Using major, minor, and trace element geochemistry of melt inclusions to study
magma dynamics and evolution at Augustine Volcano, Alaska
J. Katharine Marks
Senior Integrative Exercise
March 11, 2009
Submitted in partial fulfillment of the requirements for a
Bachelor of Arts degree from Carleton College, Northfield, Minnesota TABLE OF CONTENTS
Abstract
Introduction...... 1
Geologic Setting...... 3
The 1986 Eruption
Petrography...... 8
Mineral and Melt Geochemistry...... 11
Discussion...... 16
Conclusion...... 24
Acknowledgments...... 24
References Cited...... 25
Appendix...... 28
Using major, minor, and trace element geochemistry of melt inclusions to study magma dynamics and evolution at Augustine Volcano, Alaska
J. Katharine Marks Carleton College Senior Integrative Exercise March 11, 2009
Advisors: Cameron Davidson, Carleton College James D. Webster, American Museum of Natural History
ABSTRACT
Augustine Volcano is a stratovolcano of the Aleutian Arc in southwestern Alaska, near Anchorage. It has erupted regularly throughout recorded history in two phases: 1) an explosive phase includes ash columns and pyroclastic flows, and 2) an effusive phase consists of dome-building and block lava flows at the summit. This study analyzes plagioclase-hosted melt inclusions from one sample of andesitic pumice from the 1986 eruption for major, minor, and trace (H+, Li, Be, B) elements. Melt inclusion geochemistry is useful to determine the influence of pre-eruption magmatic processes (i.e. creation, fractional crystallization, mixing or mingling, and degassing) on the composition of the magma and the eruption style of the volcano. The melt inclusions in the study are rhyolitic (73.6-75.8 wt.% SiO2) and show fractionation trends in major element data (e.g. decreasing MgO and increasing Na2O with increasing SiO2). SO2 and Be data show two separate trends which indicate the mingling of two magmas of similar composition. This mingling most likely takes place in a system of interconnected dikes below the volcano where old magma is stored and through which new magma pulses and mingles with the old magma. It is possible that the mingling of these magmas is one factor that helps to trigger an eruption, but other factors (e.g. degassing) also contribute to eruption style.
Keywords: inclusions, magma, geochemistry, Alaska, fractional crystallization, mixing.
1
INTRODUCTION
Magmatic characteristics such as composition, and processes such as vesiculation,
fractional crystallization, and mixing determine the eruption dynamics of volcanoes.
Having a better grasp of such processes should result in an improved ability to predict
and respond to potentially dangerous volcanic activity. The composition of a magma is a
particularly important factor because it determines eruption style; the more felsic the
magma, the more viscous it becomes, and thus eruptions are more explosive. All
magmas are composed of three phases: crystals, melt, and fluid. The melt is the liquid
rock and its composition, along with the abundance and composition of the crystals
influence the viscosity of the magma. The fluid phase is composed of volatiles (e.g. CO2,
H2O, Cl, S, and F) and light elements (e.g. Li) that exsolve from the melt due to declining
temperature and pressure during ascent (Wallace, 2005).
Melt inclusions, pockets of the melt phase of the magma trapped in a growing
phenocryst, record the composition of the melt surrounding the phenocryst presumably at
a stage before eruption (Lowenstern, 1995; Lukács et al., 2005; Wallace, 2005). Melt
inclusions are found in a wide range of compositions of silicic magmas: from basaltic
(e.g. Gurenko et al., 2005; Wade et al., 2006) to dacitic (e.g. Gardner et al., 1995; Blundy
and Cashman, 2005; Witter et al., 2005) to granitic (e.g. Müller et al., 2006).
Melt inclusions have been used to determine pre-eruption volatile contents of the
melt (e.g. Wade et al., 2006; Benjamin et al., 2007). Wade and others (2006) analyzed
olivine-hosted melt inclusions from Arenal Volcano, Costa Rica for H2O, other volatiles,
and trace elements to constrain the H2O content (maximum of 4 wt.%) of the basaltic
parental magmas. Benjamin and others (2007) also analyzed olivine-hosted melt 2 inclusions, but from Irazú Volcano in Costa Rica. Previously, magmas from Irazú were thought to originate from ocean island sources because their trace element ratios are similar to those of ocean island basalts (low Ba/La and high La/Sm). However, samples taken of melt inclusions contain high levels of volatiles (>3 wt.% H2O and >2000 ppm
Cl), which are typical of arc magmas whose fluids originate from recycled water from the downgoing slab (Wallace, 2005). Thus, based on the trace element and volatile data from melt inclusions, Benjamin and others (2007) hypothesized that the magmas come from
Galápagos Islands-derived subducted material, thus reconciling the ocean island basalt characteristics with the arc magma characteristics of Irazú.
