation will trap a more evolved
melt. Therefore, mafic minerals, such as olivine and pyroxene, will trap a more primitive melt
than later-stage minerals.
Primary inclusions of primitive melts closely represent the
composition of the parental, volatile bearing magma and are thus invaluable for melt inclusion
studies. Knowing the composition of the primitive melt and the late-stage melt from groundmass
provides information about the evolutionary pathway of the magma and its degassing history.
This study will focus on primary melt inclusions trapped within olivine crystals, since they
are the first phenocrysts to crystallize from a relatively primitive magma. In cases where no
viable melt inclusions are found in olivine crystals, pyroxene-hosted inclusions will be studied.
Though pyroxene crystallizes later than olivine and evidently traps more evolved melt, these
inclusions will nonetheless provide valuable information about the evolution of magma and its
intermediary composition between primitive and eruptive.
2.2 Sample description
Whole Rock
Whole rock samples from both Cerro Negro and Las Pilas were collected in 2013 by Jeffery
Zurek and Patricia MacQueen. Cerro Negro samples were from the 1999 eruption whereas Las
Pilas samples are believed to be from the 1528 eruption.
Cerro Negro
The samples are fresh, dark grey to black, vesicular subalkaline basalts (48.5 wt % SiO2)
that are distinctively low in K2O (0.44 wt %) and high in MgO (7.27 wt %). Phenocrysts are 1-4
mm and dominated by plagioclase (An70, 15 % by volume), olivine (Fo82 – Fo77, 10 %) and
pyroxene (10 %). The average amount of phenocrysts in the sample is estimated at 25 %. Most
phenocrysts are fragmented, with some being subhedral and occurring as groups
(glomeroporphyritic). Both plagioclase and pyroxene phenocrysts are twinned and also occur as
microlites in the finer grained groundmass. Some pyroxenes exhibit weak light brown to pale
green pleiochroism. The presence of small magnetite grains was evident in the crushed sample.
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Figure 9. Representative thin section from Cerro Negro, 1999.
Phenocrysts are labeled. Olv = olivine, plag = plagioclase, pyx = pyroxene. Cross polars (XPL);
magnification 5x; width of the photo is 3.02 mm.
Las Pilas
Las Pilas tephras were collected close to the edifice from the 1528 eruption. These
samples are weathered, brown vesicular subalkaline basalts (50.94 wt % SiO2) with a much
larger grain size than Cerro Negro. Phenocrysts are 1-7 mm and dominated by plagioclase
(An33- An37, 20 % by volume), pyroxene (En64 – En82, 15 %) and olivine (1-5 %). The average
amount of phenocrysts by volume is estimated at 35 %. Phenocrysts are glomeroporphyritic and
the groundmass contains plagioclase microlites.
23!
!
Figure 10. Representative thin section of the 1528 eruption at Las Pilas
Representative thin section from Las Pilas 1528. Phenocrysts are labeled. Olv = olivine, plag =
plagioclase, pyx = pyroxene. Cross polars (XPL); magnification 2x; width of the photo is 7.57 mm.
2.3 Melt Inclusion description
Cerro Negro
Melt inclusions from Cerro Negro olivines occur either as single inclusions or as groups
with a larger inclusion found among several smaller ones. All inclusions are glassy and range in
size from a few microns up to 200 microns. Larger inclusions are more likely to have a fine wall
faceting than smaller ones. On average, inclusions occupied a volume of less than 3 % and are
elliptical to oblate in shape. Almost all inclusions are distinctively darker than their host olivine
and most contain a single shrinkage bubble either near the center of the inclusion or near the
margin. Some inclusions contain 2 smaller bubbles but only one sample contained a rim of up to
5 smaller shrinkage bubbles. The shrinkage bubbles occupied less than 2 % by volume and on
average 1 %. Melt inclusions with fractures and daughter minerals were sparse and avoided.
24!
!
Figure 11. Olivine host with melt inclusions from Cerro Negro.
Sample 99C. Fo content of the olivine host is 78. Numerous melt inclusions are present, however, the top
2 were analyzed.
Las Pilas
Since olivine phenocrysts did not contain any noticeable melt inclusions, pyroxenehosted melt inclusions were studied instead. A plethora of small inclusions are available in most
pyroxene crystals. All inclusions are glassy, of similar colour to the host pyroxene and a few
microns in diameter on average. Inclusions are irregularly shaped, but those sampled were
elongate and usually contain a single shrinkage bubble near the margin. The shrinkage bubbles
occupy less than 0.8 % by volume. Daughter minerals were rare and melt inclusions with such
minerals were not sampled.
25!
!
Figure 12. Pyroxene host with melt inclusions from Las Pilas.
Sample LP-E. En content of the pyroxene is 70. Numerous melt inclusions are present, however only a
few were large enough for analysis.
2.4 Sample preparation
Melt inclusions were prepared at the Cordilleran Geology Lab in the Department of Earth
Sciences at Simon Fraser University. Sample preparation was the same for Cerro Negro
olivines and Las Pilas pyroxenes.
Bulk rocks were crushed to a median grain size of 3 mm. Olivine crystals from Cerro
Negro and pyroxene crystals from Las Pilas were picked from these grains using a binocular
microscope. Crystals were then placed within a mound of liquid silica onto glass slides; each
glass slide contained up to 18 crystals. Once cooled, the glass slides were examined under a
petrographic microscope. Inclusions were chosen in host minerals free of any fractures or
daughter minerals. Inclusions tended to be small (200 microns or less in size) and difficult to
detect. Once all inclusion-bearing minerals were marked, they were carefully removed from the
glass slide using a heating plate. These crystals were then washed with acetone and placed
within a brass holder using epoxy. The surfaces of the holders containing the crystal were then
26!
!
carefully and individually polished using a range of 30 to 1 µm pads. Once a representative
exposure of the desired inclusions was obtained, the holders were placed into stainless steel
pucks with 8 slots. All of the holders were set such that the surface of the puck was even and
contained all exposed inclusions. Each puck was then polished using a 0.3 µm alumina
polishing compound.
Sample analysis
Electron microprobe
The major and volatile elemental compositions of melt inclusions, host crystals and
matrix glasses were analyzed at the Laboratoire Magmas et Volcans (LMV, Clermont-Ferrand,
France) using a SX-100 CAMECA electron microprobe (EMP) and a 15 kV accelerating voltage.
Calibration was based on a mixture of glasses and synthetic or natural mineral standards.
Mineral analyses were done using a 1 µm focused beam, which widened to a 5-10 µm during
glass analyses to reduce Na loss. Volatile analyses were measured with an 80 nA sample
current and a 50 s acquisition time using the LPET diffraction crystal for Cl and S and a 300 s
acquisition with TAP crystals for F. To reduce volatile loss during analyses, measurements were
taken at 20 second intervals. During sulphur analysis, variations in the wavelength of sulphur Kα
X-ray, a function of its oxidation state in silicate glasses, were considered. The precision of the
EMP is better than 5 % for major elements excluding MnO, Na2O, K2O and P2O5, which had an
EMP precision < 10 %. The approximate 2σ precision for Cl, S and F is 7 %, 4 % and 28 %,
respectively. Major element compositions of the whole rocks were analyzed by ICP AES at
LMV. The associated errors are approximately 10 %.
Every melt inclusion, host crystal and matrix glass was measured more than once to
ensure a reliable range of data. Melt inclusions were sampled at least twice; most were sampled
three times. Host crystals were also analyzed at least twice to ensure compositional
homogeneity; one analysis was done near the rim of the inclusion and the other away from the
inclusion. If the host crystal contained more than one inclusion, as was the case for some
olivines and all pyroxenes, each inclusion corresponded to two analyses of the host crystal.
Groundmass measurements were preformed five times and whole rocks were measured once.
Analyzed values are displayed in Table 1.
27!
!
Post entrapment modifications
Melt inclusions were chosen such that they exhibited no obvious compositional changes
following entrapment. However, modification due to crystallization of the host mineral and
diffusive exchanges is difficult to avoid and must be compensated by performing calculations in
order to restore the inclusion to its original composition. These calculations are well known and
established for olivine-hosted inclusions and will be applied here. However, calculations based
on pyroxene-hosted inclusions have yet to be perfected as they contain large errors and thus
will not be preformed. Nonetheless, these inclusions still provide valuable information of magma
evolution.
Olivine-melt equilibrium constant
To determine the original composition of the melt inclusion, the inclusion is corrected for
any olivine host crystallization. This calculation is based on KD, the olivine-melt equilibrium
constant between ferrous iron and magnesium, which is well established at 0.3 ± 0.03 (Roeder
and Emslie, 1970). Prior to any corrections, the KD values of our melt inclusions ranged between
0.15 – 0.34. By incrementally adding 0.1 wt % of olivine into the melt, the equilibrium
composition between the host and inclusion was determined. The amount of post-entrapment
crystallization (PEC) of olivine ranged between 0 and 11 %, with most inclusions less than 5 %.
PEC corrected values and corrected melt inclusion compositions are found in Table 1.
Diffusive exchange
Further modifications arise due to diffusive exchange of Fe and Mg during crystallization
and equilibration of an olivine overgrown along the inclusion rim. However, such compositional
changes would result in KD values significantly higher than 0.3, which was not the case.
Additionally, total FeO (FeOT) of melt inclusions would be much lower than the host rock and the
expected composition of the initially trapped inclusion. Within Cerro Negro rocks, Fe loss was
minimal as shown by the high FeOT content of the melt inclusion (8.9-14.1 wt %) encompassing
the FeOT of the whole rock (11.14 wt %).
28!
!
Table 1
Cerro Negro
All oxides are reported in weight percent.
