180
Walker, G.W., and Duncan, R.A. (1989). Geologic map of the Salem 1° by 2°
quadrangle, western Oregon. U.S. Geological Survey Miscellaneous Investigations Series
Map I-1893.
Wallace, P. J., and Carmichael, I.S.E. (1992). Sulfur in basaltic magmas. Geochimica et
Cosmochimica Acta, 56, 1863-1874.
Wallace, P.J., and Carmichael, I.S.E. (1994). S speciation in submarine basaltic glasses as
determined by measurements of SKD X-ray wavelength shifts. American Mineralogist,
79, 161-167.
Wells, R.E. (1990) Paleomagnetic rotations and the Cenozoic tectonics of the Cascade
arc, Washington, Oregon, and California. Journal of Geophysical Research, 95, 1940919417.
Winther, K.T., Watson, E.B, and Korenowski, G.M. (1998). Magmatic sulfur compounds
and sulfur diffusion in albite melt at 1 GPa and 1300-1500°C. American Mineralogist,
83, 1141-1151.
181
CHAPTER 5
GEOCHEMISTRY OF 2004-2005 MOUNT ST. HELENS ASH: IDENTIFICATION
AND EVOLUTION OF THE JUVENILE COMPONENT
Michael C. Rowe
Carl Thornber
Adam J.R. Kent
This manuscript has been submitted for publication as a U.S. Geological Survey
Professional Paper
182
ABSTRACT
Petrologic monitoring of volcanic ash is often utilized for identifying juvenile
volcanic material and observing changes in the composition and style of an eruption.
During the 2004-2005 eruption of Mount St. Helens, identification of juvenile material in
early phreatic explosions was complicated by a significant proportion of 1980-1986 lava
dome fragments and glassy tephra in addition to older lithic fragments. Compositions of
glass, mafic phases, and feldspars and groundmass textures are correlated in an attempt to
identify juvenile material in early phreatic explosions and differentiate between various
methods of producing and distributing ash. Clean glass in the ash is dominantly
correlated with older eruptive events and is not particularly useful for identifying juvenile
compositions. High Li concentrations in feldspars provide a useful tracer for juvenile
material and indicate an increase from 10% to 28% juvenile material between October 1,
2004 and October 4, 2005 prior to the emergence of hot dacite on the surface of the crater
on October 11, 2004. The presence of high Li feldspars out of equilibrium with the
groundmass and bulk dacite early in the eruption is also suggestive of vapor enrichment
to the initially erupted dacite. If excess vapor was indeed present, this may have assisted
in triggering the recent eruption. Textural and compositional comparisons between dome
collapse events, vent explosions, and dome fault gouge indicate that the fault gouge is the
likely source of ash for both types of events. Comparisons between vent explosions and
dome collapse events suggest that the fault gouge and new dome were initially very
heterogeneous, becoming increasingly homogeneous by early January 2005.
183
INTRODUCTION
The phreatomagmatic eruption on October 1, 2004 was the first of five such
events between October 1-5, 2004 which preceded the emergence of hot dacitic material
on the crater floor of Mount St. Helens on October 11, 2004. From October 5, 2004
through December 2005, two phreatic explosions and numerous dome collapse events
have sent volcanic ash over the crater rim (Scott et al., in preparation). Volcanic ash
provides a means by which changes in eruptive behavior can be observed in near real
time and, in some cases, may prove to be a precursor to changes in volcanic activity
(Taddeucci et al., 2002). Monitoring of volcanic ash is often conducted because it can be
collected safely, easily, and at low cost. Also, because the erupting crater/vent is not
always visible or accessible, volcanic ash may provide the only petrologic evidence for
the onset of, or changes in an eruption.
Juvenile magmatic glass has been observed in tephras prior to extrusion of
lavas/domes or before large magmatic eruptions (Watanabe et al., 1999; Cashman and
Hoblitt, 2004). Monitoring of glass compositions in volcanic ash has also identified
multiple magmatic components in eruptions, and has recorded compositional changes
over the course of a single eruption (i.e. Swanson et al., 1994; Swanson et al., 1995,
Pallister et al., 1992, Schiavi et al., in press). At Mount St. Helens, there are several
factors related to previous eruptive events that complicate the ash story, making
petrologic monitoring of volcanic ash enigmatic. Lava dome growth from 1980-1986
generated a thick cap (~250m) of dacitic lavas over the preexisting conduit. In addition,
crater-filling breccia and tephra from the May 18, 1980 and prior eruptions may underlie
184
the 1980-86 lava dome for 500m or more (Friedman et al., 1982). Phreatomagmatic
eruptions are therefore likely to have a significant proportion of older Mount St. Helens
material. Evaluation of ash is further complicated by processes related to mechanical
sorting, which occurs as a function of distance from the vent and requires consideration
of sampling location and wind velocity when comparing ash samples (e.g. Houghton et
al., 2000; Sparks et al., 1997).
Petrologic studies of older Mount St. Helens tephras provide a basis for
comparing products from the 2004-2005 eruption. Published glass analyses from 19801982 tephra and dome samples demonstrated trends of increasing crystallinity and
decreasing water content over the course of the eruptive period, with lower water likely
associated with progressively lower volume and intensity post-May 18 explosive
eruptions (Melson, 1983; Sarna-Wojcicki et al., 1982b). Textural comparisons of the
May 18, 1980 blast material with precursory eruptions indicate the appearance of juvenile
material as early as two months prior to the big blast and that the juvenile component was
reflected by ash particles with either glassy or microcrystalline matricies, characteristic of
shallow crystallization (Cashman and Hoblitt, 2004). The study by Cashman and Hoblitt
(2004) is significant in that it identifies the need to consider crystalline material in
addition to glassy fragments as potentially juvenile and provides a textural comparison
for current eruptive products. Juvenile material, as defined here for the 2004-2005
Mount St. Helens eruption, refers to material erupted hot with textural and geochemical
characteristics of the earliest new dome material (SH304-collected November 4, 2004)
and later dome samples.
185
Studies of volatile element concentrations in tephras have been utilized to identify
evidence for magmatic degassing associated with explosive events before and after May
18, 1980 (Thomas et al., 1982; Berlo et al., 2004). A 210Pb excess and a large Li spike in
plagioclase feldspars (up to ~23 Pg/g) in the 1980 cryptodome, followed by significantly
lower Li concentrations in the May 18 climatic eruption and following eruptive events,
led Berlo et al. (2004) to propose that anomalously high Li concentrations were the result
of vapor enrichment, originating from a deep source, to shallowly stored/stalled magma.