Melt inclusions help to understand magma chamber processes (e.g. Gardner et al.,
1995; Blundy and Cashman, 2005; Lukács et al., 2005). Gardner and others (1995) used the varied compositions of melt inclusions in plagioclase and quartz phenocrysts as evidence for significant mixing producing the heterogeneous dacites erupted at Mount St.
Helens. Other plagioclase-hosted melt inclusions from Blundy and Cashman (2005) show decreased dissolved H2O. This observation coupled with increased crystallinity of the matrix glass indicates rapid, isothermal, decompression-driven crystallization, which strongly influences eruption style of the volcano. Lukács and others (2005) compared trace, major, and minor element data from melt inclusions in plagioclase, ortho- and clinopyroxene, and quartz to each other, and also to phenocryst, lithic clast, whole rock, and matrix glass data. These various data show a complex crystallization history including the crystallization of an andesitic magma and its subsequent fractionation at depth, late-stage crystallization of accessory minerals that controls the trace element 3
composition of the melt, crustal contamination, and withdrawal of the evolved, rhyolitic
melt at shallower levels.
Trace elements can also be used to help determine the origin and crystallization
history of igneous rocks (Hanson, 1978). The distribution of trace elements provides
clues as to which igneous processes may have acted upon the sample (e.g. Berlo et al.,
2004; Norman et al., 2005). Berlo and others (2004) uses Li along with radioactive
isotope data from the 1980 Mount St. Helens eruption to show increased levels of 210Pb
and Li which suggests that the magma stopped rising through the conduit before the
eruption, leading to degassing of the magma at depth. Norman and others (2005) show
30-40% fractional crystallization from olivine tholeiite parental magmas based on melt
inclusions from lavas from two different Kilauea eruptions. Trace elements found in melt
inclusions can reconstruct the crystallization history from origin to eruption of a magma,
and can thus be used to constrain eruption styles of volcanoes (e.g. Lukács et al., 2005).
In this paper, I use the trace elements H+ (as a proxy for water), Li, Be, and B
along with major element geochemical data from melt inclusions in plagioclase
phenocrysts of one sample from the 1986 eruption of Augustine Volcano in Alaska to
study eruption dynamics of an arc volcano. These geochemical data show fractionation
and mingling trends that are important to understanding magmatic behavior prior to
eruption.
GEOLOGIC SETTING
Augustine Volcano is a subduction-related, Pleistocene stratovolcano that rises
~1200 m above sea level (e.g. Waythomas and Waitt, 1998; Roman et al, 2006). 4
Augustine forms its own island in the Cook Inlet of southwestern Alaska and belongs to the eastern Aleutian Arc near other volcanoes such as Iliamna, Redoubt, Spurr and
Katmai (Fig. 1). Augustine has erupted seven times in recorded history (1812, 1883,
1935, 1964, 1976, 1986, and 2006), in addition to many prehistoric eruptions recorded only by tephra deposits (e.g. Waythomas and Waitt, 1998; Roman et al, 2006).
Augustine’s typical recorded eruptions begin with an explosive phase that creates tall ash columns (<12 km) and pyroclastic flows (Waythomas and Waitt, 1998). This is followed by an effusive phase in which lava erupts and creates a new dome (Waythomas and Waitt, 1998; Roman et al., 2006). The most violent known eruption happened in
1883 when pyroclastic flows created a tsunami in Cook Inlet, damaging the shoreline communities (Yount et al., 1987).