T
MnO
MgO
CaO
Na2O
K 2O
P 2O 5
S (ppm)
Cl (ppm)
F (ppm)
Cl/K2O
Mg#
Kd
PEC %
16.511
11.695
0.050
7.110
11.121
1.976
0.313
0.198
1449.90
1111.32
24.44
0.355
51.997
0.30
10
0.713
16.183
12.827
0.182
7.626
10.847
1.824
0.522
0.091
-
-
-
-
51.439
0.30
10
Melt Inclusion
SiO2
TiO2
Al2O3
CN-1 99A MI-1
46.258
0.767
CN-1 99A MI-2
45.514
FeO
CN-1 99B
48.061
0.907
16.348
10.669
0.122
7.398
10.666
2.210
0.461
0.247
998.64
96.82
60.42
0.021
55.269
0.30
5
CN-1 99C MI-1
46.517
0.921
16.483
10.821
0.135
7.161
10.751
1.958
0.358
0.152
1213.63
756.56
146.02
0.211
54.109
0.30
2
CN-1 99C MI-2
46.334
0.793
16.791
10.634
0.276
6.928
11.046
1.881
0.378
0.269
1225.00
794.19
64.68
0.210
53.720
0.30
2
CN-1 99D
46.001
0.795
17.080
10.918
0.234
8.082
11.435
1.845
0.253
1.285
1157.85
699.55
189.72
0.276
56.876
0.30
7
CN-1 99E MI-1
45.318
0.778
17.216
11.323
0.319
7.179
11.258
1.786
0.242
0.105
1435.97
845.57
117.12
0.349
53.043
0.30
4
CN-1 99E MI-2
46.247
0.703
17.751
11.215
0.184
7.182
11.719
1.675
0.265
0.070
1462.23
795.76
226.31
0.301
53.295
0.30
1
CN-1 99F MI-1
45.619
0.811
16.448
14.113
0.230
7.434
10.260
2.105
0.313
0.085
1517.09
1291.39
204.88
0.413
48.414
0.30
11
CN-1 99G
45.324
1.130
18.269
9.253
0.168
6.512
11.284
2.145
0.336
0.208
1378.92
796.40
266.33
0.237
55.631
0.30
0.1
CN-1 99H
46.462
0.723
16.496
11.907
0.315
7.346
10.471
1.693
0.341
0.171
1541.98
952.01
67.90
0.279
52.362
0.30
7.5
CN2 99J MI-1
46.929
0.788
18.521
8.986
0.155
6.470
11.428
2.017
0.312
0.119
1392.60
761.40
79.20
0.244
56.194
0.31
0
CN2 99J MI-2
44.580
1.666
17.348
10.751
0.162
7.678
11.202
1.981
0.378
0.309
1110.84
783.18
155.01
0.207
55.995
0.30
3
CN2 99K
46.402
0.775
16.777
10.451
0.137
7.509
11.220
1.356
0.255
0.128
1364.16
802.62
138.18
0.315
56.143
0.30
2
CN2 99L
46.468
0.760
17.047
10.568
0.072
6.295
10.294
1.929
0.307
0.172
1611.77
721.95
237.72
0.235
51.485
0.30
0.2
CN2 99N
46.634
0.791
17.119
9.730
0.192
6.678
11.091
1.952
0.360
0.139
1277.20
810.40
17.00
0.225
55.014
0.34
0
CN2 99O
46.017
0.789
17.547
10.294
0.100
7.500
10.808
2.068
0.312
0.101
1264.86
663.48
89.10
0.212
56.487
0.30
10
T
MnO
MgO
CaO
Na2O
K 2O
Fo content
Mg#
Host Olivine
SiO2
TiO2
Al2O3
FeO
CN-1 99A
39.362
0.011
0.022
20.247
0.274
40.533
0.190
0.002
0.011
78.111
78.103
CN-1 99B
39.190
0.010
0.031
17.813
0.373
41.682
0.217
0.002
0.001
80.662
80.655
CN-1 99C
39.302
0.019
0.036
20.364
0.344
40.459
0.195
0.000
0.004
77.972
77.974
CN-1 99D
39.739
0.011
0.030
17.342
0.332
43.078
0.173
0.006
0.001
81.577
81.570
CN-1 99E
39.251
0.004
0.016
19.344
0.302
41.057
0.195
0.015
0.000
79.095
79.087
CN-1 99F
38.487
0.004
0.044
21.817
0.312
38.397
0.187
0.033
0.002
75.830
75.821
CN-1 99G
39.284
0.000
0.040
17.742
0.138
42.147
0.194
0.000
0.000
80.896
80.889
CN-1 99H
39.238
0.000
0.000
19.882
0.357
40.843
0.176
0.020
0.001
78.550
78.542
CN-2 99J
39.004
0.015
0.023
18.261
0.252
42.790
0.191
0.000
0.001
80.684
80.677
CN-2 99K
39.292
0.018
0.002
18.049
0.290
43.066
0.197
0.000
0.000
80.964
80.958
CN-2 99L
38.466
0.006
0.030
20.242
0.322
40.774
0.184
0.006
0.000
78.216
78.209
CN-2 99N
38.919
0.012
0.018
20.035
0.238
40.879
0.225
0.014
0.000
78.435
78.427
CN-2 99O
38.602
0.018
0.021
17.681
0.291
42.534
0.178
0.002
0.011
81.090
81.083
!
29!
Groundmass
GM1
GM2
GM3
GM4
GM5
Whole Rock
WR-1
SiO2
56.641
54.877
55.408
56.039
54.869
SiO2
48.5
TiO2
1.7259
1.5921
1.5951
1.6363
1.7168
TiO2
0.72
T
Al2O3
12.5373
12.5126
12.7688
12.4317
12.8924
Al2O3
17.59
FeO
13.9301
14.6382
13.6135
14.0556
14.4865
T
FeO
11.14
MnO
0.19
MnO
0.2422
0.4343
0.3343
0.3341
0.459
MgO
7.27
MgO
3.3994
3.5324
3.2629
3.8269
3.4224
CaO
12.23
CaO
7.7782
8.0152
7.6307
7.6319
7.9068
Na2O
1.86
K 2O
0.44
Na2O
2.8333
2.2539
3.2918
2.7703
2.1738
K 2O
1.039
0.9781
1.1145
1.0259
1.0436
P 2O 5
0.2738
0.2488
0.2645
0.2805
0.1613
Cl (ppm)
1146
1076
1221.8
1350.2
1187.2
F (ppm)
479.4
537.6
454.4
526.8
460.6
S (ppm)
7.2
12.8
23.2
5.6
23.8
P 2O 5
0.11
Las Pilas
T
MnO
MgO
CaO
Na2O
K 2O
P 2O 5
Total
Mg#
15.727
12.757
0.145
3.762
7.864
2.967
0.991
0.186
97.967
34.445
1.034
18.482
10.039
0.331
3.849
6.813
3.030
0.572
0.261
95.617
40.583
1.327
17.115
9.387
0.179
2.688
6.297
3.475
1.440
0.398
97.570
33.782
54.695
1.513
16.634
11.142
0.185
2.366
6.390
2.855
1.283
0.326
97.389
27.449
LPB MI-1
55.622
1.381
13.527
11.010
0.282
3.973
8.111
2.597
1.197
0.315
98.014
39.132
LPB MI-2
51.683
1.348
13.326
13.195
0.354
4.841
8.707
2.433
0.844
0.233
96.964
39.528
LPB MI-3
55.215
1.208
14.031
10.489
0.346
4.331
8.682
2.888
0.958
0.138
98.285
42.383
LPB MI-4
52.697
1.151
15.078
9.898
0.184
4.805
8.826
3.174
0.988
0.218
97.019
46.376
LPC MI-1
51.529
1.243
15.778
12.199
0.279
4.534
8.505
2.600
0.803
0.201
97.669
39.839
LPD MI-1
54.541
1.384
13.145
13.895
0.555
4.056
8.430
2.663
1.200
0.357
100.224
34.213
LPD MI-2
52.435
1.545
11.879
17.630
0.313
3.106
7.387
2.473
0.908
0.136
97.812
23.889
LPE MI-1
55.167
1.300
13.480
12.708
0.312
3.482
7.079
3.196
1.339
0.242
98.304
32.808
LPE MI-2
56.221
0.976
14.529
10.320
0.360
3.637
7.091
3.440
1.501
0.308
98.383
38.572
LPE MI-3
51.778
1.203
14.575
12.507
0.392
4.122
8.225
2.942
0.990
0.266
96.999
36.994
LPF MI-1
51.953
1.156
14.984
11.045
0.256
4.074
8.048
2.937
1.117
0.342
95.912
39.657
Melt Inclusion
SiO2
TiO2
Al2O3
LPA MI-1
52.332
1.235
LPA MI-2
51.208
LPA MI-3
55.265
LPA MI-4
FeO
LPF MI-2
52.220
1.043
14.811
12.099
0.065
4.744
8.578
2.723
0.981
0.258
97.523
41.129
LPG MI-1
58.417
1.034
14.694
8.865
0.208
2.986
6.504
3.591
2.201
0.315
98.815
37.506
LPH MI-1
49.965
1.668
13.129
16.562
0.334
3.873
8.293
2.749
0.611
0.112
97.294
29.409
LPH MI-2
50.317
1.578
14.888
15.136
0.606
3.724
7.913
2.570
0.663
0.145
97.539
30.476
LPH M-3
49.992
1.544
15.325
14.867
0.418
4.365
7.888
2.504
0.527
0.205
97.635
34.343
!
30!
Melt Inclusion
Cl (ppm)
F (ppm)
S (ppm)
LPA MI-1
1064.2
472.8
643.6
LPA MI-2
1113.600
521.600
1273.800
LPA MI-3
1227.800
362.200
375.200
LPA MI-4
1177.400
337.200
335.000
LPB MI-1
991.200
310.800
366.800
LPB MI-2
863.600
263.800
473.000
LPB MI-3
835.400
197.200
453.600
LPB MI-4
987.200
285.000
594.200
LPC MI-1
859.400
240.200
859.800
LPD MI-2
922.400
310.600
352.800
LPE MI-1
1399.000
343.800
547.000
LPE MI-2
1543.400
564.200
567.600
LPF MI-2
821.400
254.400
440.600
LPG MI-1
1769.400
696.600
370.400
LPH MI-2
919.200
538.800
712.400
T
MnO
MgO
CaO
Na2O
K 2O
Mg#
En content
3.043
6.218
0.154
16.264
21.263
0.219
0.004
15.878
82.315
2.048
12.157
0.337
14.789
18.234
0.229
0.000
14.020
68.409
4.482
6.445
0.123
15.892
21.130
0.220
0.004
15.442
81.466
1.753
13.843
0.484
14.101
17.761
0.281
0.000
13.130
64.485
0.465
2.045
10.998
0.433
15.045
18.935
0.282
0.009
14.150
70.918
0.484
3.122
9.347
0.325
15.885
18.995
0.251
0.000
15.087
75.183
51.121
0.456
2.107
10.111
0.328
14.833
19.728
0.231
0.005
13.998
72.337
51.479
0.486
2.487
9.925
0.299
15.225
19.786
0.249
0.002
14.442
73.152
Groundmass
SiO2
TiO2
Al2O3
GM1
54.405
1.3666
GM2
53.869
GM3
53.892
GM4
GM5
Host Pyroxene
SiO2
TiO2
Al2O3
LPA
51.402
0.352
LPB
51.246
0.417
LPC
51.404
0.460
LPD
51.428
0.516
LPE
51.636
LPF
51.047
LPG
LPH
Whole rock
WR-1
!
FeO
T
MnO
MgO
CaO
Na2O
K 2O
P 2O 5
Cl (ppm)
F (ppm)
S (ppm)
14.2492
10.3816
0.3048
4.6273
8.3919
3.0364
1.4039
0.3492
98.5167
749.2
562.6
1.2479
14.868
11.4399
0.2155
4.6577
8.7724
3.3245
1.3183
0.1942
99.9076
744.8
575.4
1.2982
14.731
11.7217
0.2953
4.9708
8.9335
2.7428
1.1563
0.2345
99.9762
764.8
268.6
53.41
1.3275
15.026
11.4212
0.2549
4.9243
8.9456
2.6671
1.1776
0.162
99.3193
699.2
427.8
53.148
1.3281
14.9477
11.5676
0.1413
4.8469
8.9927
2.7518
1.1546
0.1937
99.0732
706.4
615.8
SiO2
50.94
TiO2
0.88
Al2O3
16.64
FeO
T
FeO
10.63
MnO
0.19
MgO
5.94
CaO
10.98
Na2O
2.45
K 2O
0.8
P 2O 5
0.19
31!
Results
3.1 Cerro Negro
In total, 17 melt inclusions from 13 minerals were analyzed from Cerro Negro. Melt
inclusion compositions corrected for post entrapment crystallization can be found in Table 1;
uncorrected values are shown in Appendix 1.
The sampled whole rock from Cerro Negro has major oxide content of 48.5 wt % SiO2,
7.27 wt % MgO and a total alkali content (Na2O + K2O) of 2.30 wt %, thereby classifying the
rocks as relatively primitive, subalklaine basalts (Figure 13). Inclusion bearing olivines comprise
a narrow Forsterite range (Fo75- Fo81.5), which implies crystallization occurred within a limited
temperature interval (Portnyagin et al, 2012).
Figure 13. Ternary and alkalinity plot of Cerro Negro and Las Pilas whole rocks.
a) Ternary plot of whole rock data from Cerro Negro (blue) and Las Pilas (red). A= total alkalis (Na2O wt
% + K2O wt %). F= total iron (Fe2O3 wt % + FeO wt %). M= total magnesium (MgO wt %). b) Both Cerro
Negro and Las Pilas are classified as subalkaline magmas, specifically calc-alkaline. Ternary plot
template from Marshall (1996).
32!
!
b)
Cerro#Negro#
Las#Pilas#
!12!!
Na2O%wt%%+%K2O%(wt%%)%
!11!!
!10!!
!9!!
!8!!
!7!!
!6!!
!5!!
!4!!
!3!!
!2!!
