Central America Arc
Presentation materials for instructor to show in
association with class exercises
Bob Stern (U TX Dallas), Ben Edwards (Dickinson College), Mike Carr (Rutgers U),
Jim Walker ( N. Illinois U)
Dec. 2013
Part one: background
Modified from Le Maitre et al 2002
Handout Fig. 1
(Courtesy of Eric Calais, UMR CNRS http://web.ics.purdue.edu/~ecalais/haiti/context/)
Central American arc (~1100 km long) lies along the western margin of the
Caribbean plate.
Handout Fig. 2
Simplified cross section across the Central American subduction zone
Handout Fig. 3
Nations of Central America
Handout Fig. 4
(Source Wikipedia)
Mayan civilization
Wikipedia
http://www.latinamericanstudies.org/mayas.htm
Simplified tectonic map of the Central American convergent plate boundary
Handout #5
http://www.geomapapp.org
Cocos Plate Sediment (Hole 495)
Miocene &
younger pelagic
sediment
Modified from Plank & Langmuir 1998
Plank et al. 2002
Central American Subduction Zone is a hot subduction zone (subducts young ocean crust)
Tectonic setting
Note:
DSDP drillsite 495
sampled sediments
on downgoing Cocos
Plate.
Well-defined volcanic
front defined by
linear segments with
offsets. Scattered
“behind volcanic
front” volcanoes.
Volcano-tectonic
depressions, marked
by two lakes and a
gulf
Gulf of Fonseca;
Lake Managua; Lake
Nicaragua;
DSDP
495
CRDB
oblique
(Courtesy of Mike Carr).
Gravity anomalies show that the NW arc is built on continental crust (gravity
low) but the SE arc is built on oceanic crust (gravity high)
Modified from: http://www.ifg.uni-kiel.de/1267.html
Central American volcanoes are often rained on (tropical rain forest). A wide range
of people study Central American volcanoes: Professors like Mike Carr of Rutgers
and students
(Images Courtesy of Mike Carr).
Prof. Mike Carr (Rutgers) keeping dry.
Arenal (Costa Rica) in background
Rutgers students at the La Paz waterfall (on
the N side of Poás, on the trail to Arenal
(Costa Rica)
Smooth slab-red contours at 50 Km interval
Slightly oblique convergence-green lines
Slab dip increases from Mexico
through Nicaragua, then decreases
Beneath Costa Rica.
Discontinuity in contours in Costa
Rica, Quesada sharp contortion
(QSC) is controversial
QSC
From Syracuse and Abers, 2006
Convergence vectors, DeMets 2001
(Source LaGeo, 2009)
Part two:
Modes, Major Elements, and Norms:
Cerro Negro and Ilopango
1 cup raw almonds
1 cup raw cashews
1/4 cup raw pepitas/pumpkin seeds
1/4 cup raw sunflower seeds (no shell)
1/2 cup unsweetened coconut flakes
1/4 cup coconut oil
1/2 cup raw honey
1 tsp. vanilla extract
1 tsp. sea salt
1 cup raisins
Paleo Granola
recipe
1. Preheat oven to 275° F. Place the almonds, cashews and coconut flakes in a food
processor or blender and pulse a few times to break into smaller chunks.
2. Microwave the coconut oil, raw honey and vanilla extract together in a medium
sized mixing bowl for about 30-45 seconds. Add the seeds, nuts and coconut flakes
and stir to coat.
3. Spread the granola mixture evenly onto a baking sheet lined with parchment paper
and cook for about 20-25 minutes or until lightly browned, stirring once or twice.
Remove from heat and add the raisins and sprinkle with sea salt, pressing the
mixture together to form a flat surface (see photo below).
4. Allow to cool for about 20 minutes or until hardened, and then break into chunks
and enjoy.
Paleo Granola
1) How many of the 10 ingredients can you see easily? Could you identify more with a hand lens?
2) Why can’t you identify all of the ingredients visually?
3) What is the texture of this material?
4) What would you call the texture if it was an igneous rock?
5) What do you visually estimate as the % of this material that is raisins?
Modal analysis of rocks involves visual estimation of visible minerals, by eye. This analysis is
better looking with a hand lens, even better with a microscope and thin section.
Let’s do a modal analysis of Central American lava
5mm scale
Courtesy of
Bob Stern
and Ben
Edwards
Olivine is honey colored, Clinopyroxenes are green and black. Plagioclase is white.
Q1) What is texture?
Q2) What is size and percentage of olivine, clinopyroxene, plagioclase, and groundmass?