The 1986 Eruption
According to Yount and others (1987), five weeks of increased seismic activity preceded the start of the 1986 eruption. This eruption progressed similar to other recorded eruptions, beginning explosively on March 27 and creating a continuous, ash- rich eruption column that averaged 3-4.6 km high. This column was punctuated by explosions that reached a maximum height of 12.2 km and deposited ash as far away as
Anchorage, AK (Fig. 2A and B; Yount et al., 1987; Roman et al., 2006). During this phase, hundreds of pyroclastic flows (Fig. 2D) raced down the North Slope, the largest flows reaching the north shore 5 km from the summit (Yount et al., 1987). The effusive phase of the 1986 eruption produced two episodes of dome growth (Fig 2C) during April and August, the first also included a steep lava flow near the summit (Fig. 3). These 5
164°E 180° 164°W 148° 132°
66°
Anchorage
ALASKA 62° Spurr
58°
54°
Redoubt 60°
Iliamna
COOK INLET
Augustine
Douglas
Afognak Island 58°N
Katmai Mageik 50 km Kodiak
156° 154° Modi ed from Coats, 1949. 152°W
Figure 1. Map of Cook Inlet and surrounding area showing Augustine in red and other volcanoes in the area. 6
Figure 2. Photos of Augustine during the 1986 eruption. A) Aerial photo of eruption column taken by S.J. Smith. B) Eruption column, photo taken by M.E. Yount. C) Photo of the lava dome taken by the USGS. D) Pyroclastic flow down the north slope, photo taken by M.E. Yount. (All photos taken from the Alaska Volcano Observatory website.) 7 27’30” Modi ed fromModi ed et al., Roman 2006. 100 200 500 400 600 300 900
800 700 0 Scale (km) Scale 1 153°20’ 2 22’30” 59°25’ KEY: deposits. See text for explanation. eruption. Notice the sample location is in ‘pp’ Figure 3. 1976 pyroclastic ow deposits 1986 lava dome and lava ow 1986 pumiceous pyroclastic ow deposits (’pp’) Sample LocationSample avalanche1883 debris and pyroclastic ow 1986 lithic pyroclastic ow deposits (’pl’) Map showing the deposits from 1986 8 episodes were accompanied by small bursts of ash and pyroclastic flows and an eruption column that never exceeded 3.7 km in height (Yount et al., 1987).
PETROGRAPHY
Pyroclastic flows erupted during the explosive phase are labeled ‘pp’ because they are more pumiceous than those erupted during the effusive phase, denoted ‘pl’
(lithic) (Fig. 3; Waitt and Begét, 1996 in Roman et al., 2006). ‘Pl’ deposits contain clasts that are dense and juvenile, with a small amount of non-juvenile material. ‘Pp’ deposits consist of low-density clasts, bread-crusted bombs, and fine-ash particles, along with some denser, lithic clasts (Fig. 3; Waitt and Begét, 1996 in Roman et al., 2006).
One sample (AV0208) from the 1986 eruption was selected because of the amount and quality (clear glass and no crystals, cracks, or channels) of melt inclusions based on thin section examination. The sample in this study is a grey, vesiculated andesitic pumice from the top of a ‘pp’ flow (Fig. 3). In thin section, ‘pp’ samples from this eruption are vesiculated with clear matrix glass and contain phenocrysts of, in descending order of abundance: plagioclase feldspar, clinopyroxene, and orthopyroxene, magnetite and ilmenite (Fig. 4A, C). There are minor amounts of olivine and even less of hornblende, which show extensive disequilibrium textures (Harris, 1994 in Roman et al.,
2006). This sample contains glomerocrysts of predominantly pyroxene and Fe-Ti oxides
(Fig.4C), zoned plagioclase (Fig. 5A, C), and plagioclase phenocrysts with sieve-textured cores (Fig. 5C). Figure 4. Sample in thin section. A) Photomicrograph under plain polarized light showing mineral assemblage and texture. The blue is stained epoxy (E) filling the vesicles to differentiate it from plagioclase. Abbreviations: opx - orthopyroxene; cpx - clinopyroxene; ox - oxide composed of ilmenite and magnetite; pl - plagioclase feldspar. B) Photomicrograph with the same field of view as A excpet under cross polars. C) SEM backscattered electron image of a glomerocryst composed of oxide and pyroxene. Abbreviations the same as in A. 9 Figure 5. Melt inclusions in plagioclase phenocrysts (pl). A) Phenocryst AV0208c_p8-3 mounted in resin and polished under plain polarized light. Melt inclusions (examples circled) are the dark squares or dots that follow the zoning. B) Phenocryst AV0208c_p6-9 mounted and polished in resin under reflected light. Melt inclusions are darker rectangles and the even darker circles inside are bubbles. C) SEM backscat- tered electron image of a plagioclase phenocryst. Notice the zoning in the phenocryst and the melt inclusions with their bubbles. The bright spots in some inclusions are oxides that crystallized after the inclusions was trapped in the phenocryst. Abbreviations: pl - plagioclase; E - epoxy filling the vesicles; ox - oxide composed of magnetite and ilmenite; mg - matrix glass. 10 11
Melt inclusions in this sample are predominantly found in plagioclase phenocrysts
(Fig. 5), although they can also be found in pyroxene phenocrysts. In some cases, melt
inclusions follow the zoning patterns in the plagioclase phenocrysts (Fig 5A).