35#
40#
45#
50#
SiO2%(wt%%)%
55#
60#
65#
Figure 13. Continued from previous page
Melt inclusions from the same olivine host do not vary considerably in major element and
volatile contents, the variations are within errors of 4 % S, 7 % Cl and 28 % F. This suggests
that melt inclusions sufficiently represent the melt trapped at the time of mineral crystallization.
Volatile contents of inclusions do not show an obvious relationship with the presence of a
shrinkage bubble, which implies that the volatile content was not modified by post entrapment
diffusion into the shrinkage bubble. However, the two inclusions CNJ-1 and CNJ-2 show slight
variations in the host Fo content (78.5 vs 80.6), major oxides (46.9-44.5 wt % SiO2, 0.78-1.66 wt
% TiO2, 18.5-17.34 wt % Al2O3, 89-10.7 wt % FeOT) and in F concentration (79.2-115 ppm).
Additionally, CNJ-1 also contained the largest shrinkage bubble and the highest SiO2 and Al2O3
but the lowest TiO2, FeOT and F content (Table 1). The cause for this variation is a
compositional zonation and irregular growth within the host olivine, causing entrapment of two
different melts; CNJ-1 likely was the later melt since its composition is relatively evolved.
Melt inclusions have well constrained oxide contents and are basaltic in composition:
44.5-48 wt % SiO2, 16.1-18.5 wt % Al2O3, 8.9-14.1 wt % FeOT, 6.2-8.1 wt % MgO and 0.24-0.52
wt % K2O. The Fo content of host olivines and the MgO content of melt inclusions correlate
positively with Al2O3 and CaO and negatively with SiO2, Na2O, K2O and P2O5 of inclusions.
Since the latter oxides increase in concentration with magmatic evolution, this pattern implies
that more primitive olivine hosts (Fo>78) have trapped more primitive melts (MgO >7 wt%). A
33!
!
strong negative correlation exists between the Fo content of the olivine host and FeOT content
of the inclusion (Figure 14). The most primitive olivine host (Fo81.5) contains some of the lowest
FeO, CaO and K2O contents sampled (17.34 wt %, 0.173 wt % and 0.0011 wt %, respectively).
The corresponding melt inclusion contains some of the highest MgO and CaO contents (8.08 wt
% and 11.43 wt %, respectively). Conversely, the composition of the melt hosted within the least
primitive olivine (Fo75) contains some of the lowest content of most oxides; 75 wt % SiO2, 16.4
wt % Al2O3, 7.43 wt % MgO and 10.3 wt % CaO but contain the highest FeOT (10.4 wt %).
a)
Portnyagin#MI#
CN#1999#MI#
PRIM#
EVOL#
56!
Melt SiO2 (wt %)
54!
52!
50!
48!
46!
44!
68#
70#
72#
74#
76#
78#
80#
82#
84#
Olivine Fo
b)
16!
15!
Melt FeO (wt %)
14!
13!
12!
11!
10!
9!
8!
68#
70#
72#
74#
76#
78#
80#
82#
84#
Olivine Fo
34!
!
c)
14!
Melt CaO (wt %)
13!
12!
11!
10!
9!
8!
7!
68#
70#
72#
74#
76#
78#
80#
82#
84#
Olivine Fo
d)
1!
Melt K2O (wt %)
1!
1!
0!
0!
0!
68#
70#
72#
74#
76#
78#
80#
82#
84#
Olivine Fo
Figure 14. Major oxides of melt inclusions versus the Fo content of host olivines.
Melt inclusion data, including data from Portnyagin et al. (2012). Graphs a, b and d exhibit a negative
correlation between Fo content and SiO2, FeO and K2O. This shows that primitive olivines (Fo>78) trap
less evolved melts. Graph c shows a positive correlation exists between the Fo content of the host olivine
and the CaO content of the melt inclusions. The values of PRIM and EVOL represent the most primitive
and evolved melt inclusion compositions, respectively (discussed below).
Volatile concentrations (S, Cl and F) of inclusions range between 998-1611 ppm, 961291 ppm, and 17-266 ppm, respectively. The relative enrichment, especially of sulphur,
35!
!
supports the fact that the melt inclusions represent either a partially degassed or non-degassed
magma as S, Cl and F are incompatible elements and thus increase in concentration within the
melt during differentiation. The most primitive inclusions, hosted in olivines with Fo>78, contain
an average of 1325 ppm S, 759 ppm Cl and 125 ppm F. The most primitive melt inclusion,
hosted in olivine Fo81.5, contain lower S and Cl (1157 ppm and 699 ppm, respectively) and
higher F (189 ppm) than average. There is a rough negative correlation between inclusion
contents of Cl and host olivine Fo such that the inclusion with the highest Cl content was
trapped within the least primitive olivine (1291 ppm Cl and Fo75). Additionally, variations of
volatile contents relative to the K2O content of melt inclusions and groundmass illustrate the
process of degassing between primitive and evolved melts during ascent and olivine
crystallization (Figure 15). Similar to Figure 14, a trend can be delineated from relatively volatile
rich melt inclusions towards the degassed and evolved groundmass. Referring to Figure 15a,
the primitive melt, represented by the melt inclusions, are initially sulphur rich and progressively
deplete during magma evolution. Figure 15b, c show the opposite; the primitive melt becomes
enriched in Cl and F with evolution indicating incompatible behavior. The average Cl/K2O ratio is
0.25, which is typical of Nicaraguan arc basalts (Ezra et al, 2007). The decrease in sulphur and
the increase in chlorine and fluorine during differentiation are due to respective solubilities. The
solubility of Cl increases with the ratio of Na + K/Al whereas F is known to be very soluble in
silicate melts (Wallace and Anderson, 2000).
a)
CN99_MI
CN99_GM
S (ppm)
250#
1#
0#
0.2#
0.4#
0.6#
0.8#
1#
1.2#
1.4#
K2O (wt %)
36!
!
b) 2000#
Cl (ppm)
1500#
1000#
500#
0#
0#
0.2#
0.4#
0#
0.2#
0.4#
0.6#
0.8#
K2O (wt %)
1#
1.2#
1.4#
1#
1.2#
1.4#
c) 700#
600#
F (ppm)
500#
400#
300#
200#
100#
0#
0.6#
0.8#
K2O (wt %)
Figure 15. Volatiles vs K2O wt % of melt inclusions and groundmass
a) S (log scale, ppm) vs K2O wt %. Melt inclusion values are between 1000ppm and 1610 ppm. The
primitive melt (melt inclusion) is volatile rich compared to the degassed evolved melt (groundmass). b) Cl
(ppm) vs K2O wt %. c) F (ppm) vs K2O wt %. Enrichment of both Cl and F during magma evolution
illustrates their incompatible behavior. Error bars are 2σ.
Groundmass values plot within the expected trend such that they are the most evolved
amongst whole rock and melt inclusion data (Table 1). Groundmass composition vary from 54.856.6 wt % SiO2, 12.4-12.8 wt % Al2O3, 13.6-14.6 wt % FeOT, 3.2-3.8 wt % MgO, 7.6-8 wt % CaO
and 0.97-1.11 wt % K2O; these narrow ranges imply that the melt has a uniform composition.
Compared to the whole rock, groundmass glass has higher contents of SiO2 and CaO but lower
37!
!
values of MgO, Al2O3, FeOT and K2O. Compared to melt inclusions, groundmass glass has higher
contents of SiO2, FeOT and K2O but lower concentrations of Al2O3 and MgO. The average S content
(14 ppm) is lower than the least primitive olivine host, implying considerable sulphur degassing from
the melt during fractionation. However, the average Cl and F content, 759 ppm and 491 ppm,
respectively, are higher than primitive olivine hosts (Fo>78), implying minimal Cl and F fractionation
during magma ascent.
Major oxides of Cerro Negro
Figure 16 compares the major oxides against K2O contents of melt inclusions, whole rock
and groundmass at Cerro Negro. A strong correlation can be seen between all oxides such that the
melt inclusion data represents the primitive end member of magmatic evolution. Groundmass values
consistently plot towards the evolved, fractionated portion of the trend. Whole rock data deviates
from its theoretical plot near groundmass data and instead plots near the melt inclusions. A distinct
magmatic pathway from primitive to evolved can be discerned from the melt inclusions and whole
rock towards the groundmass.
38!
!
Figure 16. Comparison of major oxides at Cerro Negro.
Melt inclusions (blue diamond), groundmass (cyan triangles) and whole rock (orange square) compared
to K2O wt%. There is a distinct compositional difference between the melt inclusion and whole rock trend
T
with the groundmass data. The groundmass is enriched in SiO2, TiO2 and FeO relative to melt inclusions
(a, b, c). Conversely, melt inclusions are enriched in Al2O3. In each graph, the groundmass is more
evolved (i.e. K2O rich) than the melt inclusions (i.e. K2O poor). Melt inclusions consistently define the
primitive end member of the compositional trend. Whole rock data plot close to melt inclusions,
suggesting primitive (or high Mg) basalt. Error bars are 2σ. See Table 1 for details.
39!
!
Figure 16. Continued from previous page.
Melt inclusions are enriched in CaO and MgO compared to groundmass (e, g). Both melt inclusions and
groundmass contain similar Na2O and P2O5 contents. Melt inclusions consistently define the primitive end
member (i.e. K2O poor) compared to the evolved groundmass.
3.2 Las Pilas
In total, 20 melt inclusions from 8 pyroxene crystals were analyzed; compositions are
shown in Table 1. Las Pilas whole rock data contain 50.94 wt % SiO2, 5.94 wt % MgO and a
total alkali content of 3.25 wt %, classifying Las Pilas as an evolved subalkaline basalt (Figure
13). Pyroxene hosts have a relatively broad range of enstatite and Mg#, En64-En82 and 23-46
respectively, suggesting a wide crystallization interval during magma ascent.
Melt inclusions contain 49.9-58.4 wt % SiO2, 11.8-18.4 wt % Al2O3, 8.8-17.6 wt % FeOT,
2.4-4.8 wt % MgO, 6.3-8.8 wt % CaO and 0.5-2.2 wt % K2O. The melts are basaltic, similar to
the whole rock. The En content of the host mineral is positively correlated with SiO2, FeOT, CaO
and K2O but negatively with Al2O3 and MgO. The former increases in concentration with
evolution whereas the latter deplete. Thus this pattern shows that more primitive pyroxenes
40!
!
(En>72) trap a more primitive melt (MgO >3 wt %). The highest En content host (En82) does not
appear to have any relative enrichments or depletions of oxides, which could be a result of post
entrapment crystallization.
Volatile contents of melt inclusions ranged from 335-1273 ppm S, 821-1769 ppm Cl and
197-696 ppm F. The average volatile contents of inclusions trapped within least modified
pyroxenes (En>72) were 1189 ppm Cl, 612 ppm S and 433 ppm F. Inclusions within evolved
pyroxenes (En<70) contained 919 ppm Cl, 448 ppm S and 273 ppm F, on average and the
inclusion with the highest S content was hosted within the pyroxene with the greatest En content
(Table 1). As can be seen, decreasing the En and MgO wt % content of the host pyroxene
correlates to a decrease in volatiles within melt inclusions (Figure 17). The melt inclusion LP-G
contained the highest Cl and F values as well as the highest SiO2 (58.4 wt %) and K2O (2.2 wt
%) and lowest MgO (2.9 wt %) contents suggesting minimal Cl and F fractionation during
magma ascent.
Variations of volatile contents relative to the K2O wt % content illustrate the degassing
history tracked by both melt inclusions and groundmass (Figure 17). Referring to S (ppm)
versus K2O wt % (Figure 17a), there is a distinct drop in sulphur content from melt inclusions to
groundmass. A similar trend of decreasing volatile content during magma evolution is shown by
the Cl (ppm) content versus K2O wt % (Figure 17b). However, unlike sulphur there is no distinct
jump but rather a smooth decrease in Cl (ppm) from melt inclusions to groundmass. Fluorine
data is more variable but appears to be in similar concentrations between melt inclusions and
groundmass.