QAPF diagram for
volcanic rocks
(Q = quartz, A = alkali
feldspar, P = plagioclase, F =
feldspathoid (nepheline,
leucite)
IUGS rock classification system
Depends on proportion of visible
minerals of quartz, feldspars, or
feldspathoids.
Useless for aphyric lavas and
problematic for porphyritic lavas
Q: using modal analysis, what is name of
Central American lava in last slide?
QAPF classification
diagram for volcanic
rocks. (Source,
http://bc.outcrop.org/
GEOL_B11/)
Major elements
Major Elements vs. Trace Elements
Major elements comprise >0.1% of rock and are reported as
oxides e.g., SiO2, Al2O3, TiO2, etc. These are reported as weight
percent (wt. %). Major element compositions are generally
manifested by lava modes. We use major element compositions
to classify lavas.
Trace elements comprise <0.1% of rock and are reported as
elements e.g., Rb, Cr, Zr. These are usually reported as parts per
million by weight (ppm); sometimes parts per billion (ppb) or
even parts per trillion (ppt). Trace elements are important for
understanding how igneous rocks form and evolve but their
abundances are not visible in the mode.
Let’s explore major
element analyses,
using Cerro Negro as
an example
Courtesy of
Mike Carr
Handout #6
Major Element Analysis of Cerro Negro sample CN-4
SiO2
TiO2
Al2O3
FeO*
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Total
5mm scale
Courtesy of Bob Stern and Ben Edwards
This lava erupted during 1957-1960 activity of Cerro Negro.
48.6 wt. %
0.65
14.4
11.0
0.2
9.97
11.3
1.78
0.35
0.06
0.08
98.39 wt. %
Volcanic rocks are generally too fine-grained to identify
many minerals; use diagrams based on chemistry
Total alkalies vs. silica
(TAS diagram)
IUGS diagram for volcanic rocks
Oxidation state of Iron is important but challenging to determine!
Iron exists in igneous rocks as both Fe2+ (ferrous) and Fe3+ (ferric).
Most major element analyses ignore oxidation state of iron
total iron is assumed to be either FeO or Fe2O3
reported in major element analysis as “FeO*” or “Fe2O3T”.
Oxidation state of iron in the magma is not preserved in the volcanic rocks, because Fe is
easily oxidized by interaction with groundwater or atmosphere during eruption,
increasing the amount of ferric iron (Fe2O3) at the expense of ferrous iron (FeO).
Kelley and Cottrell (2009) analyzed small bubbles of melt (now glass) trapped in olivines.
Technique: synchrotron-based Fe K-edge micro–x-ray absorption near-edge structure
(μ-XANES) to determine ratios of oxidized iron to total iron Fe3+/ΣFe = Fe3+/(Fe3+ + Fe2+).
Fe3+/ΣFe ratios are lower for basalts erupted from mid-ocean ridges: 0.13 to 0.17
Fe3+/ΣFe ratios are higher for arc basalts (like CAVA): 0.18 to 0.32).
Arc magmas are more oxidized than those erupted from mid-ocean ridges.
EXPLANATION: the mantle wedge above subduction zones (where arc lavas are
generated) is more oxidized than the mantle beneath mid-ocean ridges.
This becomes important for Mg# and for calculating normative
compositions (norms).
Normative Mineralogy
Converts major element chemical analysis into theoretical
mineralogy.
Easy to do using on-line software.
Important normative minerals include olivine (OL),
clinopyroxene (diopside; DI); orthopyroxene (hypersthene; HY);
plagioclase (both endmembers: anorthite AN and albite AB); Kfeldspar (orthoclase OR); apatite (AP); iron oxides (hematite
HM and magnetite MT); quartz (QZ).
No hydrous minerals amphibole or biotite
Rocks are made of minerals, so there should be a simple relationship between the
composition of a rock and its constituent minerals.
We can use the major element analysis to predict what minerals to expect in a lava.
We can look at minerals as a guide to lava compositions.
Five most common minerals in igneous rocks: olivine, clinopyroxene, plagioclase, alkali
feldspar, and quartz.
These minerals have chemical formula that require most of the major elements. For
example, olivine has the chemical formula (Mg, Fe)2SiO4. Note that the elements in
parentheses indicate solid solution between Mg-rich endmember “forsterite” and Fe-rich
endmember “fayalite”.
To form olivine requires that a rock
contains a significant amount of three
of the major elements: SiO2, MgO,
and FeO. In fact, olivine generally
contains much more MgO than FeO!
Wikipedia
Clinopyroxene has the chemical formula Ca (Mg, Fe) Si2O6 ,
requiring four of the major elements.