MINERAL AND MELT GEOCHEMISTRY
Plagioclase phenocrysts were placed in resin on round slides; the slides were
ground down until melt inclusions emerged on the surface, and then polished. Electron
microprobe analyses were conducted with a Cameca SX-100 electron microprobe at the
American Museum of Natural History. Melt inclusions (n = 12) and their respective
plagioclase phenocrysts were analyzed for Na, Fe, K, Si, F, Al, Ti, Ca, Mg, Mn, P, Cl, S
using an accelerating voltage of 15 keV and a beam diameter of 5 µm for 30 s. The
elements Na, Fe, K, Si, and F were analyzed with a 2 nA beam current that was moved
over the sample for the duration of acquisition, and Al, Ti, Ca, Mg, Mn, P, Cl, and S with
a still 10 nA beam current. The same slides and melt inclusions were analyzed with a
Cameca ion probe at Woods Hole Oceanographic Institute at 10 keV with 0.5-1 nA beam
current, -90 volt offset, and 40 volt energy window.
Chemical analyses of the plagioclase phenocrysts are listed in Table 1, showing
that the anorthite content ranges from An47 to An71. The wide variation in An captures
either 1) core-rim differences, since phenocryst analyses were taken beside each melt
inclusion which means the distance from the core varies among the analyses and/or 2)
when the phenocryst crystallized. The silica content of the melt inclusions, normalized
on an anhydrous basis, ranges from 73.6-75.8 wt.% (Table 2). Figure 6 presents most
major and minor element Harker plots, which show normal trends in the oxides and TABLE 1. COMPOSITION OF PLAGIOCLASE PHENOCRYSTS Sample AV0208c.p3-3.1 AV0208c.p3-3.2 AV0208c.p4-5 AV0208c.p5-6 AV0208c.p6-1 AV0208c.p10-1 AV0208b.p3.1 AV0208b.p3.2 AV0208b.p10 AV0208b.p12.1 AV0208b.p12.2 Oxides in weight %
SiO2 56.22 54.93 56.02 56.36 54.91 57.02 53.82 50.03 56.03 54.39 52.35
Al2O3 28.16 28.66 28.25 27.59 28.57 27.47 29.61 31.16 27.75 28.92 30.25
TiO2 0.03 0.03 0.02 0.04 0.04 0.01 0.02 0.00 0.00 0.02 0.01 FeO 0.48 0.56 0.32 0.50 0.21 0.50 0.47 0.34 0.35 0.46 0.31 MgO 0.04 0.05 0.05 0.02 0.02 0.05 0.03 0.03 0.02 0.03 0.04 MnO 0.00 0.00 0.02 0.02 0.01 0.00 0.00 0.03 0.03 0.01 0.00 CaO 10.85 11.44 10.76 10.16 11.29 9.96 12.43 14.38 10.34 11.29 12.59
K2O 0.15 0.15 0.16 0.14 0.10 0.19 0.09 0.06 0.13 0.09 0.07
Na2O 5.43 5.00 5.12 5.56 5.05 5.98 4.34 3.22 5.45 5.05 4.35 TOTAL: 101.35 100.82 100.73 100.39 100.22 101.18 100.82 99.25 100.11 100.26 99.97
Cations based on 8 oxygens Si 2.50 2.46 2.50 2.53 2.47 2.54 2.42 2.30 2.52 2.45 2.38 Al 1.48 1.52 1.49 1.46 1.52 1.44 1.57 1.69 1.47 1.54 1.62 Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Fe(2+) 0.02 0.02 0.01 0.02 0.01 0.02 0.02 0.01 0.01 0.02 0.01 Mg 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Ca 0.52 0.55 0.52 0.49 0.54 0.48 0.60 0.71 0.50 0.55 0.61 K 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.00 0.01 0.01 0.00 Na 0.47 0.43 0.44 0.48 0.44 0.52 0.38 0.29 0.48 0.44 0.38 TOTAL: 5.00 5.00 4.98 4.99 4.99 5.00 4.99 5.00 4.99 5.00 5.01
XCa 0.52 0.55 0.53 0.50 0.55 0.47 0.61 0.71 0.51 0.55 0.61 XK 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.01 0.01 0.00 XNa 0.47 0.44 0.46 0.49 0.44 0.52 0.38 0.29 0.48 0.45 0.38 Note: Data collected using electron microprobe. Beam was positioned adjacent to, but not touching, the melt inclusion. 12 TABLE 2. MAJOR, MINOR, AND TRACE ELEMENT COMPOSITIONS OF MELT INCLUSIONS