41!
!
a)
LP_MI
LP_GM
1400#
S (ppm)
1200#
1000#
800#
600#
400#
200#
0#
0.4#
0.6#
0.8#
1#
1.2#
1.4#
1.6#
1.8#
2#
2.2#
2.4#
2.6#
K2O (wt %)
b)
1800#
Cl (ppm)
1500#
1200#
900#
600#
300#
0#
0.4#
0.6#
0.8#
1#
1.2#
1.4#
1.6#
1.8#
K2O (wt %)
2#
2.2#
2.4#
2.6#
Figure 17. Volatile variations versus K2O.
a) S (ppm) vs K2O wt % b) Cl (ppm) vs K2O wt % c) F (ppm) vs K2O wt % of Las Pilas melt inclusions
(green triangle) and groundmass (red circle). Degassing trends can be delineated based on S and Cl
content (a and b). Melt inclusions are S and Cl rich and progressively deplete towards the groundmass
values. The relatively similar F concentrations between melt inclusion and groundmass data imply that
there was minimal F fractionation during magmatic evolution.
42!
!
c)
700#
600#
F (ppm)
500#
400#
300#
200#
100#
0#
0.4#
0.6#
0.8#
1#
1.2#
1.4#
1.6#
K2O (wt %)
1.8#
2#
2.2#
2.4#
2.6#
Figure 17. Continued from previous page.
Melt inclusions hosted in the same pyroxene crystal show minimal variations in volatile
or major elements contents. The exception is pyroxene LPA, which hosts 4 melt inclusions.
Variations are shown in FeOT content (9.3-12.7 wt %) and Mg# (27.4-40.5), but the most
significant is the S content varying from the highest value recorded to the lowest (335 ppm and
1273 ppm S), contained within one crystal (Table 1). This pyroxene crystal is likely zoned and
trapped different melts during growth. Additionally, inclusions without shrinkage bubbles (LPB
and LPF) contain the lowest Cl and F contents (821 ppm and 197 ppm, respectively). In order to
determine whether post entrapment diffusion is an important process, the volatile contents
should be lowest in inclusions with a shrinkage bubble. Since this is not the case, diffusion likely
did not significantly modify volatile contents within inclusions.
The range of major oxides within the groundmass are: 53.1-54.4 wt % SiO2, 10.3-11.7 wt
T
% FeO , 4.6-4.9 wt % MgO, 8.3-8.9 wt % CaO and 1.2-1.4 wt % K2O. This narrow range shows
a homogeneous composition. Comparing to whole rock values, groundmass values are
relatively enriched in SiO2 and K2O but depleted in MgO and CaO. Compared to melt inclusions,
the groundmass is relatively enriched in SiO2, Al2O3, FeO and CaO and depleted in MgO and
K2O. Unlike Cerro Negro, Las Pilas whole rock, groundmass and melt inclusion oxides are all
very similar (Figure 18). However, the volatile contents within the groundmass are distinct: 1959 ppm S, 699-764 ppm Cl, and 268-615 ppm F.
43!
!
Major oxides of Las Pilas
Figure 7 compares the major oxides of melt inclusion, groundmass and whole rock data
against K2O wt % at Las Pilas. The strongest positive correlation is seen between CaO wt %
and K2O wt %. This correlation is interesting because the melt inclusion, groundmass and whole
rock data are scattered along the trend, unlike Cerro Negro, which shows two distinct clusters. A
rough negative correlation can be delineated based on the contents of SiO2, TiO2 and K2O wt %
versus K2O wt % (Figure 18). Referring to SiO2 wt % versus K2O wt %, this trend shows that the
pyroxenes have trapped progressively differentiated melt.
Figure 18. Comparison of major oxides at Las Pilas.
Melt inclusion (green triangle), groundmass (red circle) and whole rock (red triangle) at Las Pilas. The
melt inclusion data define the relatively primitive end member of the compositional trend (i.e. K2O poor).
Since groundmass data represent the late stage, fractionated melt, areas where groundmass and melt
inclusion data overlap suggest the host pyroxenes crystallized from an evolved melt. Error bars are 2σ.
44!
!
Figure 18. Continued from previous page.
Comparing the MgO wt% and FeOT wt% against the Mg# of melt inclusions,
groundmass and whole rock data reaffirms the trends seen from the major oxide diagrams:
many melt inclusions plot away from groundmass and whole rocks data towards the evolved
end member (Figure 19). This suggests that inclusions with MgO concentrations of 3 wt% or
less and Mg# less than 37 have undergone compositional modifications after entrapment or
were trapped at a later time. The difference between groundmass and the melt inclusion trend
implies that post entrapment crystallization is a dominant factor in the variability within melt
inclusions and that their apparent evolved state is not only due to later entrapment. Conversely,
the group of melt inclusion near the groundmass and whole rock data could also reflect the
compositional variability of the melt during entrapment. This implies that pyroxene was able to
continue crystallizing from an evolving and degassing melt.
45!
!
MgO (wt %)
a)
MI!
7!
6.5!
6!
5.5!
5!
4.5!
4!
3.5!
3!
2.5!
2!
20#
25#
30#
GM!
WR!
35#
40#
45#
50#
55#
40#
45#
50#
55#
Mg#
b) 20!
FeOT (wt %)
18!
16!
14!
12!
10!
8!
20#
25#
30#
35#
Mg#
Figure 19. Comparing the Mg# of melt inclusions against FeOT wt % and MgO wt % of
melt inclusions (blue diamond), groundmass (orange square) and whole rock data (green
triangle) at Las Pilas.
The melt inclusions that plot near the groundmass and whole rock data are in equilibrium. Melt inclusions
that plot away from this cluster (with Mg# <37 and MgO <3 wt%) likely have undergone post entrapment
modifications. Error bars are 2σ.
Do the melt inclusions represent undegassed magma at depth?
The olivine-hosted melt inclusions from Cerro Negro represent a partially degassed to an
undegassed melt based on the correlation between the volatiles and incompatible element
abundances such as K2O. The relatively high average sulphur content (1325 ppm) within
olivine-hosted melt inclusions supports the undegassed state of the melt (Streck and Wacaster,
2006). The average Cl/K2O ratio at Cerro Negro is 0.25, which is similar to the Cl/K2O ratio of
46!
!
Nicaragua (0.2-0.4; Sadofsky et al., 2008). Pyroxene-hosted melt inclusions, however,
crystallized from a partially degassed melt.
Additional data
Additional whole rock, groundmass and melt inclusion data from the 1867, 1971 and
1992 Cerro Negro eruptions are taken from Roggensack (2001), Sadofsky et al. (2008) and
Portnyagin et al. (2012). These data are PEC corrected using the models of Danyushevsky and
Plechov (2011) and Borisov and Shapkin (1990) and reported in Appendix 2. Additional whole
rock data for Las Pilas was available from the database of Carr et al. (2013) and are shown in
Appendix 2.
47!
!
Discussion
4.1 Major oxides and volatiles of Cerro Negro and Las Pilas
Comparing the major oxides and volatile contents between these two systems reveals a
similar pathway and evolutionary trend for ascending magma. Figure 20 illustrates the variations
in oxide contents plotted against K2O wt % for all melt inclusions, groundmass and whole rock
data for Cerro Negro and Las Pilas. A very strong trend can be delineated from a primitive melt
at < 0.6 K2O wt % towards a more evolved, K2O-enriched melt. The most striking pattern is the
consistency of melt inclusions from Cerro Negro to represent the mafic end member while Las
Pilas melt inclusions define the evolved end member. This is best seen on the plot of SiO2 wt %
and MgO wt % versus K2O wt %. Since the Cerro Negro melt inclusions define a tight cluster,
the system likely stopped crystallizing olivine past a threshold of 0.6 wt % K2O and began
crystallizing pyroxene. The few overlapping melt inclusions from Cerro Negro and Las Pilas
provide the general compositional range of the melt at the point of co-crystallization of olivine
and pyroxene: 0.71-1.66 wt % TiO2, 1.8-3.01 wt % Na2O, 10-16.6 wt % FeOT and 15.3-18.1 wt
% Al2O3.
Sulphur degassing is well tracked by this system showing a clear trend of sulphur rich,
olivine-hosted melt inclusions towards a sulphur poor, degassed melt represented by pyroxene
–hosted inclusions. Additionally, the sulphur content within the groundmass of both Cerro Negro
and Las Pilas are exceedingly similar. Overall it can be seen that the Las Pilas data fills in the
gap between Cerro Negro melt inclusions and groundmass values, implying that Las Pilas
represents the intermediary composition. In terms of volatile contents, the degassing trend is
clearly shown between the olivine-hosted melt inclusions towards the groundmass of Las Pilas
(Figure 21). The major oxides and volatile contents of both olivine-hosted and pyroxene-hosted
melt inclusions define a robust trend of magma fractionation from depth to the surface.
Therefore, Cerro Negro and Las Pilas magmas likely have the same primitive source. However,
Cerro Negro has compositional variability between its melt inclusion, groundmass and whole
rock data that is not seen within Las Pilas.
48!
!
Figure 20. Comparison of the major oxides vs K2O of all melt inclusions, groundmass
and whole rock data from this study.
Cerro Negro melt inclusions consistently plot at the primitive end member whereas the evolved end
member is represented by Las Pilas groundmass and melt inclusion data. The overlap between the most
evolved Cerro Negro melt inclusions and the least evolved Las Pilas melt inclusions could provide the
composition of the melt at the point where olivine stopped crystallizing and pyroxene began. Cerro Negro
groundmass data have a similar composition as melt inclusions, groundmass and whole rock data of Las
Pilas, implying the same evolved melt composition. Error bars are 2σ.
49!
!
Figure 20. Continued from previous page.
50!
!
1750#
CN99_MI
LP_MI
LP_GM
CN99_GM
1500#
1250#
S (ppm)
1000#
750#
500#
250#
0#
0.2# 0.4# 0.6# 0.8#
1#
1.2# 1.4# 1.6# 1.8#
2#
2.2# 2.4# 2.6#
K2O (wt %)
2000#
Cl (ppm)
1500#
1000#
500#
0#
0.2# 0.4# 0.6# 0.8#
1#
0.2# 0.4# 0.6# 0.8#
1#
1.2# 1.4# 1.6# 1.8#
K2O (wt %)
2#
2.2# 2.4# 2.6#
700#
600#
F (ppm)
500#
400#
300#
200#
100#
0#
1.2# 1.4# 1.6# 1.8#
K2O (wt%)
2#
2.2# 2.4#
Figure 21. Comparison of volatile contents of melt inclusions, groundmass and whole
rock between Cerro Negro and Las Pilas.
Increasing K2O wt% indicates further magmatic evolution. Cerro Negro melt inclusions consistently define
the degassed, primitive end member whereas Las Pilas groundmass marks the evolved, degassed end
member. The gap between Cerro Negro melt inclusions and groundmass is filled in by Las Pilas data,
implying they track the same magmatic evolution (similar to Figure 20). Error bars are 2σ.
51!
!
4.2 Relevant prior work
Olivine-hosted melt inclusions from Cerro Negro have been previously studied by
Portnyagin et al. (2012). In this study, samples from the 1992 eruption were analyzed and
reported with past data from Sadofsky et al. (2008) and Roggensack (2001). The dataset for the
1867, 1971, 1992 and 1995 tephra and lava samples will be used to compare with the 1999
data from this study.
Portnyagin et al. (2012) examined major oxides and volatile content from olivine-hosted melt
inclusions. The olivines were slightly more primitive (Fo72 – Fo82) than those used in this study.
Crystallization was estimated to begin at 450 MPa, which corresponds to a depth of 14 km, and
continued until near-surface depths beneath Cerro Negro (Portnyagin et al, 2012). Similar to our
findings, the whole rock data defined a separate array that intersected the melt inclusion data
and ranged from high Mg to low Mg basalts. Essentially, the whole rock composition was not
represented by the melt trapped within the inclusions.