The goals of this study of 2004-2005 Mount St. Helens ash are to 1) identify
juvenile eruptive material in phreatomagmatic events associated with the reactivation of
Mount St. Helens, and 2) combine textural and geochemical observations to distinguish
between the processes and products of ash generation, including a comparison between
dome collapse events and phreatomagmatic vent explosions. Accomplishing these goals
requires a detailed examination of the textures and geochemical characteristics of the
erupted ash samples in comparison to dome rock petrology and geochemistry provided by
other studies (Pallister et al., in preparation; Kent et al., in preparation; Thornber et al., in
preparation).
METHODS
Sample Collection
Following the October 1, 2004 phreatomagmatic eruption, 27 ash collection
stations were established around the perimeter of Mount St. Helens (Fig. 33). Ash
samples collected prior to the establishment of stations were made from relatively clean
186
Figure 33. Locations map of ash collection stations, JL-1 and A02 to A27, established
around Mt. St. Helens, Washington. Samples collected from locations other than
established stations are not shown here (see Rowe et al- Open file report in preparation
for latitude and longitude of sample collection sites). Inset map of Washington with black
box depicting Mt. St. Helens and vicinity.
187
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189
flat surfaces (Fig. 34a). Collection stations were located with line of sight to the volcano
ranging from 2.4 to 10 km from the vent (Fig. 33). Each station is comprised of a rebar
suspended double-bucket. The inner bucket, with drainage slits approximately a third of
the way up from the bottom, is suspended within the outer bucket to allow excess water
to drain without significant loss of ash (Fig. 34b). Collection intervals varied from less
than 3 days to greater than 15 days at individual stations until December 2, 2004 (Fig.
35). Ash collection at the established stations was a function of accessibility of the
station, and predominant wind directions for the period preceding collection (Fig. 35).
Following December 2, 2004, most ash collection has been of discrete ash producing
events (either dome collapses or phreatic explosions) on snow-covered or otherwise clean
surfaces. Snow-covered surfaces provide easy identification of new ash, and allow for
tracking of deposited ash to its source (Fig. 34c). Additional ash samples collected in
August 2005 were made from petrology spyders located within the crater (Fig. 34d).
Fifteen samples collected between October 1, 2004 and March 9, 2005 were
utilized in this study (Table 13). Of the 15 samples studied here, 12 are ash samples
spanning October 1, 2004 to March 9, 2005, two are dome fault-gouge (collected
November 4, 2004 and February 22, 2005) and one sample is of a crater debris flow
(October 20, 2004) associated with collapse of the initial spine (Table 13). See Thornber
et al. (in preparation) and Pallister et al. (in preparation) for description of dome fault
gouge and crater debris flow collection.
Of the five early explosive events, only the three events (October 1, 4 and 5)
included in this study resulted in downwind ash fallout between October 1 and October 5,
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Table 13. Ash, gouge, and crater debris samples included in this study
Sample
MSH04E1DZ_1
MSH04E2A03_A1
MSH04E3RANDLE_2
MSH04A20_10_11
MSH04A20_10_16
MSH04A09_10_12
MSH04A21_10_20
MSH04A04_11_2
MSH04MR_11_4
MSH05JP_1_14A
MSH05JV_1_19
MSH05DRS_3_9_4
SH303-1
SH307-1
SH300-1
1
Type
ash
ash
ash
ash
ash
ash
ash
ash
ash
ash
ash
ash
Gouge
Gouge
Crater
Debris2
Date collected
10/1/2004
10/4/2004
10/5/2004
10/11/2004
10/16/2004
10/12/2004
10/20/2004
11/2/2004
11/4/2004
1/14/2005
1/19/2005
3/9/2005
11/4/2004
2/22/2005
10/20/2004
Eruption date1
10/1/2004
10/4/2004
10/5/2004
10/5/04-10/11/04
10/11/04-10/16/04
10/4/04-10/12/04
10/15/04-10/20/04
10/16/04-11/2/04
11/4/2004
1/13/2005
1/16/2005
3/8/2005
11/4/2004
2/7/2005
10/20/2004
Eruption dates for crater debris and gouge samples estimate by John Pallister. 2Only fine
material collected from the crater debris flow is included in this study. Further details of
ash samples are available in Rowe and others (Open File Report, in preparation).
192
2004 (Major et al., 2005). Samples among the complete suite of Mount St. Helens 20042005 ashes (Rowe et al., Open File Report in preparation) were selected for this study
based on most detailed sampling intervals, wind directions, and volume of material.
Analytical methods
Major element compositions of groundmass glass (and melt inclusions where
possible), feldspars, and mafic phenocryst phases (amphibole, clinopyroxene, and
hypersthene) were measured for all 12 ash samples by electron microprobe analysis
(EMPA). In order to reduce sampling biases, 25-50 feldspars, ~20 mafic phenocrysts,
and 10-20 glasses were analyzed from each sample, proportional to their relative
abundances in ash samples. Backscatter electron (BSE) images were taken of all the ash
and dome gouge samples in order to document heterogeneity of ash particles within and
between samples. Trace element concentrations of the feldspars from 14 of the samples
were obtained by laser ablation inductively-coupled plasma mass spectrometry (LA-ICPMS). Feldspar analyses (major and trace) only were made from the 2 dome fault-gouge
samples and only feldspar trace element concentrations were measured for the crater
debris sample (SH300-1; Table 13).
Analyzed samples were not sieved and phenocrysts and glasses of all sizes were
analyzed to reduce the possible biases created by mechanical sorting during transport
(Houghton et al., 2000). Phenocrysts for LA-ICP-MS and EMPA were targeted
indiscriminately, as groundmass or phenocryst edges were often not preserved in ash
grains. EMPA measurements were made on a Cameca SX-100 at Oregon State
University. Glass EMPA measurements were conducted using a modified procedure to
193
that suggested by Morgan and London (1996). Na and K were counted for 60 seconds
using a 2 nA beam current, 15 keV accelerating voltage and a 10 Pm beam diameter. Si,
Al, Ti, Ca, Mn, Mg, Fe, P, S, and Cl, were analyzed using a 30 nA beam current, 15 keV
accelerating voltage and 10 Pm beam diameter. Count times ranged from 10-50 seconds
depending on the element of interest. The small beam size (~20 Pm is optimal) is
required in this case due to the small size of glassy fragments present in the ash (Morgan
and London, 1996). Feldspars (30 nA) and mafic phases (50 nA) were measured using a
15 keV accelerating voltage and 5 Pm and 1 Pm beam diameters, respectively. When
possible, analyses of phenocryst phases were made within 15 Pm of the grain boundary.