AV0208c AV0208c AV0208c AV0208c AV0208c AV0208c AV0208c AV0208b AV0208b AV0208b AV0208b AV0208b Sample Name p2-11 p3-3 p3-3 p4-5 p5-6 p6-1 p10-1 p6 p6 p10 p12 p12 mi1 mi1 mi2 mi1 mi1 mi1 mi1 mi1 mi2 mi1 mi1 mi2 Major and minor elements*
SiO2 74.38 74.77 73.55 75.22 74.04 75.26 75.55 74.42 74.44 75.34 75.67 75.81
Al2O5 13.21 12.76 12.94 12.64 13.11 13.01 12.57 13.08 13.04 12.76 13.01 12.82 CaO 2.19 2.01 2.21 1.87 2.24 1.94 2.04 2.18 2.05 2.02 1.76 1.79 MgO 0.58 0.53 0.68 0.49 0.57 0.45 0.47 0.45 0.62 0.48 0.36 0.44 MnO 0.04 0.13 0.07 0.05 0.10 0.05 0.05 0.08 0.08 0.01 0.06 0.04 FeO 2.20 2.15 2.67 2.46 2.56 1.77 2.02 2.16 2.08 2.18 1.75 1.57
Na2O 4.47 4.29 4.09 4.34 4.04 4.45 4.38 4.41 4.41 4.36 4.54 4.47
K2O 2.08 2.19 2.27 2.07 2.12 2.26 2.10 2.22 2.15 2.14 2.11 2.13
TiO2 0.30 0.39 0.39 0.38 0.33 0.17 0.32 0.32 0.34 0.24 0.19 0.28
§ P2O5 745 483 634 529 468 838 495 544 381 597 291 320 Cl§ 4462 4977 7029 4081 6879 4755 3937 5261 5601 3614 3851 4128
§ SO2 268 441 190 235 405 478 422 502 237 428 271 405 F§ 0 2087 3465 0 1047 405 0 495 1657 0 887 1723 WBD# 2.52 4.08 4.28 3.84 3.98 4.40 3.04 3.84 5.05 4.15 4.82 4.83
Trace elements** H§§ 2.33 3.67 4.22 3.58 1.47 0.42 5.31 3.99 3.64 3.69 3.71 4.21 Li§ 20.21 23.02 29.15 24.78 14.74 16.97 20.44 12.79 15.30 21.07 17.97 19.62 Be§ 1.48 1.36 1.45 1.30 1.16 1.35 1.29 0.91 0.80 0.79 0.84 0.85 B§ 20.86 22.46 25.17 20.96 13.32 8.62 17.69 18.06 14.06 12.62 8.74 10.44 Note: Trace elements analyzed using ion probe. * Major and minor elements analyzed using electron microprobe. In weight percent. § In ppm. # WBD = Water by difference. It is found by subtracting the total amount of elements analyzed in one sample from 100. This assumes that the excess, if there is any, is mostly water, which is not always correct, but it can be a good approximation. ** Trace elements analyzed using in probe. §§ In weight percent. 13 14
13.30 2.50
13.20
2.00 13.10
13.00 1.50
12.90 (wt.%) 3
O CaO (wt.%) 2 1.00 Al₂O₃ (wt.%) 12.80Al
12.70 0.50
12.60
12.50 0.00 73.00 73.50 74.00 74.50 75.00 75.50 76.00 73.00 73.50 74.00 74.50 75.00 75.50 76.00 SiO₂ (wt.%) SiO₂ (wt.%)
0.80 0.14
0.70 0.12
0.60 0.10
0.50 0.08
0.40
0.06 MnO (wt.%) MgO (wt.%) 0.30 MnO (wt.%)
0.04 0.20
0.02 0.10
0.00 0.00 73.00 73.50 74.00 74.50 75.00 75.50 76.00 73.00 73.50 74.00 74.50 75.00 75.50 76.00 SiO₂ (wt.%) SiO₂ (wt.%)
3.00 0.45
0.40 2.50 0.35
2.00 0.30
0.25 1.50 (wt.%) 0.202 TiO₂ (wt.%) TiO₂ FeO (wt.%) 1.00 0.15
0.10 0.50 0.05
0.00 0.00 73.00 73.50 74.00 74.50 75.00 75.50 76.00 73.00 73.50 74.00 74.50 75.00 75.50 76.00 SiO₂ (wt.%) SiO₂ (wt.%)
Figure 6. Part 1, see the rest of the figure on the next page. 15
4.60 2.30
4.50 2.25
4.40 O 2 2.20 Na 4.30
Na₂O (wt.%) K₂O (wt.%) 2.15 2 4.20 K
2.10 4.10
4.00 2.05 73.00 73.50 74.00 74.50 75.00 75.50 76.00 73.00 73.50 74.00 74.50 75.00 75.50 76.00 SiO₂ (wt.%) SiO₂ (wt.%)
8000 600
7000 500
6000
400 5000
4000 300 2 Cl (ppm)
SO₂ (ppm) SO 3000 200
2000
100 1000
0 0 73.00 73.50 74.00 74.50 75.00 75.50 76.00 73.00 73.50 74.00 74.50 75.00 75.50 76.00 SiO₂ (wt.%) SiO₂ (wt.%)
Figure 6. Continued from previous page. Harker plots: major and minor elements versus silica. Major and minor elements analyzed with electron microprobe. 16 elements. The P2O5 data does not show any major trends and the F data are sporadic and thus unreliable, so neither were included.