The magma pathway is not constant for Cerro Negro, as shown by the 1992 and 1995
eruptions during which magma was emplaced within the mid-lower crust and the upper crust,
respectively (Roggensack et al, 1997). Therefore, H2O-induced crystallization during
decompression is adopted in order to explain the trend of crystallization at decreasing pressure
and increasing liquidus temperatures.
Cerro Negro
Cerro Negro data from this study strongly align with that of Portnyagin et al. (2012)
showing a temporally homogeneous magma beneath Cerro Negro. In terms of an array of high
Mg and low Mg basalts, the whole rock data from this study plots towards the high Mg end
member (Figure 22). The olivine hosts in this study, though slightly more evolved, entrap melts
that plot within a narrow range of compositions (Figure 22). Progressive fractionation and
degassing follow very similar trends towards the evolved and degassed groundmass data.
Volatile contents differ slightly such that Portnyagin et al. (2012) noted a gradual decrease in
sulphur content from the 1867 to the 1971 and 1992 eruptions, indicating lower oxygen fugacity
and greater magmatic degassing with time (Figure 23). The 1999 data plot within the range of
1971 data, reflecting a return to a volatile rich system.
Las Pilas
52!
!
The Las Pilas data fit the trend such that the melt inclusion and groundmass data remain
at the evolved end member of the cumulative Cerro Negro trend (Figure 22). With a greater
spread of melt inclusion data from Portnyagin et al. (2012), there is greater overlap between
melt trapped within olivine and pyroxene hosts. The depletion of sulphur within the pyroxenehosted melt inclusions support the notion that sulphur was considerably degassed prior to
pyroxene crystallization (Figure 23). The groundmass data of Las Pilas plot within the range of
Cerro Negro but is slightly more evolved (Figure 22 & 23). Las Pilas whole rock plots within the
melt inclusion-groundmass trend and thus do not exhibit the same compositional variability seen
within Cerro Negro.
Figure 22. Major oxide comparison for all Cerro Negro and Las Pilas Data (including Portnyagin
et al. (2012) and additional Las Pilas whole rock).
The similarity between the 1999 Cerro Negro samples from this study and the 1867, 1971, 1992 and 1995 samples
indicate that the magma chamber beneath Cerro Negro is invariant with time. A distinct compositional trend can be
seen between Cerro Negro and Las Pilas. Error bars are 2σ.
53!
!
Figure 22. Continued from previous page.
54!
!
S (ppm)
a)
CN99_MI
Port_MI
LP_MI
Port_GM
LP_GM
CN99_GM
2500#
2250#
2000#
1750#
1500#
1250#
1000#
750#
500#
250#
0#
0#
0.2# 0.4# 0.6# 0.8#
0#
0.2# 0.4# 0.6# 0.8#
1# 1.2# 1.4# 1.6# 1.8#
K2O (wt %)
2#
2.2# 2.4#
b) 2500#
Cl (ppm)
2000#
1500#
1000#
500#
0#
1# 1.2# 1.4# 1.6# 1.8#
K2O (wt %)
2#
2.2# 2.4#
1# 1.2# 1.4# 1.6# 1.8#
K2O (wt %)
2#
2.2# 2.4#
c) 700#
600#
F (ppm)
500#
400#
300#
200#
100#
0#
0#
0.2# 0.4# 0.6# 0.8#
Figure 23. Volatile contents of all Cerro Negro and Las Pilas data.
The data from Portnyagin et al. (2012) define the least degassed end member in terms of S (ppm) and Cl (ppm).
The magmatic degassing trend is distinct with Las Pilas (specifically the groundmass data) defining the evolved
and degassed end member of the trend. Error bars are 2σ.
55!
!
4.3 Magma composition during evolution
As can be seen, the magma chamber beneath Cerro Negro has remained relatively
invariant since at least 1867 and possibly since its formation in 1850 (Figure 22 & 23).
Characterizing the system is a unique compositional variability between its melt inclusion and
whole rock data. This variability is not seen within the whole rock sample from Las Pilas.
Portnyagin et al. (2012) calculated the composition of the parental and evolved melt
beneath Cerro Negro. The same method will be applied here and compared with Las Pilas.
Referring to Figure 24, the average of all olivine-hosts with Fo content greater than 80 yields the
most reasonable composition of the primitive melt (PRIM). The PRIM composition is estimated
at 7.33 wt % MgO and 0.29 wt % K2O. The evolved melt (EVOL) is the average of evolved
inclusions hosted in olivines with Fo <77. The EVOL composition is estimated at 6.45 wt % MgO
and 0.49 wt % K2O.
The EVOL composition is comparable to the average melt composition within pyroxenehosted inclusions at Las Pilas, which is 3.8 wt % MgO and 1.05 wt % K2O. Reported in Figure
24 are the melt inclusion, groundmass and whole rock compositions from Cerro Negro and Las
Pilas as well as previous data from Cerro Negro. The Las Pilas composition could be explained
by the same differentiation trend at Cerro Negro, with Las Pilas melts being more evolved.
56!
!
Figure 24. Comparison of MgO and K2O between all data compiled at Cerro Negro and Las
Pilas.
The primitive melt composition, PRIM, is defined by the average melt inclusion composition trapped within
primitive olivines (Fo>80). Error bars are 2σ. The evolved melt composition, EVOL, is defined by the melt
inclusion composition trapped within evolved olivines (Fo<77). PARM is the intersection between the whole rock
array and the evolutionary path. Las Pilas and Cerro Negro data have the same magmatic source and thus are
shown to originate from the same primitive composition and pass through the same EVOL composition. Since
the whole rock and melt inclusion data exhibit compositional variability, there must be some mechanism
controlling these compositions i.e. phenocryst sorting. This mechanism is only seen within Cerro Negro data.
57!
!
4.4 Mechanisms of an interconnected magma chamber
Figure 25 represents the primitive magma chamber beneath Cerro Negro and Las Pilas.
The pathway from chamber to edifice can be considered as two separate dykes. Along the
Cerro Negro pathway, Portnyagin et al. (2012), based on the work by Walker and Carr (1986)
and Carr and Walker (1987), reasoned that the settling velocities and subsequent redistribution
of phenocrysts within ascending magma controls the contrasting compositions between whole
rock and melt inclusion data.
Four different cases are proposed by Portnyagin et al. (2012) to describe the processes
occurring within the dyke beneath the Cerro Negro edifice (Figure 25). Entering the dyke in each
case is magma similar to the PRIM composition. The magma ascends, decompresses and
releases water, which thereby induces crystallization. Each case varies based on an open,
closed or intermediate system crystallization trend and is dependent upon the relative settling
(or floatation) velocities of major phenocrysts.
Figure 25. The 4 cases proposed by Portnyagin et al. (2012) to explain the compositional
variability between Cerro Negro rocks and melt inclusions.
Case 1 involves closed system crystallization where all phenocrysts are suspended until eruption and
erupted magma composition is similar to PRIM. Case 2 is an open crystallization system where all
phenocrysts are removed from the melt, possibly by accumulating along the dyke wall. Case 3 involves
the evolution of magma from PRIM via the partial removal of phenocrysts from the magma during initial
fractionation, producing a composition similar to PARM. Case 4 involves compositional zoning of magma
within the dyke due to the flotation of plagioclase phenocrysts and the settling of olivine and pyroxene.
From Portnyagin et al. (2012).
58!
!
The first case is closed system crystallization where all phenocrysts remain suspended
until eruption. The erupted magma composition is similar to PRIM. This is likely not the case
beneath Cerro Negro since the PRIM composition does not lie along the whole rock trend. The
second case is an open system crystallization such that all phenocrysts are separated from the
melt by either settling or accumulating along the dyke wall. This is also unlikely as the resultant
magma is EVOL in composition and aphyric. The most reasonable mechanism is therefore an
intermediate between an open and closed system, represented by the third and fourth cases.
The third case involves the evolution of magma from PRIM to EVOL due to the partial removal
of phenocrysts from the magma during the initial stages of crystallization. The composition
following early stage fractionation is PARM, which is slightly more evolved than PRIM. The
composition of PARM is defined as the intersection of the whole rock array and the melt
inclusion trend, containing 6.77 wt % MgO and 0.39 wt % K2O. Further evolution of PARM
results in the flotation of newly crystallized phenocrysts. This is not the case beneath Cerro
Negro because this trend does not explain the variability of high Mg and low Mg basalts. The
cause of Mg variability within whole rocks relies upon the dominance of Fe-Mg phenocrysts
such as olivine and pyroxene over plagioclase. Therefore, the only difference between case 3
and 4 is a vertical compositional zoning of magma within a dyke due to the flotation of
plagioclase phenocrysts and the settling of olivine and pyroxene at shallow depths. Based on
plagioclase hosted fluid inclusions, Portnyagin et al. (2012) determined that plagioclase
phenocrysts were able to rise in evolved Cerro Negro magmas at pressures and depths below
200 MPa and 7 km. The bulk magma composition is similar to PARM and either deviates
towards the leading edge of the dyke to A (low Mg, enriched in plagioclase) or to deeper
portions of the dyke to B (high Mg, enriched in Fe-Mg silicates). In the case of the 1999 eruption
(this study) the composition deviates towards B and becomes relatively enriched in Fe-Mg
silicates. As a result, the re-distribution of phenocrysts within this dyke produces the array of
whole rock compositions seen at Cerro Negro. Case 4 therefore represents the most
reasonable mechanism within the dyke beneath Cerro Negro.
The mechanism controlling the composition of Cerro Negro does not account for the Las
Pilas melt compositions based on the relative compositional homogeneity between melt
inclusions and whole rock. Using the four cases proposed by Portnyagin et al. (2012), Las Pilas
is well represented by case 1, a closed system fractionation trend with suspended phenocrysts
of plagioclase, pyroxene and olivine and predicts that whole rock data plots within the same field
as melt inclusion data, which is the case at Las Pilas.
59!
!
A number of scenarios are proposed in order to investigate these compositionally
different dykes (Figure 26). The following diagrams represent the subsurface structures beneath
Cerro Negro and Las Pilas. Surrounding the interconnected magma chamber is a series of
shallow intrusive complexes, some of which are likely inactive conduits.
Figure 26. Schematic models illustrating the potential plumbing system beneath Cerro
Negro and Las Pilas.
Each scenario comprises a 14 km deep magma chamber. Scenarios 1 and 2 include dykes sourced at
the same depth from the magma chamber and extend towards the edifice. Scenario 3 and 4 illustrate
branching dykes that diverge at 2 km depth. At 7 km depth phenocrysts begin to separate. The dyke
beneath Cerro Negro involves phenocryst sorting whereas the dyke beneath Las Pilas does not.
The location of the magma chamber and feeding dykes follow the work by Portnyagin et
al. (2012) and MacQueen (2013). Based on H2O and CO2 contents, the depth of the magma
chamber is estimated at 14 - 15 km beneath Cerro Negro and plagioclase phenocrysts begin to
float and separate from sinking olivine and pyroxene crystals at 7 km (Portnyagin et al., 2012).
The geophysical work by MacQueen (2013) showed a possible connection at 2 km depth. This
is initially interpreted to represent the depth at which the dyke branches.
60!
!
Scenarios 1 and 2 comprise dykes sourced at the same depth from the magma chamber
toward the edifice of Cerro Negro and Las Pilas (Figure 26). This would suggest that,
theoretically, the whole rock compositions should be similar and that Las Pilas should have a
greater proportion of olivine crystals. As this is not the case, these two diagrams are likely not a
correct representation of the subsurface structures.