A rhyolite glass standard (USNM 72854 VG-568), a feldspar standard (Labradorite
USNM 115900), and a pyroxene standard (Kakanui augite USNM 122142) were
analyzed prior to each analytical session. Glass standard statistics are presented in Rowe
et al. (USGS OFR- in prep).
Trace element concentrations of feldspars (Li, Ti, Sr, Ba, La, Ce, Pr, Nd, Eu and
Pb) were measured by LA-ICP-MS in the W.M. Keck Collaboratory for Plasma
Spectrometry, Oregon State University. Analyses were performed using a stationary laser
(70 Pm spot size) to ablate a progressively deepening crater in the sample materials,
requiring targeted feldspars to have a diameter greater than ~80 Pm. Due to the large spot
size required and fine grain size of the feldspars in the ash, laser ablation analyses were
made close to the center of inclusion free grains. Each individual analysis represents 40
seconds of data acquisition during ablation with background count rates measured for 30
seconds prior to ablation. A pulse frequency of 4 Hz resulted in an ablation crater ~15 Pm
194
deep. Trace element abundances were calculated relative to the NIST 612 glass standard,
which was analyzed under identical conditions throughout the analytical session. USGS
glass BCR-2G was also analyzed as a secondary standard. Counts were normalized to
29
Si and concentrations determined following the method of Kent et al. (in preparation).
Precision of trace element analysis is presented in Kent et al. (in preparation), however Li
concentrations, most relevant to this study, have a precision of 8% (1 standard deviation).
RESULTS
Glass
Ash sample fragments were dominated by crystalline to microcrystalline textures
with groundmass crystallization resulting in the formation of crystalline silica and
feldspar microlites (Fig. 36). Matrix textures of the juvenile dome are described by
Pallister et al. (in preparation). Al2O3 and SiO2 concentrations in the initial 179 glass
analyses are used to discriminate between analyses that have been significantly modified
as a result of the groundmass crystallization of feldspar and quartz microlites (Fig. 36).
The resulting 81 reliable glass analyses are hereafter referred to as “clean glass.” Most
clean glass analyses are of glass adhered to phenocrysts or from pumice fragments.
Major element concentrations of clean glass are highly variable, due to localized effects
of melt crystallization and lithic heterogeneity of the ash. Clean glass analyses range
from 52-79 wt% SiO2 but are dominantly between 72-78 wt% SiO2 (Table 14). In
addition to a wide compositional range there is no apparent temporal trend to suggest a
195
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196
systematic change in the melt composition over the course of the 2004-2005 sampling
(Fig. 37).
Mafic phases
Mafic phenocrysts analyzed from the 12 ash samples included hypersthene,
amphibole, and clinopyroxene.
Mafic phases were distinguished from feldspar and
groundmass during backscatter imaging on the electron microprobe. Hypersthene is the
dominant mafic phenocryst in the ash and displays a wider range in Mg# (50.8-75.2) than
observed in new dome samples (Mg# 54-71). While discrete clinopyroxene phenocrysts
are present in the early ash samples, they are rare in the new dome material, often
associated with xenolith fragments. Clinopyroxene was not observed in ash deposits
generated after Oct. 20, 2004. Amphibole compositions are also highly variable, with a
mean Mg# of 59.8 ± 13 (1 standard deviation) and Al2O3 from 6 to ~16wt%, comparable
to the range observed in both new dome dacite (mean Mg# of 64.7± 4.9; 6 to 14wt%
Al2O3) and in 1980-1986 dome rock (Thornber et al., in preparation). In addition,
thicknesses of the outermost reaction rims of amphiboles are variable, with thick reaction
rims present on some grains. Rim thickness on amphiboles becomes less variable with
time and begins to resemble the new dome (only rare thick rims by March 8, 2005
tephra). The presence of thick-rimmed amphibole grains and clinopyroxene is
characteristic of 1980-1986 magmas and distinct from the uniformly thin (~5 Pm)
decompression rim around 2004-2005 dacitic amphiboles, thus suggesting that older
Mount St. Helens material is mixed with the juvenile ash (Rutherford and Hill, 1993;
Rutherford and Devine, in preparation). Due to the high compositional variability in the
197
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(MSH04, MSH05) have been removed- see Table 13 for complete sample names. Total Fe as FeO.
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Notes: 1Juvenile glass as described in text is based on MgO and K2O fields defined by SH304 and SH305 glass and inclusion analyses. 2Sample prefixes
98.71
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Total
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FeO3
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Sample2 E2AO3-1A E2AO3-1A A20-10-11 A20-10-16 A20-10-16 A21-10-20 A04-11-2
Major element oxides
Table 14: Electron microprobe analyses of clean, juvenile glass 1.
198
199
ash and in the case of hypersthene and clinopyroxene, lack of comparison with older
Mount St. Helens tephras and lavas, the mafic phases will not be discussed further at this
time. See Thornber et al. (in preparation) for more discussion of textural and
compositional variability of Mount St. Helens 2004-2005 amphiboles.
Feldspars
Feldspar phenocrysts in ash, gouge, and crater debris display a wide
compositional variation. Feldspar crystals in the ash range from An87-28, overlapping
both the new dome (An53-33) and dome samples from 1980-1985 (An51-34). Rim
compositions of the new dome have a more restricted range of An46-34, again similar to
that observed in the 1980’s dome rocks (Rowe et al., 2005; Streck et al., in preparation).
A total of 266 LA-ICP-MS analyses were completed on 14 of the samples included in
this study. Ba and Sr concentrations are the most variable, ranging from 14.5-224.6 Pg/g
and 532-1603ҏ Pg/g respectively. La (1.0-9.7ҏPg/g), Pb (0.3-7.0 ҏPg/g) and Li (6.9-48.5
ҏPg/g) also are highly variable (Rowe et al., Open File Report- in preparation). La has a
well defined, and Pb a weakly defined positive correlation with Ba, while Sr is negatively
correlated with Ba (Fig. 38). Li concentrations do not correlate with other major or trace
elements. While variability in most of the trace elements is within the range predicted by
changes in feldspar/melt partition coefficients with An content, higher Li concentrations
in a small percentage of the feldspars would require ~200ҏ Pg/g Li in the melt,
significantly greater than observed Li in the bulk dacite (21-28ҏ Pg/g; Fig. 38; Bindeman
et al., 1998, Thornber and Pallister, same issue; Kent et al., manuscript in prep). In ash
samples collected after November 4, 2004 a significant change occurs in the Li anomaly,
200
Figure 38. Selected trace element concentrations of feldspars analyzed from ash samples.