Siddal, 2014
Photomicrograph of clinopyroxene
1mm, cross-polars
An unusually large clinopyroxene
Plagioclase has the chemical formula (NaSi, CaAl)AlSi2O8; this requires four of
the major elements, three of which were not needed to form olivine.
Kurt Hollocher
Photomicrograph of zoned plagioclase,
~2mm, cross-polarized light. Note
polysynthetic (lamellar) twinning
Wikipedia
Large plagioclase crystal, note cleavage
Alkali feldspar (Sanidine, Orthoclase, Microcline) has the formula (Na,
K)Al2Si3O8
Large rhombs of alkali feldspar
C.E. Jones
http://www.pitt.edu/~cejones/GeoImages/1Minerals/1Igneo
usMineralz/Feldspars.html
Perthite: texture in alkali feldspar formed
when albite (white) exsolves from potash
feldspar (pink)
http://www.mindat.org/min-6683.html, Hudson
Institute of Mineralogy
Quartz has the formula SiO2
Wikipedia
Nearly all phosphorus goes into making apatite, which is also a
common minor mineral in igneous rocks.
Wikipedia
Mafic vs. Felsic
Arc lavas include all three varieties: mafic, intermediate, and felsic
Alkaline vs. sub-alkaline suites
CAVA erupts subalkaline lavas
Mg# (Mg-number)
Major element analysis of mantle peridotite (from Workman and Hart 2005) :
SiO2
TiO2
Al2O3
FeO*
MnO
MgO
CaO
Na2O
K2 O
P2O5
Total
44.7 wt. %
0.13
4.0
8.2
0.13
38.7
3.2
0.13
0.01
0.02
99.09 wt. %
Calculate Mg# = 100 * Mg/(Mg+Fe+2); let’s first assume that all Fe = Fe+2
1) Convert wt. % to atomic proportions by dividing wt% by molecular weight: atomic
proportion Mg =38.7/40.3 = 0.96; atomic proportion Fe = 8.2/71.845 = 0.11
2) Plug atomic proportions into formula: Mg# = 100* 0.96/(0.96+0.11) = 89
FYI: Mg# for most mantle peridotites range from 89 to 92 or so
Exercise: Calculate Mg# of Cerro Negro sample CN-4
SiO2
TiO2
Al2O3
FeO*
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Total
5mm scale
Courtesy of Bob Stern and Ben Edwards
Primitive (unfractionated) basalts have Mg# >65
Q: Is this a primitive basalt?
48.6 wt. %
0.65
14.4
11.0
0.2
9.97
11.3
1.78
0.35
0.06
0.08
98.39 wt. %
Answer: Mg# of Cerro Negro sample CN-4
SiO2
TiO2
Al2O3
FeO*
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Total
48.6 wt. %
0.65
14.4
11.0
0.2
9.97
11.3
1.78
0.35
0.06
0.08
98.39 wt. %
5mm scale
Courtesy of Bob Stern and Ben Edwards
This sample has Mg# = 62. Almost primitive but not quite.
Wait a minute! What about the oxidation state of iron?
We should only be counting Fe2+ atoms, not all Fe atoms, which includes Fe3+ !
Q: What is the Mg# if we adjust according to Kelley & Cottrell (2009) who found Fe3+/ΣFe
for arc basalts = 0.18 to 0.32). Redo calculation using Fe3+/ΣFe = 0.25
Cerro Negro, 1968 eruption
Cerro Negro Volcano, Nicaragua. Ash and cinders erupt from the summit crater in 1968 as a
vent near the base of the 150 meter-high cinder cone feeds a basaltic lava flow (R. Decker)
Guatemala El Salvador Nicaragua
Costa Rica
400
Volcano volume Km3
Atitlán Santa Ana
Irazú
300
Masaya
Tecapa
Barva
200
San
I
Cristóbal
Rincón
100
Mv
T
I
0
0
LP
500
Arenal
1000
Distance Km
Relative volumes of 39 Central American volcanic centers (Stoiber & Carr 1973).
I: Ilopango; T: Telica; LP: Las Pila-El Hoyo (site of Cerro Negro).
Let’s look at Cerro Negro and its basalts
Cerro Negro, Nicaragua: Central America’s youngest volcano
Established 1850, how many eruptions since?
Cerro Negro is part of
the Pilas-El Hoyo
volcanic complex in
western Nicaragua
(also known as the
Marabios Range)
Walker & Carr 1986
Courtesy of Mike
Carr
Handout #6
Prof. Mike Carr at Cerro Negro, 1982 (thanks to J. Walker NIU)
Volcanic Geology of Cerro Negro
Not locations where samples
CN-4 and CN-6 were taken.