Scenarios 3 and 4 thus compensate for the considerable difference in whole rock
compositions by introducing a dyke that diverges at 2 km depth. At 7 km depth within the
shared conduit, phenocrysts begin to separate. Scenario 3, however, illustrates that the
package of magma that diverts into the dyke beneath the Las Pilas edifice travels a shorter
distance prior to eruption and would be compositionally homogeneous. The magma that
remains in the dyke beneath Cerro Negro travels further and thus experiences a greater degree
of phenocryst sorting prior to eruption. The depth at which the Las Pilas conduit branches could
also be the depth within the Cerro Negro conduit at which the phenocryst proportions match the
values seen within the Las Pilas whole rock.
Scenario 4 is a slightly different with the dominant dyke beneath Las Pilas and the area
prior to divergence is larger and extends from 2 km to 7 km depth. Additionally, this expanse
could be interpreted as a smaller, secondary magma chamber or an area of active and inactive
dyke swarms. Within this 5 km area, phenocrysts would begin to separate, establishing a
distinct compositional stratification within the conduit. This magma would preferentially rise
towards Cerro Negro since the dyke is oriented parallel to the direction of minimum principle
stress (La Femina et al, 2002). Therefore, only a small proportion of magma would divert
towards Las Pilas since the conduit is essentially oriented parallel to maximum stress.
The occurrence of only one magmatic eruption documented at Las Pilas places
significant temporal restrictions on the subsurface morphology. Since the last eruption was 300
years before Cerro Negro formed, it is more likely that a conduit was initially established
beneath Las Pilas, thus discounting Scenario 3.
Scenario 4 is further supported by the concept of dyke capture by Gaffney et al. (2007).
Dyke capture best occurs when intrusive conduits preferentially follow steep faults that
propagate in resistant rocks. In accordance with the work by La Femina et al. (2002), who
proposes east-west extension within blocks along NNW trending structures, dyke capture along
a high angle normal fault beneath Cerro Negro is likely its dominant conduit. Finally, the concept
of diverging and converging dyke intrusions beneath volcanic complexes, initially proposed by
61!
!
Petronis et al. (2013), could explain the lack of activity at Las Pilas. Shifting along the local
stress regime would influence whether a dyke diverges or becomes blocked. In the case of Las
Pilas, subtle shifts within the tectonic regime following the 1954 eruption could have caused the
divergence of the conduit to the northwest and the manifestation of the new dyke was the
formation of Cerro Negro.
4.5 Implications for future eruptions at the Cerro Negro-Las Pilas-El
Hoyo Complex
The implications of an interconnected magma chamber include the proper classification
of Cerro Negro and the future eruptions within this complex. Since the two systems are
connected, Cerro Negro should be considered as the newest edifice within the Las Pilas-El
Hoyo Complex. Similar to a volcanic field, this complex has no prominent peak and contains
edifices distributed over a large area. However, future eruptions at Cerro Negro may cause it to
build with time, allowing it to become the prominent center for activity. Proposed mechanisms
such as dyke capture and converging and diverging dykes suggest that new conduits may
develop and create cinder cones within the complex.
62!
!
Conclusion
Olivine- and pyroxene-hosted melt inclusions from Cerro Negro and the Las Pilas-El
Hoyo Complex suggest an interconnected plumbing system. Water and carbon dioxide contents
from previous studies indicate the depth of the magma chamber is 14-15 km whereas
geophysical data suggest that a connection likely exists at 2 km depth (Portnyagin et al, 2012;
MacQueen, 2013). Melt inclusion, groundmass and whole rock data from Cerro Negro and Las
Pilas define a linear evolutionary trend from the primitive melt, represented by Cerro Negro,
towards the evolved and degassed melt represented by Las Pilas. Crystallization is believed to
be induced by the release of water from ascending magma.
Beginning at 7 km depth,
phenocryst settling is a proposed mechanism that accounts for the compositional variability
between melt inclusions and whole rocks at Cerro Negro (Portnyagin et al, 2012). Las Pilas data
do not exhibit the same variability, therefore branching dykes must be present beneath these
systems. The most likely scenario is a dyke that originally connected the magma chamber to
Las Pilas but due to local stress changes and dilation, subsequent dyke branching formed Cerro
Negro in 1850. The proper classification of Cerro Negro as the newest edifice on the Cerro
Negro-Las Pilas-El Hoyo Complex implies that the eruptive intensity will increase. This poses
significant hazards to local towns and villages who, in the past, have been devastated by ashfall
causing infrastructure and agricultural damage. The lack of eruptions at Las Pilas implies the
conduit is likely blocked and that future eruptions are improbable. However, dyke branching and
divergence may occur in the future and create new vents. The model proposed here also has
global implications such that volcanic fields comprised of closely spaced edifices could
potentially share a magma chamber and that eruptions at one edifice could precede others
nearby.
5.1 Further research
In order to better constrain the composition of the common magma source and the depth
and divergence of the dyke, trace elements, H2O and CO2 contents of melt inclusions should be
analyzed. Additionally, olivine-hosted melt inclusions from Las Pilas should be used in order to
compare primitive melt compositions with melts from Cerro Negro. This supplementary data will
allow for the more precise determination as to whether Cerro Negro and Las Pilas do, in fact,
share the same magmatic source.
63!
!
References
Benjamin, E.R., Plank, T., Wade, J.A., Kelley, K.A., Hauri, E.H., and Alvarado, G.E. 2007. High water
contents in basaltic magmas from Irazu volcano, Costa Rica. Journal of Volcanology and Geothermal
Research, 168: 68-92.
Bice, D.C. 1980. Tephra stratigraphy and physical aspects of recent volcanism near Managua, Nicaragua.
United States.
Bundschuh, J. and Alvarado Induni, G.E. 2007. Central America: Geology, resources and hazards. Taylor
& Francis, New York.
Carr, M.J. 1984. Symmetrical and segmented variation of physical and geochemical characteristics of the
Central American volcanic front. Journal of Volcanology and Geothermal Research, 20: 231-252.
Carr, M.J., Feigenson, M.D., Patino, L.C., and Walker, J.A. 2003. Volcanism and geochemistry in Central
America; progress and problems. Geophysical Monograph, 138: 153-174.
Carr, Michael J.; Feigenson, Mark D.; Bolge, Louise L.; Walker, James A.; Gazel, Esteban.
RU_CAGeochem v.2, a database and sample repository for Central American volcanic rocks at Rutgers
University, doi:10.1594/IEDA/100403, obtained from the EarthChem Library
(http://www.earthchem.org/library).
Connor, C.B., Hill, B.E., Winfrey, B., Franklin, N.M., and La Femina, P.C. 2001. Estimation of volcanic
hazards from tephra fallout. Natural Hazards Review, 2: 33-42.
Gaffney, E.S., Damjanac, B., and Valentine, G.A. 2007. Localization of volcanic activity; 2, Effects of preexisting structure. Earth and Planetary Science Letters, 263: 323-338.
Hill, B.E., Connor, C.B., Jarzemba, M.S., La Femina, P.C., Navarro, M., and Strauch, W. 1998. 1995
eruptions of Cerro Negro volcano, Nicaragua, and risk assessment for future eruptions. Geological
Society of America Bulletin, 110: 1231-1241.
La Femina, P.C., Connor, C.B., Hill, B.E., Strauch, W., and Saballos, J.A. 2004. Magma-tectonic
interactions in Nicaragua; the 1999 seismic swarm and eruption of Cerro Negro volcano. Journal of
Volcanology and Geothermal Research, 137: 187-199.
64!
!
La Femina, P.C., Dixon, T.H., and Strauch, W. 2002. Bookshelf faulting in Nicaragua. Geology [Boulder],
30: 751-754.
MacQueen, P.G. 2013. Geophysical investigations of magma plumbing systems at Cerro Negro volcano,
Nicaragua. Masters thesis. Simon Fraser University, Burnaby, British Columbia.
Marshall, D. 1996. TernPlot template. Available from
http://serc.carleton.edu/NAGTWorkshops/petrology/plot_programs.html.
McBirney, A.R. 1955. Thoughts on the eruption of the Nicaraguan volcano Las Pilas. Bulletin of
Volcanology, : 113-117.
McKnight, S.B. 1995. Geology and petrology of Cerro Negro volcano, Nicaragua. United States.
McKnight, S.B. and Williams, S.N. 1997. Old cinder cone or young composite volcano? The nature of
Cerro Negro, Nicaragua. Geology [Boulder], 25: 339-342.
Metrich, N. and Wallace, P.J. 2008. Volatile abundances in basaltic magmas and their degassing paths
tracked by melt inclusions. Reviews in Mineralogy and Geochemistry, 69: 363-402.
Newhall, C.G. and Self, S. 1982. The volcanic explosivity index (VEI) an estimate of explosive magnitude
for historical volcanism. Journal of Geophysical Research, 87: 1231-1238.
Petronis, M.S., Delcamp, A., and van Wyk, d.V. 2013. Magma emplacement into the Lemptegy scoria
cone (Chaine des Puys, Fance) explored with structural, anisotropy of magnetic susceptibility, and
paleomagnetic data. Bulletin of Volcanology, 75.
Portnyagin, M.V., Hoernle, K., and Mironov, N.L. 2012. Contrasting compositional trends of rocks and
olivine-hosted melt inclusions from Cerro Negro volcano (Central America): implications for
decompression-driven fractionation of hydrous magmas. International Journal of Earth Sciences.
Roedder, E. (1984): Fluid Inclusions, Min. Soc. Am. Rev. in Min. 12, 644 pp.
Roeder, P.L. and Emslie, R.F. 1970. Olivine-liquid equilibrium. Contributions to Mineralogy and Petrology,
29: 275-289.
65!
!
Roggensack, K. 2001. Unraveling the 1974 eruption of Fuego volcano (Guatemala) with small crystals
and their young melt inclusions. Geology [Boulder], 29: 911-914.
Roggensack, K., Hervig, R.L., McKnight, S.B., and Williams, S.N. 1997. Explosive basaltic volcanism
from Cerro Negro volcano; influence of volatiles on eruptive style. Science, 277: 1639-1642.
Sadofsky, S.J., Portnyagin, M., Hoernle, K., and van den Bogaard, P. 2008. Subduction cycling of
volatiles and trace elements through the Central American volcanic arc: Evidence from melt inclusions.
Contributions to Mineralogy and Petrology, 155: 433-456.
Salazar, J.M.L., Hernandez, P.A., Perez, N.M., Melian, G., Alvarez, J., Segura, F., and Notsu, K. 2001.
Diffuse emission of carbon dioxide from Cerro Negro volcano, Nicaragua, Central America. Geophysical
Research Letters, 28: 4275-4278.
Streck, M.J. and Wacaster, S. 2006. Plagioclase and pyroxene hosted melt inclusions in basaltic
andesites of the current eruption of Arenal volcano, Costa Rica. Journal of Volcanology and Geothermal
Research, 157: 236-253.
Walker, J.A. and Carr, M.J. 1986. Compositional variations caused by phenocryst sorting at Cerro Negro
volcano, Nicaragua. GSA Bulletin, 97: 1156.
Wallace, P.J, Anderson, A.T, Sigurdsson, H., Houghton, B.F., McNutt, S.R., Rymer, H., Stix, J., and
Ballard, R.D.[. 2000. Encyclopedia of volcanoes. Academic Press : San Diego, CA, United States.
!
66!
!
Appendix 1
Uncorrected values for olivine-hosted inclusions from Cerro Negro
All oxides are given in weight percent.