This study includes two methods of water content analysis for each sample.
Hydrogen (H+) was collected by ion probe and is used here as a proxy for water content.
The water by difference (WBD) method subtracts the analytical total in weight percent from 100 and assumes whatever is left is water. Neither of these methods is exact because not all hydrogen in a sample is from water, and water is not the only phase that comprises the rest of the sample. Figure 7 “Water” compares the two different data sets; they are generally similar and show a generally horizontal trend, meaning the water content is relatively constant over the range of silica contents. These approximated water contents, mostly ranging from ~3-5 wt.%, are a little below the typical amounts (~4-6 wt.%) for silicic magmas, which suggests that the magma languished at shallow depths for a period of time (Stix et al., 2003).
The other three trace element plots in Figure 7—Li, Be, and B—show that the sample with ~73.5 wt.% SiO2 (circled) is outlying the rest of the data. Excluding this datum, Be and B trend negatively, whereas Li trends positively.
DISCUSSION
Augustine is an arc volcano, created by the subduction of the Pacific plate beneath the North American plate. Dewatering of the downgoing slab induces melting in the mantle wedge above, which generates the magma that feeds the volcano. Rather than magma creation, the data in this study speak more to the transport and storage of magma and magma chamber dynamics. However, other studies have used melt inclusions to H⁺ 35.00 Water WBD 6.00 30.00
5.00 25.00
4.00 20.00
3.00 Li (ppm) 15.00 2 HH₂O (wt.%) 2.00 10.00
1.00 5.00
0.00 0.00 73.00 73.50 74.00 74.50 75.00 75.50 76.00 73.00 73.50 74.00 74.50 75.00 75.50 76.00 SiO₂ (wt.%) SiO₂ (wt.%)
1.60 30.00
1.40 25.00 1.20
20.00 1.00
0.80 15.00 B (ppm) Be (ppm) 0.60 10.00 0.40
5.00 0.20
0.00 0.00 73.00 73.50 74.00 74.50 75.00 75.50 76.00 73.00 73.50 74.00 74.50 75.00 75.50 76.00 SiO₂ (wt.%) SiO₂ (wt.%)
Figure 7. Trace element Harker plots. Trace elements analyzed using ion probe. Water plot compares two different methods of estimating water content of melt inclusions. Red circled datum is same sample in each plot and is signaled as an outlier in these three plots. 17 18 infer composition and origin of parent magmas (e.g. Wade et al., 2006; Benjamin et al.,
2007).
Fractional crystallization is intimately involved with the transport and storage of the magma, and melt inclusions can be tracers of the geochemical evolution of the magma through its possibly punctuated ascent to Earth’s surface (Kent et al., 2007). The trends in major element geochemical data are consistent with differentiation of the magma (for example, MgO decreases as SiO2 increases; Fig. 6). This process happens through changes in temperature and pressure, which implies movement of the magma.
As the magma rises through the crust, pressure and temperature both decrease, and therefore different minerals form and their chemical compositions change.
The data in this study illustrate evidence for fractional crystallization in the magmas beneath Augustine in three ways. First, the wide range of An-content in the plagioclase phenocrysts (24% difference; Table 1) probably show more primitive and more evolved compositions and thus track part of the fractionation history of the magma.