Abbreviated sample names are listed (see Table 13 for complete identifications).
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Figure 39. Histograms of Li concentrations in feldspars for each of the 14 samples
examined. Li concentrations greater than 30 Pg/g (dashed line) in ash samples prior to
November 4, 2004 (MR_11_4) coincide with the juvenile feldspars of dome sample
SH304. Eruption dates of samples are in parentheses. Abbreviated sample names
correspond to Table 13.
203
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with Li concentrations more homogenous and consistently lower compared to earlier ash
samples, with a maximum concentration of 30ҏ Pg/g (Fig. 39). This reduction in Li
abundance is also observed in dome samples following the collection of Mount St.
Helens dome sample SH304 (Kent et al., in preparation; Kent et al., manuscript in prep).
DISCUSSION
October 1-5, 2004 ash explosions
Identification of a juvenile component
Identification and quantification of juvenile material in early eruptive events can
be critical to evaluating the potential for continuation or waning of an eruption. Juvenile
material in the early ash component is evident from specific glass compositions and high
feldspar Li content. Most glass major element compositions overlap with 1980-1982
(Melson, 1983; Sarna-Wojcicki et al., 1982b) and pre-1980 eruptions (Sarna-Wojcicki et
al., 1982b). However, matrix glass and melt inclusion analyses of 2004-2005 Mount St.
Helens samples SH304 and SH305 are distinctly lower in MgO (<0.5 wt%) and higher in
K2O (up to 5.5 wt% K2O) at comparable SiO2 concentrations (Fig. 37; Table 14; Pallister
et al., in preparation). Based upon the combined variations of MgO and K2O, glass
analyses can be categorized into what appear to be older glassy ash fragments and
juvenile material. None of the clean glass analyses from the October 1 and October 5,
2004 eruptions are juvenile while ~20% of the October 4 glasses have juvenile
characteristics. The estimated percentage of juvenile material drops significantly, to less
than 9% for October 4 and 5% overall for the early explosive events however because of
205
high proportion of highly crystallized groundmass present (Fig. 36). In addition, all of
the analyzed pumice fragments fall within the compositional fields defining the older
volcanics. Backscatter electron images in Figure 40b and 40d provide a textural
comparison between juvenile crystalline groundmass (40b) and a pumiceous glass
fragment compositionally similar to the older volcanics (40d). Both textural and
compositional comparisons between the various groundmass types are critical to avoiding
a common misconception that clean glass particles (for example Figure 40d) are always
representative of the juvenile component associated with the eruption (for example,
Watanabe et al., 1999; Swanson et al., 1995; Sarna-Wojcicki et al., 1982a).
Previous studies of 1980’s tephra and dome samples indicate that feldspars from
1980’s lavas and tephras have a maximum Li concentration of ~30ҏ Pg/g (Berlo et al.,
2004, Kent et al., 2005). However, feldspars of early 2004 dome sample SH304 (erupted
~October 18, 2004) have concentrations ranging from 30-40 ҏPg/g Li, distinctly higher
than that of older Mount St. Helens feldspars (Berlo et al., 2004; Kent et al., 2005), thus
allowing us to distinguish between juvenile and older feldspars present in the October 1,
2004 to October 5, 2004 ash (Fig. 39). Based on this evidence, 10% of the feldspars from
the October 1 eruption are juvenile while 28% of the October 4 feldspars are juvenile.
Despite only two feldspars from the October 5 ash providing reliable Li concentrations,
do to the small grain size of the sample, one of the two has a high Li concentration. Both
the glass and feldspar compositions suggest an increase in the proportion of juvenile
material from October 1 to October 4, 2004, however the feldspar compositions suggest
the presence of a significant juvenile component as early as October 1, whereas the glass
206
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analyses do not indicate the presence of juvenile material until October 4. The nearly 3x
increase in juvenile material from October 1, 2004 to October 4, 2004 is suggestive of
upward movement of magma within the shallow conduit, consistent with the appearance
of hot dacite on the surface of the crater floor on October 11, 2004.
Considering issues of matrix interference, higher estimates of the percentage of
juvenile material based upon feldspar trace element analyses compared with estimates
using the glass compositions is a result of quartz and feldspar microlite crystallization
evident from early dome samples (Fig. 36, 40). This same microcrystalline texture is
observed in all of the ash samples suggesting that shallow magmatic crystallization has
affected textures of juvenile ash particles, similar to observations by Cashman and
Blundy (2004) with regard to early 1980 eruptions.
Evidence for a volatile eruption trigger
The high Li concentrations observed in early erupted feldspars from the ash (max
of 48.5ҏ Pg/g Li) and dome samples (max of 41.5ҏ Pg/g Li) may have implications for the
initiation of the recent eruption. As previously discussed, the high Li content observed in
feldspars would require a melt concentration of roughly 200 ҏPg/g Li based on
mineral/melt partition coefficients, despite the lower 21-28ҏ Pg/g Li observed in the bulk
dacite. As suggested by Berlo et al. (2004), the discrepancy between the high Li
feldspars and low Li groundmass could be explained by vapor enrichment of shallowstored magma, followed by the loss of Li in the groundmass as a result of degassing.
Kent et al. (manuscript in prep) predict that vapor enrichment must have occurred less
than ~1 year (and potentially within just a few days) before eruption since enrichment
208
prior to that time would have resulted in diffusional equilibration of feldspars in gabbroic
inclusions- which have lower Li than groundmass plagioclase. A CO2 output of 2415
tons/day, measured on Oct. 7, 2004, followed by a steady decline in ensuing months also
may indicate a greater volume of gas early in the eruption, although the higher gas output
could also result from higher extrusion rates early in the eruption (McGee et al., in
preparation). The high Li concentrations in feldspars in conjunction with higher CO2
output measured on October 7 may indicate the presence of a significant volume of
excess gas prior to the onset of the eruption. Hence, Li-enriched plagioclase in the
earliest eruption products indicates that high vapor pressure could have triggered the
recent eruption.