Melt inclusion
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2 O
P2 O5
S ppm
Cl ppm
F ppm
Mg#
CN-1 99A MI
47.126
0.852
18.345
10.790
0.056
3.377
12.335
2.195
0.348
0.220
1611.0
1234.8
271.6
35.8
CN-1 99A MI-2
46.206
0.792
17.981
11.957
0.202
3.989
12.032
2.027
0.580
0.101
0.0
0.0
0.0
37.3
CN-1 99B MI-2
48.532
0.955
17.208
10.296
0.129
5.598
11.217
2.327
0.485
0.260
1051.2
1019.2
63.6
49.2
CN-1 99C MI-1
46.660
0.940
16.819
10.654
0.137
6.459
10.966
1.998
0.366
0.155
1238.4
772.0
149.0
51.9
CN-1 99C MI-2
46.473
0.810
17.134
10.464
0.282
6.222
11.267
1.919
0.385
0.275
1250.0
810.4
66.0
51.4
CN-1 99D MI
46.472
0.855
18.366
10.435
0.251
5.448
12.283
1.984
0.272
0.138
1245.0
752.2
204.0
48.2
CN-1 99E MI1
45.577
0.811
17.933
10.989
0.332
5.768
11.719
1.861
0.252
0.110
1495.8
880.8
122.0
48.3
CN-1 99E MI2
46.316
0.711
17.930
11.133
0.186
6.840
11.835
1.692
0.267
0.071
1477.0
803.8
228.6
52.3
CN-1 99F MI-1
46.500
0.911
18.481
13.160
0.258
3.607
11.505
2.365
0.352
0.096
1704.6
1451.0
230.2
32.8
CN-1 99G MI-1
45.331
1.131
18.287
9.246
0.168
6.476
11.295
2.147
0.337
0.208
1380.8
797.2
266.6
55.5
CN-1 99H MI
47.048
0.782
17.833
11.261
0.340
4.630
11.306
1.831
0.369
0.185
1667.0
1029.2
73.4
42.3
CN2 MI99J
46.929
0.788
18.521
8.986
0.155
6.470
11.428
2.017
0.312
0.119
1392.6
761.4
79.2
56.2
CN2 MI99J-2
44.748
1.718
17.885
10.517
0.167
6.588
11.543
2.042
0.389
0.319
1145.2
807.4
159.8
52.7
CN2 MI99K
46.547
0.791
17.119
10.296
0.140
6.783
11.445
1.384
0.260
0.131
1392.0
819.0
141.0
54.0
CN2 MI99L
46.484
0.761
17.081
10.549
0.072
6.226
10.314
1.933
0.308
0.172
1615.0
723.4
238.2
51.3
CN2 MI99N
46.634
0.827
17.258
9.730
0.192
6.678
11.091
1.952
0.360
0.139
1277.2
810.4
17.0
55.0
CN2 MI99O
46.841
0.876
19.497
9.473
0.111
3.608
11.989
2.298
0.347
0.112
1405.4
737.2
99.0
40.4
67!
!
Appendix 2
Additional data for Cerro Negro
All oxides are reported in weight percent.
Melt Inclusion
Data source
Roggensack, 2001
Roggensack, 2001
Roggensack, 2001
Roggensack, 2001
Roggensack, 2001
Roggensack, 2001
Roggensack, 2001
Roggensack, 2001
Roggensack, 2001
Eruption
1867
1867
1867
1867
1867
1867
1867
1867
1867
PEC%
1.00
3.00
3.00
5.00
0.00
6.00
6.00
1.00
2.01
SiO2
47.80
49.25
50.45
48.09
49.57
48.91
49.06
49.51
47.62
TiO2
0.77
0.69
0.75
0.69
0.72
0.75
0.76
0.60
1.27
Al2O3
18.55
17.48
17.76
17.55
18.47
17.90
17.41
18.02
17.90
FeO
10.98
10.77
9.86
11.25
10.32
10.25
10.61
9.86
10.74
MnO
0.15
0.18
0.00
0.22
0.15
0.00
0.00
0.18
0.19
MgO
7.20
7.49
6.74
7.75
6.64
7.23
7.59
7.20
7.54
CaO
12.09
11.71
11.79
12.29
11.83
12.36
12.38
12.38
11.84
Na2O
1.93
1.93
2.03
1.70
1.93
2.13
1.83
1.89
2.24
K2 O
0.21
0.19
0.36
0.24
0.25
0.23
0.23
0.25
0.29
P2 O5
0.17
0.16
0.15
0.08
Roggensack, 2001
Roggensack, 2001
Roggensack, 2001
Roggensack, 2001
Roggensack, 2001
Roggensack, 2001
Roggensack, 2001
Roggensack, 2001
1867
1867
1867
1867
1867
1867
1867
1867
0.00
2.01
2.01
3.00
5.00
0.00
0.00
0.00
50.16
47.89
50.08
48.73
48.42
49.56
48.64
49.40
0.80
0.88
0.92
0.70
0.76
1.00
0.72
0.74
17.76
17.52
16.85
18.09
17.43
18.02
18.39
18.30
9.77
12.61
11.76
10.48
10.28
10.05
10.53
9.98
0.16
0.19
0.33
0.17
0.12
0.16
0.29
0.13
7.13
6.40
6.35
7.44
7.79
6.78
7.07
7.35
11.71
11.63
9.91
11.97
12.94
11.31
12.08
11.58
2.02
2.24
2.93
1.95
1.79
2.20
1.93
2.03
0.27
0.28
0.53
0.29
0.25
0.38
0.21
0.25
0.10
0.17
0.15
0.05
0.09
0.41
Data source
Roggensack, 2001
Roggensack, 2001
Roggensack, 2001
Roggensack, 2001
Roggensack, 2001
Sadofsky et al., 2008
Sadofsky et al., 2008
Sadofsky et al., 2008
Sadofsky et al., 2008
Sadofsky et al., 2008
Sadofsky et al., 2008
Sadofsky et al., 2008
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Eruption
1867
1867
1867
1867
1867
1971
1971
1971
1971
1971
1971
1971
1971
1971
1971
1971
1971
1971
1971
1971
!
PEC %
5.00
3.00
0.00
1.00
4.01
2.03
1.02
4.07
2.79
2.10
6.56
5.20
6.41
6.84
4.99
5.65
3.05
5.11
6.05
0.90
SiO2
49.11
50.85
51.01
48.01
48.74
47.85
48.44
48.94
49.18
49.72
49.46
48.06
50.90
49.08
49.08
48.18
48.87
49.44
48.01
50.48
TiO2
0.81
1.13
1.08
0.75
0.75
0.76
1.15
0.56
0.70
0.66
0.57
0.75
0.82
0.80
0.90
0.72
0.93
0.48
1.21
0.77
Al2O3
17.78
16.03
16.29
17.48
17.98
18.14
18.21
18.12
17.52
17.85
17.23
17.44
16.12
17.24
17.98
18.25
18.13
17.95
17.73
18.02
FeO
10.97
13.03
12.03
12.29
10.70
10.88
11.26
10.40
10.09
9.72
11.13
11.21
11.85
12.34
11.49
10.88
11.43
10.46
11.03
9.74
MnO
0.18
0.31
0.24
0.24
0.18
0.18
0.18
0.23
0.17
0.17
0.16
0.19
0.23
0.18
0.18
0.19
0.24
0.19
0.18
0.20
MgO
7.13
5.63
5.60
7.09
6.95
7.41
6.76
7.62
7.69
7.47
7.61
8.01
7.17
6.86
6.89
7.32
6.67
7.73
7.84
6.66
CaO
11.34
9.39
10.00
11.75
12.17
12.46
11.20
11.78
12.27
12.04
11.43
12.13
10.45
10.67
10.90
12.27
10.90
11.86
11.86
11.82
Na2O
2.18
2.62
2.77
1.95
2.11
1.79
2.12
1.81
1.81
1.78
1.78
1.66
1.80
1.99
1.92
1.73
2.09
1.55
1.65
1.77
0.09
0.22
Fo host
80.00
81.00
81.00
81.00
80.00
82.00
82.00
82.00
82.00
Mg# MI
52.82
52.09
51.29
49.58
53.43
48.18
48.96
55.46
53.37
Kd
0.28
0.26
0.25
0.23
0.29
0.20
0.21
0.27
0.25
Cl ppm
800
600
800
800
600
900
800
400
600
S ppm
2133
1467
2467
2467
82.00
76.00
77.00
82.00
83.00
81.00
81.00
82.00
56.55
45.19
46.65
52.52
51.79
54.61
54.47
56.77
0.29
0.26
0.26
0.24
0.22
0.28
0.28
0.29
500
900
1100
600
600
500
800
600
2000
1333
0.12
K2 O
0.28
0.58
0.55
0.20
0.18
0.26
0.33
0.25
0.29
0.28
0.28
0.24
0.37
0.31
0.33
0.25
0.39
0.15
0.26
0.31
P2 O5
0.05
0.21
0.24
0.07
0.10
0.13
0.21
0.16
0.16
0.19
0.21
0.15
0.13
0.36
0.18
0.08
0.20
0.08
0.10
0.11
Fo host
80.00
73.00
74.00
78.00
80.00
81.06
78.98
81.93
82.57
82.55
80.70
81.65
78.32
77.19
78.63
80.74
78.29
81.69
81.72
80.52
Mg# MI
47.69
39.84
45.35
49.63
48.83
52.60
50.52
51.92
54.56
55.48
46.69
50.03
43.81
40.79
45.38
47.31
47.31
50.84
48.48
53.91
Kd
0.23
0.24
0.29
0.28
0.24
0.26
0.27
0.24
0.25
0.26
0.21
0.22
0.22
0.20
0.23
0.21
0.25
0.23
0.21
0.28
Cl ppm
600
1300
900
700
900
960
776
689
749
945
604
706
1054
824
830
794
1166
314
616
813
2133
2133
1467
1800
2333
2467
2000
2333
2133
S ppm
2133
2000
2333
2133
1231
1221
1031
1255
1292
899
1453
951
1508
1576
1685
1367
721
1422
1479
68!