The second is shown in the trends exhibited by the major, minor and trace element data in the melt inclusions. According to Webster (1999), as pressure decreases in a magma, Cl becomes less soluble in the melt and thus exsolves to the fluid phase, as shown by the decrease in Cl concentration with increasing SiO2 in Figure 6. On the other hand, the Mg shows a decreasing trend because Mg partitions into the crystal phase (Fig. 6). Finally,
Roman and others (2006) present data that show the silica content of the whole rock to be
56.7-63.2 wt.% and of the matrix glass to be 75.7-76.6 wt.%. The melt inclusions and matrix glass are much more rhyolitic than the whole rock data, as shown in the Total
Alkali-Silica diagram (Fig. 8; Le Bas et al., 1986). This reinforces the claim that the Comparison between matrix glass, Matrix glass (Roman et al., 2006) Whole rock (Roman et al., 2006) whole rock, and melt inclusion compositions Melt inclusions (this study) 16
14 Phonolite Trachyte 12 Tephri- phonolite Foidite
10 Phono- tephrite Trachyandesite Trachydacite Rhyolite 8 Basaltic Tephrite trachy- andesite 6 Basanite Trachy- Na₂O + K₂O (wt.%) Na₂O + basalt
4 Dacite
Basaltic 2 Picro- Basalt andesite Andesite basalt
0 40 44 48 52 56 60 64 68 72 76 80 84 SiO₂ (wt.%)
Figure 8. Total Alkali-Silica diagram based on Le Bas and others (1986). Whole rock data is much more mafic than the matrix glass and melt inclusions, which are similar. This is evidence for fractional crystallization of the magma before eruption since the melt is more evolved than the whole rock. 19 20 magma underwent extensive fractional crystallization before eruption, showing that the melt is more evolved than the magma as a whole.
Fractional crystallization is not the only process to act upon the magma stored beneath Augustine. Because of the nature of the ‘chambers’ where this magma is stored, magma mingling also influences the composition of the melt and magma. Instead of one large chamber, it is probable that magma is stored beneath Augustine in a series of interconnected dikes that source from depth (Roman et al., 2006). These interconnected dikes allow new pulses of magma to interact with older magma from previous eruptions, creating heterogeneous compositions from two subtly different magmas (Fig. 9).
This mingling can potentially be observed in the SO2 and Be data that show two distinct, generally negative trends (Fig. 10). Both trends have similar slopes, though at different amounts of each oxide or element. These plots could show two slightly different compositions of magma mingling together, one that is more primitive and less degassed than the other (higher SO2 and Be contents mean more primitive melt compositions). However, data from other elements and oxides do not readily show similar trends. Though the Al2O3, FeO, TiO2, and K2O data vary widely, it is possible to discern two distinct trends, but there is not a large difference among the actual quantities of each oxide in the inclusions. Alternatively, the highly diverse nature of the data could signify the compositional heterogeneity of Augustine magmas, as argued by Roman and others (2006). It is also possible that oxides with widely varying data or no easily discernable trends had at one time recorded two separate trends, but, through more integrated mixing of the magmas and/or diffusion of the elements throughout the melt, the trends have become less defined. For example, Li diffuses quickly through 21
Eruption after new pulse Pre-eruption of magma injected
KEY:
Residual 1976 magma
Intermediate, mingled magma
New pulse of mafic magma
Figure 9. Two-part figure showing the system of dikes beneath Augustine. The residual 1976 magma is stored pre-eruption in the dikes, and syn-eruption, the magma mingles with the new, more mafic pulse of magma in 1986. Modified from Roman and others (2006). 22
600
500
400
300 2
SO₂ (ppm) SO 200
100
0 73.00 73.50 74.00 74.50 75.00 75.50 76.00 SiO₂ (wt.%)
1.60
1.40
1.20
1.00
0.80 Be (ppm) 0.60
0.40
0.20
0.00 73.00 73.50 74.00 74.50 75.00 75.50 76.00 SiO₂ (wt.%)
Figure 10. Harker plots of SO₂ and Be vs. SiO₂ with an outlying data point removed and two different trends estimated. The fact that two distinct trends can be discerned in these plots signifies the mingling of two magmas of slightly different composition. 23
phenocrysts and melts (Kent et al., 2007). Therefore, it is possible that as the two
magmas were mingling, Li was exchanged more readily between the magmas, thus
erasing any sign of difference. Additionally, B shows widely ranging concentrations, so
it is possible that at one point there were two distinct B signals but those signals have
now been partially mixed and/or equilibrated. Those oxides for which two trends may be
discerned (i.e. Al2O3, FeO, TiO2, K2O) may at one time have also shown more distinct
trends, but diffusion between the mingling magmas may have begun to approach
equilibrium, thereby erasing any separate compositions and/or trends.