Ash generation
Airborne ash is produced by both vent explosions and dome collapse events at
Mount St. Helens. Two general mechanisms are typically described for the generation of
ash during vent explosions. In magmatic explosions, exsolution and expansion of gases in
magma during ascent leads to frothing of the magma, vesiculation and fragmentation
(Heiken, 1972, Cashman et al., 2000). In contrast, phreatomagmatic eruptions primarily
result from the rapid expansion of, in the case of Mount St. Helens, meteoric water as it
interacts with magma and/or hot rock at depth (Mastin et al., in preparation; Morrissey et
al., 2000). Mount St. Helens 2004-2005 vent explosions result in a greater area of
dispersal than dome collapse events with fallout reported as far as Ellensburg, WA
following the March 8, 2005 vent explosion (Mastin et al. 2005; Scott et al., in
preparation). In principle, particle shape is controlled by the shape and density of the
209
vesicles in magmatic eruptions while glassy fragments in phreatomagmatic eruptions are
often pyramidal or blocky (Heiken, 1972). In both cases however, relatively rapid
cooling should result in the generation of glassy material when magma is present.
Intracrater pyroclastic block and ash flows of this eruption are associated with
dome collapse events that result from dome growth and oversteepening (Vallance et al.,
in preparation). Dome collapse events with elutriation of fine ash particles, while
occurring much more frequently than vent explosions, are typically less significant,
sending ash up to 30, 000 ft into the air and falling proximal to the edifice (Scott, in
preparation). In addition to vent explosions and dome collapse events small volumes of
airborne ash may be distributed by elutriation during steaming of the dome, a relatively
low energy, and passive mechanism for ash transportation.
Mechanisms of ash generation and distribution of the recent Mount St. Helens
eruption are not discernable using traditional tools of grain shape, volume of glass, or
textural and compositional heterogeneity of ash particles (see above descriptions). This
eruption provides a case in which very little glassy material is present in the ash, and only
a small percentage of that is considered to be juvenile. Juvenile groundmass observed in
the dome and in the ash is crystalline (quartz and feldspar) with localized vesiculation
and devitrification (Fig. 40a,b). Water by difference measurements of melt inclusions
indicate that the juvenile magma is extensively degassed prior to eruption, and as
supported by gas emission data for this eruption, suggests that expansion of magmatic
gas, either during vent explosions or dome collapses, is not responsible for ash generation
(Pallister et al., in preparation; McGee et al., in preparation). In addition, the high
210
proportion of lithics and small proportion of clean juvenile glass shards and/or pumice
fragments suggests these explosions are dominantly phreatic in origin and that
crystallization of the rising magma had occurred prior to the explosive events.
By comparing geochemical analyses of multiple phases in conjunction with the
BSE imaging of ashes derived from dome collapse events and vent explosions, we have
attempted to distinguish between those processes. Major element concentrations of
matrix glass and inclusions from the January 13, 2005 dome collapse correlate well with
juvenile compositions established by glass analyses in dome samples SH304-305 (Fig.
37). In contrast, only 40% of the glass analyses from the January 16, 2005 vent
explosion correlate with the juvenile fields. This discrepancy demonstrates that the vent
eruptions contain a higher percentage of exotic fragments derived from crater debris in
the vent area. However, a distinct contrast is observed when comparing glass
compositions from the January 13, 2005 and November 4, 2004 dome collapse events.
Whereas the glass from the January 13, 2005 event is dominated by juvenile glass, the
November 4, 2004 ash is dominated by older glass (Fig. 37). This observation suggests
that the part of the early dome (or uplift area) responsible for ash generation contained a
significant proportion of older material but by January 2005 was dominantly composed of
juvenile dacitic material. This conclusion is also supported by field observations and preNovember 2004 dome samples (Pallister et al., in preparation; Reagan et al., in
preparation).
Juvenile ash fragments from dome collapse events (November 4, 2004 and
January 13, 2005) and vent explosions (January 16, 2005 and March 8, 2005) have nearly
211
identical groundmass textures, as described above (Fig. 41). Grain shape between
samples is also strikingly similar. Ash particles dominated by large phenocrysts often
reflect the general shape of that phenocryst and are more angular while particles
dominated by groundmass typically vary from round to subangular. Due to the
similarities in grain shape and texture for the juvenile matrix, commonly cited methods
for ash generation related to different types of events do not explain the observations (i.e.
Heiken, 1972; Cashman et al., 2000; Morrissey et al; 2000). The likely predominant
source of ash therefore is the dome fault gouge, which, comprising the outer 1-3m of the
dome, becomes increasingly disaggregated towards the exterior of the dome. Gouge
particles are dominantly rounded to subangular, similar to the ash (Fig. 41e,f). Particle
size distribution of dome gouge and ash samples generated from both dome collapse and
vent explosions also suggests that they are of similar origin (Thornber et al., in
preparation). Groundmass textures of dome gouge extruded on November 4, 2004
(SH303-1) are very heterogeneous, suggesting that at this time the gouge was comprised
of a high percentage of exotic wallrock lithics. This heterogeneity also explains the
dominance of older glass compositions in the November 4 dome collapse ash. In
addition, feldspar Li concentrations in the November 4 ash match those of the gouge and
do not correlate well with either the SH304 or SH305 dome samples (Fig. 39; Kent et al.,
in preparation). Dome fault gouge collected on February 22, 2005 (SH307-1) is much
more homogenous, with groundmass textures resembling the growing dome, and
groundmass textures and grain shapes identical to the ash. This homogeneity is reflected
in the January 13, 2005 dome collapse ash in which glass compositions match that of the
212
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growing dome (Fig. 37). Textural and geochemical evidence presented here confirms
that airborne ash is generated from the dome fault gouge, likely entrained with large
pieces of crater debris or dome dacitic blocks during vent explosions and dome collapse
events.
Ash collected more distally as a result of passive steaming is dissimilar to that of
the dome collapse and vent explosions. Texturally, the ash collected between October 6
and November 2, 2004 at established collection stations has a high proportion of lithic
fragments. Pumice fragments, compositionally identical to 1980’s pumice, are the
dominant groundmass type rather than the juvenile crystalline groundmass commonly
found during discrete ash producing events. Only a small proportion of clean juvenile
glass is present in all of the ash samples collected during this interval and there is no
systematic variation in proportion of juvenile glass with time, despite the appearance of
hot dacite on October 11, 2004. Similar results are observed for Li concentrations in
feldspars during this time interval, with high Li concentrations in only ~5% of the
analyzed feldspars despite identifying ~21% juvenile feldspars in the fine material
collected from the October 20, 2004 crater debris flow sample (SH300-1). The low
proportion of juvenile glass, groundmass textures, and feldspars in the distal ash suggests
these samples have experienced a greater degree of contamination. The low energy
transport of ash by steaming of the dome appears to only entrain the lowest density ash
particles, resulting in a higher proportion of pumice fragments in these ash samples.