Walker & Carr 1986
Tourists enjoy sledding down scoria-covered slopes of Cerro Negro.
Wikipedia
Cerro Negro (CN-4) 1957-1960 eruption
Courtesy of Bob
Stern and Ben
Edwards
5mm scale
Olivine is honey colored, Clinopyroxenes are green and black. Plagioclase is white.
This is a primitive basalt (MgO ~9%) although some phenocryst accumulation has occurred.
Courtesy of Bob Stern and Ben Edwards
CN-4 contains olivine composed of 77%
forsterite endmember, 23% fayalite
endmember. Shorthand for this is FO77.
Handout Fig. 8
CN-6 contains plagioclase with 75%
anorthite endmember (AN75). Q: What is
the name of plagioclase with this
composition?
Cerro Negro CN-6 (1968 eruption)
Courtesy of Bob
Stern and Ben
Edwards
5mm scale
Olivine is honey colored, pyroxenes are green and black. Plagioclase is white.
Q1: what is texture? Q2: what is mode?
What happened between 1960 and 1968
beneath Cerro Negro?
Schematic illustration of the magma
storage system beneath Mount St.
Helens showing earthquake locations
(open circles) in country rock (high
velocity) surrounding earthquakefree zones of magma storage
(low velocity). From Cashman and
Sparks 2013
1957-60: mantle melting and rapid
rise of primitive magma to erupt
(CN-4).
1960-1968: storage and
fractionation of magma beneath
volcano
1968: eruption of fractionated lava
(CN-6)
Cashman and Sparks, 2013
Photomicrographs of CN-6
Field of view = 4.6 mm
(Images Courtesy of Mike Carr and Jim Walker NIU).
Top is plane-polarized light,
bottom is with crosspolarized light
What do you see?
Vesicles are clear in Plane-polarized
light(PPL) but black (opaque) with Xpolarized light (XPL).
Several plagioclase phenocrysts.
One is large (~3mm across).
Note small (few hundred microns) olivine
near top. It is more obvious under XPL
because it has higher birefringence
(Images Courtesy of Mike Carr and Jim Walker NIU).
Note ‘dusty rim’ around large
plagioclase phenocryst. This
indicates disequilibrium between
magma and plagioclase at this time
(late in the growth history of the
plagioclase). Q: Something changed
in the magma, what could it be?
Jim Walker (NIU) and Sandino, Masaya Nicaragua, ~1982
(Image Courtesy of Jim Walker NIU).
Pacaya , Guatemala 2006. Pacaya has been erupting since 1965.
(Source http://volcano.si.edu/)
Let’s look at an example of a Central American felsic lava: Ilopango
Ilopango (El Salvador)
Courtesy of
Mike Carr
Lago Ilopango from above
Lago Ilopango from
the ground
http://blogs.laprensagrafica.com/scientia/?p=110
Ilopango is a broad, low volcano
with a ~10 km diameter caldera
Wikipedia
1880 eruption of Islas
Quemadas, source of IL4
Wikipedia
(Courtesy of
Mike Carr).
(Courtesy of
Mike Carr).
(Courtesy of
Mike Carr).
(Courtesy of
Mike Carr).
Additional slides (mostly about trace
elements)
Marine sediment is a key to some incompatible element variations
that show up in the lavas
moderate
variance
lowest
variance
maximum in
carbonate
maximum in
hemipelagic
Hemipelagic
0
Carbonate
200
300
400
Morb
Depth in meters
DSDP 495
100
500
.1
1
10
U/Th
10
100
Ba/La
--------Regional--------
100 1000 10000
Ba/Th
.01
.1
U/La
1
---------Local--------Modified from Carr et al 2003
Geochemical changes with time at Cerro Negro
11
7
NIC-CN2000-1
NIC-CN8
NIC-CN1
10
NIC-CN-92-2
NIC-CN7
6
NIC-CN12
NIC-CN3
NIC-CN11
NIC-CN13
Na O/K O
2
K/Ba
2
9
NIC-CN10
NIC-CN2
NIC-CN13
NIC-CN10
NIC-CN9
NIC-CN5
NIC-CN5 NIC-CN6
NIC-CN2
NIC-CN4 NIC-CN6
NIC-CN9
NIC-CN7
NIC-CN8
NIC-CN3
NIC-CN1
5
NIC-CN4
NIC-CN2000-1
NIC-CN11
8
NIC-CN12
NIC-CN-92-2
4
1920
1940
1960
Age
1980
2000
7
1920
1940
1960
1980
2000
Age
(Courtesy of Mike Carr)
Cerro Negro basalts show increase in K2O over
time – fractionation?