Data source
Eruption
PEC %
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2 O
P2 O5
Fo host
Mg# MI
Kd
Cl ppm
S ppm
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
1971
1971
1971
1971
1971
1971
1971
1971
1971
1971
1971
1971
3.23
2.94
1.81
2.42
4.87
1.67
2.63
7.21
4.49
4.70
2.33
2.69
49.47
49.25
49.45
49.41
49.54
49.84
49.38
50.20
52.27
49.76
49.43
47.99
0.73
0.73
0.75
0.81
0.90
0.80
0.85
0.84
0.89
0.70
0.96
1.21
17.67
17.65
18.26
18.34
17.71
17.52
17.57
17.49
17.00
17.32
17.89
18.51
10.36
10.36
10.42
9.47
10.60
11.21
11.48
10.53
9.45
10.47
10.34
11.00
0.15
0.20
0.20
0.19
0.18
0.21
0.21
0.20
0.17
0.21
0.18
0.19
7.66
7.56
7.00
7.02
7.31
6.95
7.16
7.29
6.57
7.80
7.11
6.94
11.88
12.10
11.57
12.14
11.41
10.87
10.92
11.16
10.91
11.55
11.49
11.46
1.66
1.73
1.90
1.85
1.82
1.95
1.87
1.73
2.04
1.71
2.01
1.97
0.23
0.22
0.24
0.38
0.28
0.34
0.29
0.30
0.43
0.25
0.33
0.40
0.06
0.07
0.08
0.27
0.12
0.15
0.13
0.12
0.16
0.10
0.14
0.17
81.86
81.81
80.37
82.12
80.87
79.04
79.15
80.69
80.71
82.02
81.03
79.94
53.28
53.25
52.41
54.07
49.14
50.64
49.71
45.46
49.25
51.64
52.38
49.72
0.25
0.25
0.27
0.26
0.23
0.27
0.26
0.20
0.23
0.23
0.26
0.25
741
759
516
728
658
768
762
779
910
715
706
763
1356
1414
993
1475
1359
1429
1434
1439
910
1402
1545
1632
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Sadofsky et al., 2008
Sadofsky et al., 2008
Sadofsky et al., 2008
1971
1971
1971
1971
1971
1992
1992
1992
5.36
7.22
2.25
1.19
2.74
6.73
2.88
4.40
47.99
47.59
48.28
50.36
49.13
49.11
46.70
45.90
1.35
1.45
0.85
0.85
0.91
1.04
0.94
1.62
18.19
17.74
19.15
18.11
19.22
15.12
17.58
16.82
11.08
11.81
9.53
9.85
8.83
14.34
11.77
12.95
0.23
0.13
0.17
0.14
0.19
0.25
0.17
0.21
7.15
7.53
6.38
6.87
6.12
7.55
7.85
8.41
11.60
11.43
12.99
11.28
12.45
9.40
12.90
11.07
1.78
1.68
2.13
1.97
2.34
2.28
1.50
1.58
0.35
0.34
0.29
0.32
0.52
0.53
0.24
0.53
0.12
0.12
0.11
0.12
0.19
0.15
0.19
0.71
80.22
80.06
81.02
81.02
81.48
76.58
80.99
80.63
46.61
43.75
51.48
54.04
51.40
40.57
51.24
49.04
0.22
0.19
0.25
0.28
0.24
0.21
0.25
0.23
785
792
830
751
731
1284
579
660
1541
1667
1569
1450
1500
838
255
396
Data source
Sadofsky et al., 2008
Sadofsky et al., 2008
Sadofsky et al., 2008
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Eruption
1992
1992
1992
1992
1992
1992
1992
1992
1992
1992
1992
1992
1992
1992
1992
1992
1992
1992
1992
PEC %
0.45
0.85
5.50
4.07
6.95
1.53
2.50
0.00
2.35
4.52
3.48
6.17
2.76
2.51
0.57
1.28
1.94
1.91
5.04
SiO2
48.08
48.73
49.41
50.52
51.45
48.16
47.44
50.51
49.55
49.13
52.06
50.55
48.55
52.02
49.88
50.24
49.85
50.33
48.11
TiO2
0.45
0.91
1.11
0.84
0.99
0.80
0.68
0.83
0.63
0.89
0.97
0.91
0.88
1.11
0.72
1.07
0.86
0.87
1.02
Al2O3
18.43
16.98
15.82
15.28
15.04
16.91
17.94
17.33
17.10
17.06
15.39
15.33
16.44
15.13
16.89
16.08
16.38
16.27
15.91
FeO
10.24
12.53
14.06
13.69
13.45
13.16
10.84
10.74
12.14
11.81
13.11
14.17
13.54
12.28
13.05
12.99
12.95
12.72
13.94
MnO
0.17
0.24
0.22
0.31
0.21
0.24
0.18
0.20
0.21
0.18
0.18
0.23
0.28
0.18
0.26
0.29
0.25
0.26
0.26
MgO
7.54
6.56
5.81
7.10
6.29
7.06
8.07
6.61
7.06
7.28
5.78
6.33
6.98
5.80
6.13
6.30
6.74
6.63
6.58
CaO
13.18
11.15
9.97
9.55
9.29
11.48
12.77
10.94
10.68
11.14
8.93
9.26
10.51
10.01
10.43
9.92
9.89
9.88
10.95
Na2O
1.52
2.17
2.50
1.98
2.29
1.65
1.63
2.13
1.90
1.80
2.55
2.32
2.09
2.47
1.96
2.29
2.32
2.36
2.44
K2 O
0.12
0.40
0.58
0.43
0.62
0.27
0.23
0.42
0.36
0.35
0.66
0.53
0.41
0.65
0.39
0.49
0.40
0.38
0.45
P2 O5
0.16
0.14
0.16
0.08
0.15
0.08
0.09
0.16
0.21
0.20
0.16
0.14
0.10
0.15
0.11
0.12
0.15
0.11
0.11
Fo host
82.02
76.52
72.00
75.57
73.81
76.57
82.58
78.00
77.94
78.96
72.67
72.86
76.10
74.34
73.74
74.80
76.15
76.08
74.95
Mg# MI
56.32
47.34
37.03
43.59
36.26
47.28
54.40
52.33
48.32
46.99
39.66
36.66
44.89
42.57
44.97
44.91
46.01
46.03
39.48
Kd
0.28
0.28
0.23
0.25
0.20
0.27
0.25
0.31
0.26
0.24
0.25
0.22
0.26
0.26
0.29
0.27
0.27
0.27
0.22
Cl ppm
289
879
1421
1042
1139
887
926
1050
622
908
1239
1161
1066
1149
1008
1029
986
1310
1136
S ppm
356
1192
748
917
403
1449
1536
1198
440
1135
655
960
1231
621
912
1059
1299
1593
1162
!
!
69!
Data source
Portnyagin et al, 2012
Eruption
1992
PEC %
3.98
SiO2
50.91
TiO2
1.22
Al2O3
15.87
FeO
12.82
MnO
0.26
MgO
6.83
CaO
8.89
Na2O
2.31
K2 O
0.57
P2 O5
0.13
Fo host
76.47
Mg# MI
44.08
Kd
0.24
Cl ppm
2036
S ppm
1372
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
1992
1992
1992
1992
1992
1992
1992
1992
1.08
1.58
2.44
10.37
5.64
2.98
7.01
3.93
54.83
48.16
52.68
49.89
49.02
46.58
47.87
49.58
0.62
0.84
0.78
1.09
0.69
2.05
0.84
1.05
15.49
17.95
16.34
16.18
18.16
18.60
16.13
15.61
10.83
11.11
11.66
14.39
10.34
10.95
13.44
13.91
0.16
0.20
0.21
0.22
0.15
0.17
0.15
0.28
5.74
7.67
5.80
6.24
7.24
6.75
7.59
6.84
8.59
11.90
8.60
8.29
12.09
11.48
11.58
9.95
2.63
1.70
2.69
2.67
1.82
2.47
1.78
1.99
0.73
0.25
0.87
0.65
0.29
0.63
0.29
0.31
0.22
0.08
0.18
0.12
0.08
0.14
0.12
0.27
75.90
81.14
75.16
72.82
81.34
80.69
77.92
74.68
47.23
53.50
43.86
28.21
48.23
48.46
41.71
42.35
0.28
0.27
0.26
0.15
0.21
0.22
0.20
0.25
1561
836
1119
1623
1089
1387
1006
795
628
1458
1344
1029
1640
925
954
1410
Groundmass
Source
Year
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2 O
P2 O5
Roggensack et al, 1997
1992
50.58
1.21
15.26
13.52
0.23
4.53
9.18
2.69
0.68
0.15
Roggensack et al, 1997
1992
54.49
1.44
14.43
14.58
0.27
3.46
9.44
2.91
0.85
0.18
Roggensack et al, 1997
1995
55.66
1.54
12.8
14.18
0.25
3.51
8.3
2.62
0.87
0.25
Roggensack, 2001
1867
53.94
1.68
13.29
14.73
0.19
3.41
8.97
2.83
0.77
0.19
Roggensack, 2001
1867
52.90
1.22
14.38
13.36
0.28
3.86
9.81
3.21
0.74
0.22
Portnyagin et al, 2012
1971
56.19
1.64
12.87
14.04
0.29
2.72
7.72
2.55
1.11
0.30
Portnyagin et al, 2012
1971
56.85
1.68
12.84
14.69
0.38
3.03
7.77
2.34
1.11
0.32
Portnyagin et al, 2012
1971
54.14
2.05
12.11
15.88
0.23
3.88
7.16
2.96
1.02
0.25
Portnyagin et al, 2012
1971
55.40
1.67
11.82
15.29
0.35
3.68
7.70
2.51
1.08
0.25
Portnyagin et al, 2012
1971
56.27
1.69
12.82
14.32
0.21
2.89
7.49
2.60
1.02
0.26
Portnyagin et al, 2012
1971
55.90
1.72
12.08
14.61
0.25
2.78
7.57
2.58
1.09
0.32
Portnyagin et al, 2012
1992
55.56
1.59
12.58
13.84
0.25
3.08
7.53
2.78
1.13
0.23
Portnyagin et al, 2012
1992
55.47
1.57
12.96
13.78
0.30
3.03
7.59
3.29
1.03
0.26
Portnyagin et al, 2012
1992
55.76
1.63
12.77
14.28
0.29
3.03
7.64
3.10
1.05
0.18
Portnyagin et al, 2012
1992
55.39
1.58
12.59
14.60
0.33
3.22
7.76
2.79
1.06
0.23
Portnyagin et al, 2012
1992
54.85
1.55
12.40
14.75
0.33
3.42
7.77
3.20
1.08
0.23
Portnyagin et al, 2012
1992
56.05
1.59
12.67
14.18
0.23
3.07
7.59
2.83
1.12
0.23
Portnyagin et al, 2012
1992
56.25
1.62
12.36
13.55
0.33
3.07
7.15
3.88
1.18
0.23
Year
1992
1992
1995
1971
1992
1971
1992
1999
SiO2
50.12
48.84
50.17
47.5
48.86
47.50
48.86
49.19
TiO2
0.78
0.74
0.77
0.61
0.68
0.61
0.68
0.75
Al2O3
19.03
18.02
17.99
15.74
21.26
15.74
21.26
18.41
FeO
10.17
10.11
10.14
10.31
8.86
11.45
9.84
11.25
MnO
0.19
0.19
0.2
0.19
0.16
0.19
0.16
0.19
MgO
5.17
6.07
6.46
10.03
4.1
10.03
4.10
5.98
CaO
11.75
11.79
11.44
12.44
12.45
12.44
12.45
12.05
Whole Rock
Source
Roggensack et al, 1997
Roggensack et al, 1997
Roggensack et al, 1997
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
Portnyagin et al, 2012
!
!
Na2O
2.2
2.02
2.25
1.5
2.12
1.50
2.12
2.10
K2 O
0.48
0.43
0.46
0.27
0.4
0.27
0.40
0.42
P2 O5
0.11
0.1
0.11
0.1
0.13
0.10
0.13
0.12
70!
Additional whole rock data for Las Pilas
Source
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2 O
P2 O5
Carr et al, 2013
53.6
1.67
16
9.8
0.18
4.75
8.7
3.03
0.85
0.22
Carr et al, 2013
50
0.76
17.3
9.4
0.18
6.27
12.2
2.14
0.63
0.16
Carr et al, 2013
53.08
0.86
16.67
9.26
0.184
5.61
10.91
2.6
0.908
0.196
Carr et al, 2013
53.61
1.59
16.19
9.84
0.175
5.65
9.17
2.94
0.924
0.233
Carr et al, 2013
52.71
1.52
15.95
9.69
0.17
5.8
9.04
2.9
0.853
0.224
Carr et al, 2013
60.7
0.72
16.37
6.49
0.16
2.08
5.7
3.7
2.49
0.28
Carr et al, 2013
60.05
0.71
16.29
6.5
0.16
2.14
5.67
3.8
2.27
0.263
Carr et al, 2013
64.71
0.64
15.16
5.15
0.162
1.17
3.79
3.93
2.94
0.241
Carr et al, 2013
51.62
0.87
16.4
9.48
0.183
6.4
11.3
2.47
0.728
0.171
Carr et al, 2013
52.45
0.92
16.98
9.62
0.186
5.48
11.05
2.56
0.812
0.184
Carr et al, 2013
54.83
0.85
17.03
8.99
0.18
4.75
9.63
2.84
1.05
0.2
Carr et al, 2013
54.74
0.87
17.09
9.08
0.19
4.74
9.66
2.82
1.05
0.19
Carr et al, 2013
54.5
0.83
17.68
8.94
0.18
4.54
9.62
2.92
1.06
0.2
Carr et al, 2013
62.1
0.89
15.66
7.33
0.21
1.96
5.46
4.18
1.78
0.3
Carr et al, 2013
63.38
0.87
15.87
7.25
0.2
1.96
5.27
4.26
1.85
0.32
Carr et al, 2013
55.24
0.84
17.09
8.94
0.18
4.42
9.88
3
1.15
0.22
Carr et al, 2013
55.23
0.85
17.33
9.11
0.18
4.77
9.9
2.95
1.11
0.21
Carr et al, 2013
54.78
0.84
16.48
9.04
0.18
4.59
9.71
2.83
1.09
0.2
Carr et al, 2013
62.16
0.84
16.01
6.84
0.19
1.74
4.8
4.37
1.86
0.3
71!
!