Also, as a result of the small volume of material that can be transported, the affects of
contamination from other sources, such as windblown particulates, is more significant.
214
CONCLUSIONS
This textural and geochemical examination of 2004-2005 Mount St. Helens ash
has provided several important observations and conclusions with regard to the nature of
early eruptive events and processes of ash generation and transportation.
1. High Li concentrations in feldspars suggest that the earliest phases of the recent
eruption may have contained a more significant volatile/vapor phase than originally
thought and may therefore provide evidence for a possible eruption trigger resulting,
in part, from the presence of an excess volatile phase.
2. Lithium concentrations in feldspars indicate a potential increase in the proportion
juvenile material from October 1 to October 5, 2004, tracking the rise of magma prior
to the emergence of hot dacite on October 11, 2004. High Li concentrations in
feldspar is only useful as a tracer for juvenile material during the beginning stages of
the eruption since after ~November 20, 2004 (estimated eruption date of SH305) Li
concentrations drop to levels comparable to 1980-86, suggesting that the volatile
enriched magma was only a shallow cap on the larger magma body.
3. Clean glass in ash samples is dominated by older volcanic material while juvenile
groundmass is typically crystalline (quartz and feldspar) with localized devitrification
and patchy, very fine vesiculation. This conclusion is significant in that it is typically
the clean glass that is characterized as juvenile during an eruption.
215
4. Groundmass textures, grain shapes, and compositional similarities between ash from
vent explosions and dome collapse events and dome fault gouge particles indicate that
the gouge is a predominant source of ash. Heterogeneity observed in early ash
samples correlates with observations that the early extruded dome material and fault
gouge contained a significant proportion of older volcanic material. Increasing
homogeneity of ash with time is consistent with a similar trend in dome and fault
gouge samples.
ACKNOWLEDGEMENTS
The authors would like to thank all of the CVO staff and volunteers who assisted
in the establishment of ash stations and collection of Mount St. Helens ash. We would
especially like to thank A. Eckberg, S. McConnell, and T. Herriot who assisted with
drying, sorting, and weighing of ash samples, Dan Gooding and Dave Ramsey for
assistance in the field and with the collection site map, and F. Tepley (OSU microprobe
lab) for providing microprobe time. The Mount St. Helens petrology working group
provided valuable discussion and feedback for this study. This work was supported in
part by the National Science Foundation grant EAR 0440382.
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219
CHAPTER 6
SUMMARY OF CONCLUSIONS
The overlying objective of this dissertation is to examine the major-, trace-, and
volatile-element and fO2 variability in basaltic melt inclusions from across the Oregon
segment of the Cascade volcanic arc, using microanalytical and experimental techniques,
to evaluate the impact of addition of a subduction component on the genesis of primitive
basalts in the Cascades. The first two chapters of this dissertation are dedicated to melt
inclusion rehomogenization experiments focusing on techniques of rehomogenization and
potential problems that may arise with respect to determining melt composition and
oxidation state. The third chapter details application of the techniques described in
chapters 1 and 2 to Cascade olivine-hosted melt inclusions with regard to determining
compositional variability and oxidation state in order to examine the effects of addition of
a subduction component to the subarc mantle.
In the first chapter of this dissertation, heating experiments on olivine-hosted melt
inclusions from a forearc shoshonitic basalt in the Oregon Cascades (QV03-1) document
a previously unreported phenomenon in melt inclusion rehomogenization studies.
Significant Fe-gain in melt inclusions (up to 21 wt% FeO) is determined to be an artifact
of magnetite dissolution during rehomogenization. Magnetite grains coexisting with melt
did not result from crystallization of the inclusion and likely are found to coexist with
melt because of 1) co-entrapment of melt and magnetite during crystallization since early
crystallization of a spinel phase is predicted by the higher water contents of the melt
inclusions, or 2) forming as a result of hydrogen diffusion out of the water rich inclusions
220
during slow cooling. Crystallization of a magnetite dust has been observed as a result of
hydrogen diffusion however not to the extent reported here (Danyushevksy et al., 2002).
These experiments illustrate the importance of having reasonable constraints on the
oxidation state, liquidus temperature of the melt, and water content prior to
rehomogenization. Additionally, since magnetite grains in these melt inclusions had
formed prior to, rather than as a result of rehomogenization, care must be taken to avoid
potentially problematic melt inclusions when rehomogenization is necessary.
Techniques and problems associated with determination of sulfur speciation and
oxidation state in olivine-hosted melt inclusions are detailed in the second chapter of this
dissertation. Utilizing naturally quenched and crystalline melt inclusions, heating
experiments are used to determine the variation in sulfur speciation observed in melt
inclusions resulting from natural magmatic processes and rehomogenization and analysis.
x
In naturally quenched melt inclusions, degassing and hydrogen diffusion have the
greatest potential to alter melt oxidation states. While hydrogen diffusion out of
the inclusion will result in oxidation of the melt, degassing may either oxidize or
reduce the inclusion, depending on the initial melt oxidation state.
x
Fractional crystallization (oxidation) or melting (reduction) and Fe-loss due to the
re-equilibration of the inclusion and host (oxidation) may alter the melt oxidation
state, albeit to a lesser extent than degassing or hydrogen diffusion.
x
These rehomogenization experiments have shown that at short experimental run
times (~10 minutes) and at temperatures that reproduce the original entrapment
conditions, melt oxidation states can be accurately determined once other
221
variables (degassing, hydrogen diffusion, crystallization, and Fe-loss) have been
accounted for.
Applying sulfur speciation measurement techniques to naturally quenched and
rehomogenized melt inclusions (in conjunction with major-, trace-, and volatile-element
variability) from basaltic lavas in the Oregon Cascades allows for several overlying
conclusions to be drawn with regard to (1) impact of a subduction component from the
Juan de Fuca plate and (2) basalt petrogenesis in the Cascade volcanic arc.
x
The overall range in basaltic fO2 is from less than -0.25 log units to +1.9 log units
(> 2.25 log units) relative to the FMQ oxygen buffer and that this range is
believed to be representative of the overall variation in mantle source fO2.
x
OIB-like and shoshonitic basalts have the greatest across arc geochemical
variations while LKT and CAB melt inclusions are relatively similar between the
arc and backarc.
x
Correlation of fluid mobile elements and volatiles with sulfur speciation and fO2
suggests that the addition of a subduction component added to the subarc mantle
(increasing from backarc to forearc) resulted in the generation of more highly
oxidized, fluid-mobile element rich basaltic compositions nearer to the
deformation front.
x
The absence of a significant across-arc variation in CAB melt compositions may
indicate that CAB trace element signatures present in the backarc are the result of
a prior enrichment event and not related to current subduction. High degree
222
melting, shallow mantle segregation depths and low measured fO2 may indicate
that CAB melts re-equilibrated with a more reduced composition prior to eruption
and are therefore not representative of their initial mantle oxidation states
predicted as a result of the addition of a subduction component.
In the final chapter of this dissertation, electron microprobe and LA-ICP-MS
techniques, similar to those utilized on Cascade basaltic melt inclusions, are applied to
phenocryst and glass ash fragments from the 2004 eruption of Mount St. Helens. The
two main objectives of this chapter are to identify the juvenile magmatic component in
early phreatomagmatic explosions (October, 2004) and to track changes in the proportion
and character of juvenile material in the ash over the course of the first year of the
eruption (2004-2005). Several conclusions from trace- and major-element analysis of ash
fragments are summarized here.
x
High Li concentrations in feldspars suggest that the earliest phases of the recent
eruption may have contained a more significant volatile/vapor phase than
originally thought. An increased vapor pressure may have contributed to the
onset of the eruption.
x
Lithium concentrations in feldspars indicate a potential increase in the proportion
juvenile material from October 1 to October 5, 2004, likely tracking the rise of
magma prior to the emergence of hot dacite on October 11, 2004. The high Li
concentration in feldspar is only useful as a tracer for juvenile material during the
beginning stages of the eruption; after ~November 20, 2004 (estimated eruption
223
date of SH305) Li concentrations drop to levels comparable to 1980-86,
suggesting that the volatile enriched magma was only a shallow cap on the larger
magma body.
x
Based on major-element compositions, clean glass in ash samples appear to be
dominantly older volcanic material while juvenile groundmass is typically
crystalline (quartz and feldspar) with localized devitrification and patchy, very
fine vesiculation. This is a significant observation since it is typically the clean
glass that is characterized as juvenile during an eruption.
x
Groundmass textures, grain shapes, and compositional similarities between ash
from vent explosions and dome collapse events and dome fault gouge particles
indicate that the gouge is a predominant source of ash. Heterogeneity observed in
early ash samples correlates with observations that the early extruded dome
material and fault gouge contained a significant proportion of older volcanic
material. Increasing homogeneity of ash with time is consistent with a similar
trend in dome and fault gouge samples.
224
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APPENDICES
240
APPENDIX A
METHODS
Sample preparation
Basalt samples were lightly crushed in a steel piston to avoid significant
fracturing of olivine grains along melt inclusions. Olivine phenocrysts, separated from
groundmass with a hand magnet, were hand-picked under a binocular microscope.
Grains with iddingsite alteration were avoided during picking. Following inclusion
rehomogenization, olivine grains are mounted in 1 inch epoxy rounds, then polished to
0.05 Pm for electron microprobe and laser ablation ICP-MS analysis. For samples with a
small volume of olivine or with few melt inclusions, handpicked grains were examined in
ethanol to identify melt inclusion-bearing grains and then individually mounted to
provide the greatest number of intact inclusions.
Melt inclusion rehomogenization
Olivine grains were examined in ethanol under a binocular microscope to
determine if rehomogenization was required. In most cases melt inclusions in olivine
grains were found to be partially (<5%) to completely crystalline, requiring melt
rehomogenization. To rehomogenize inclusions, olivine phenocrysts were heated in a 1
atm vertical atmosphere-controlled Deltec furnace. Olivine grains (~10-100/experiment
depending on grain size) were wrapped in platinum foil and suspended in the vertical
furnace by platinum wire. Oxygen fugacity was maintained at or below the FayaliteMagnetite-Quartz (QFM) oxygen buffer with a mixture of H2 and CO2 gas in order to
241
inhibit magnetite crystallization along grain fractures and on the surface of olivines.
Olivine grains were heated rapidly, reaching the maximum run temperature in less than 5
minutes. Olivine grains are restricted to <15 minutes at temperatures above 1000°C and
are held for a maximum of 10 minutes at the designated run temperature (± 3°C) to
reduce the potential for H+ diffusion and Fe-loss occurring during longer
rehomogenization times (Danyushevksy et al., 2000; 2002; Hauri, 2002; Qin et al., 1992).
Following rehomogenization, olivine grains are rapidly quenched in water to produce
glassy melt inclusions.
Electron microprobe analysis
The major element (and for glasses, S and Cl) concentrations of melt inclusions,
groundmass glass, and phenocryst phases (including olivine, chromite, feldspar,
pyroxene, and amphibole) were measured via electron microprobe using a Cameca SX50
and SX100 at Oregon State University. The basalt glass analytical procedure utilized for
melt inclusion analysis was optimized for analysis of S (30 sec peak count time) and Cl
(100 sec peak count time). Analyzed elements, counting times, and crystal configurations
are summarized in Table A1 for each phase analyzed by electron microprobe on the OSU
SX50 and SX100.
Beam conditions
Olivine, chromite, pyroxene, and amphibole grains were analyzed using a focused
(1Pm) beam, with a 15 KeV accelerating voltage and 50 nA beam current. Basaltic melt
242
inclusion analyses were made with a beam diameter of 4-7Pm (dependent on inclusion
size) with a 15 KeV accelerating voltage and 30 nA beam current. Feldspars were
analyzed with the same beam conditions as melt inclusions with a 5Pm beam diameter.
Rhyolite glass from Mt. St. Helens was analyzed using a modified procedure to that
suggested by Morgan and London (1996). Na and K were counted for 60 seconds using a
2 nA beam current, 15 keV accelerating voltage and a 10 Pm beam diameter. Si, Al, Ti,
Ca, Mn, Mg, Fe, P, S, and Cl, were analyzed using a 30 nA beam current, 15 keV
accelerating voltage and 10 Pm beam diameter. The small beam size (~20 Pm is optimal)
is required in this case due to the small size of glassy fragments present in the ash
(Morgan and London, 1996).
Detection limits based on reported count times for each phase on the SX 100 and SX
50 microprobes are based on the 3-sigma lower limit of detection:
Sp
3 * 2 * ( R B TB ) *
C
R P TP
equation 1
Where RB and RP are the count rates on background and peak (peak-background)
respectively. TB and TP are the count times on background and peak, respectively, and C
is the concentration of the measured element.
Mineral and glass standards
Several mineral and glass standards were utilized throughout this study. Mineral
standards, San Carlos olivine (USNM 11312/444), Springwater meteorite olivine (USNM
2566), chromite (USNM 117075), Minas Gervais magnetite (USNM 114887), and
243
chromite (USNM 117075) were analyzed repeatedly over the course of the Oregon
Cascade melt inclusion study. Analyses of Makaopuhi Lava Lake basaltic glass (USNM
113498/1 VG-A99) in addition to external glass standards BCR-2G, BHVO-2G and LO02-04 were made repeatedly to monitor precision and accuracy of glass measurements. It
should be noted that sulfur concentrations reported by Kent et al (1999a) for LO-02-04
(700ppm) are significantly lower then the average value of multiple analyses performed
at Oregon State University on both a Cameca SX50 and Cameca SX100, which yielded
sulfur concentrations of ~1400ppm. Sulfur concentrations reported by Kent et al (1999a)
however are considered unreliable (A. Kent personal communication). A rhyolite glass
standard (USNM 72854 VG-568), a feldspar standard (Labradorite USNM 115900), and
a pyroxene standard (Kakanui augite USNM 122142) were analyzed prior to each
analytical session of Mt. St. Helens ash.
Long-term reproducibility of melt inclusion analyses is a significant concern
given the duration of this study. Analyses of Makaopuhi Lava Lake basaltic glass made
prior to analysis of melt inclusions allows for monitoring our ability to compare
measurements over several years. Most major elements (SiO2, TiO2, Al2O3, FeO, MgO,
and CaO) have an error (1 s)of less than 3%. Na2O and K2O have a slightly greater error
in reproducibility but are still below 10% (Table A2). Similarly, repeated analysis of
Springwater meteorite olivine standard can be used to monitor long term reproducibility
of olivine compositions for SiO2, FeO, MgO and to a lesser extent MnO (Table A3).
Error estimation for major element compositions during the course of a single
analytical session are based on repeat analysis of BCR-2G, BHVO-2G, and LO-02-04
244
glasses and are generally below 1-2% for major elements although K2O (2.75%), P2O5
(3.5%) and S (8.9%) are slightly greater. Similar comparisons for spinels, olivines, and
rhyolitic glass all indicate reasonably low error for major elements (Tables A6-A8).
Laser Ablation ICP-MS
Melt inclusions
Trace element concentrations in melt inclusions and glasses were determined by
LA-ICP-MS in the W.M. Keck Collaboratory for Plasma Spectrometry, Oregon State
University. Analyses were performed using a NewWave DUV 193 nm ArF Excimer
laser and VG PQ ExCell Quadrupole ICP-MS under conditions similar to that reported in
Kent et al. (2004). Ablation was carried out using He as the carrier gas, and this was
mixed with Ar immediately prior to the plasma torch. Background count rates were
measured for 30 s before ablation commenced for each analysis. Plasma torch conditions
were optimized so that ThO/Th ratios were < 2%. Analyses were performed using the
spot mode – using a stationary laser to progressive ablate a progressively deepening
crater in the sample materials. Each individual analysis represents 40 s data acquisition
during ablation using 40-50 µm spot for inclusions 80 µm spot for glass. Pulse rates
varied from 3-4 Hz with the lower pulse rate correlating with the larger spot. Trace
element abundances were calculated relative to the USGS glass standard BCR-2G, which
was analyzed throughout the analysis session (see Kent et al. 2004 for the values used for
calibration).
43
Ca was used as the internal normalizing isotope in conjunction with CaO
245
contents measured by electron microprobe. Trace element concentrations were
determined following equation 2:
std
º
ª[CaO]unk
ppm xstd
cpsCa
wt %
cps xunk * «
*
*
std »
std
unk
cps x
[CaO] wt % ¼
¬ cpsCa
ppm xunk
equation 2
USGS glass BHVO-2G was also analyzed under identical conditions to the unknowns to
monitor accuracy and precision. Average BHVO-2G compositions, 1 V variability, and
deviation from accepted values shown in Table A9, generally indicate the error on trace
element analyses is less than 10%.
Feldspars
Feldspar trace element concentrations (Li, Ti, Sr, Ba, La, Ce, Pr, Nd, Eu and Pb)
were measured LA-ICP-MS under similar conditions as melt inclusions at Oregon State
University. Analyses were performed using a stationary laser (70 Pm spot size) requiring
targeted feldspars to have a diameter greater than ~80 Pm. Due to the large spot size
required and fine grain size of the feldspars in the ash, laser ablation analyses were made
close to the center of inclusion free grains. A pulse frequency of 4 Hz resulted in an
ablation crater ~15 µm deep. Trace element abundances were calculated relative to the
NIST 612 glass standard, which was analyzed repeatedly throughout the analytical
session. USGS glass BCR-2G was also analyzed as a secondary standard under
conditions identical to unknown feldspars (Table A10). Counts were normalized to 29Si
and concentrations determined following the method of Kent et at (in preparation).
246
Briefly described here, the calculated ratio CaOwt%/SiO2wt% was determined using
equation 3:
Ca Si u
Ca Si meas .
43
known
§ CaOwt % ·
¨
¸
© SiO 2 wt % ¹ std
29
calc
feld
meas .
43
29
§ CaOwt % ·
¨
¸
© SiO 2 wt % ¹ feld
(equation 3)
std
The concentration of SiO2 in the feldspar crystals was then calculated based on
the CaOwt%/SiO2wt% ratio (equation 3) and the empirical ratio CaOwt%/SiO2wt%
determined stiochiometrically and by multiple microprobe analyses of Mt. St. Helens
feldspars. Trace element concentrations were calculated using measured 29Si in
conjunction with calculated SiO2 wt% values using equation 2 (substituting Si for Ca).
Due to the large beam diameter and resulting pit of the laser ablation the SiO2 determined
from microprobe analysis was determined to not be representative of the 29Si count rate,
therefore requiring the stiochiometric relationship between SiO2 and CaO to be utilized.
Precision of trace element analysis from repeat analysis of BCR-2G for two analytical
sessions is presented in Table A10.
SKD X-ray wavelength shifts
The SKD wavelength (O>SkD@shift analyses were conducted following the
methods of Wallace and Carmichael (1994) and Carol and Rutherford (1988). Analyses
were performed at Oregon State University on a Cameca SX100 electron microprobe and
at the University of Oregon on a Cameca SX50 electron microprobe. SKD shifts were
measured simultaneously on three PET crystals to determine the reproducibility of the