The final stage in the ‘life’ of a magma is its eruption. There are may factors that
influence the exact progression of an eruption, but trace element geochemical data in melt
inclusions do not speak readily to answering these questions. However, eruption can be
induced by a rapid decrease in pressure causing the volatiles, either in the fluid phase or
still dissolved in the magma, to become gases and burst forcefully from underground. As
the magma ascends, the pressure gradually decreases and reaches a level at which
volatiles are no longer stable as liquids and thus vaporize. The pressure then increases
rapidly inside the chamber or dike and the magma can no longer be contained under the
earth and therefore erupts onto the surface.
Chlorine favors partition to the fluid phase (Webster, 1992a, b in Stix et al., 2003)
and Webster (1999) claims that Cl encourages other volatiles to exsolve from the melt
phase to the fluid phase. My data reiterate the high-Cl contents found in Augustine
magmas, ranging ~3800-7000 ppm (e.g. Symonds, 1990). It is probable that, due to
volatiles and fluids in magmas and due to their importance in eruptions, Cl and its
dynamics within the magma have a significant influence on eruptions at Augustine. Melt 24 inclusions show only that Augustine is Cl-enriched, but not how exactly that influences eruption.
CONCLUSION
Major and trace element geochemical analyses of melt inclusions in plagioclase phenocrysts from one sample of the 1986 eruption of Augustine Volcano, Alaska display fractionation trends common of arc volcanoes. The two distinct trends of SO2 and Be indicate mingling between two magmas of similar composition. The mingling is likely due to the storage conditions of Augustine’s magma: a system of interconnected dikes below the volcano. This system allows new pulses of more primitive magma to mingle with residual, more evolved magma from previous eruptions. The interaction between these magmas is a probable cause of eruption among others (e.g. degassing).
It is important to understand magma dynamics below the volcano to more accurately predict eruptions. The data in this study add to a larger data collection and thus expand the knowledge of subsurface magma chamber dynamics at Augustine
Volcano.
ACKNOWLEDGEMENTS
First, I would like to express my utmost gratitude to the National Science
Foundation for funding the Research Experience for Undergraduates (REU) program, which allowed me to be involved in research over the summer. Along those same lines, I must thank the American Museum of Natural History (AMNH) and the Earth and
Planetary Sciences (EPS) and Astrophysics departments for hosting the REU program. 25
Second I wish to extend my fervent thanks to my advisors, Dr. Jim Webster (also
one of the REU program directors) at AMNH and Dr. Cam Davidson at Carleton College
(See Appendix) for all of their guidance and advice throughout the data collection and
analysis for, and writing of this project; to Beth Goldoff for all the hands-on tutorials in
the lab and days in the probe room; to Nanette Nicholson for taking care of everyday, yet
essential business (and for the excursions to the outside world); to other members of EPS
and Astro for the warm welcome, lunches, conversations, and weekly libations: Charles
Liu (the other REU program director), Charlie Mandeville, Joe Boesenberg, Denton Ebel,
and Jamie Newman; and to Jacqui, Kriss, Andy, Mike, Patrick, Sweta, Jacob, Monica,
and the other REU interns for the fascinating times around New York City.
Finally, many big thanks must go to the Carleton College Geology Department
for providing wonderful opportunities and support; all the other geology majors for the
comical camaraderie, priceless procrastination, and fabulous friendship; and to my family
and friends from all the varied facets of my life for their encouragement, love, and joy
throughout this process.
REFERENCES CITED
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Blundy, J., and Cashman, K., 2005, Rapid decompresion-driven crystallization recorded by melt inclusions from Mount St. Helens volcano: Geology, v. 33, no. 10, p. 793-796.
Coats, R. R., 1950, Volcanic Activity in the Aleutian Arc: U.S. Geological Survey Bulletin 974-B.
26
Gardner, J. E., Carey, S., Rutherford, M. J., and Sigurdsson, H., 1995, Petrologic diversity in Mount St. Helens dacites during the last 4,000 years: implications for magma mixing: Contributions to Mineralogy and Petrology, v. 119, p. 224-238.
Gurenko, A. A., Belousov, A. B., Trumbull, R. B., and Sobolev, A. V., 2005, Explosive basaltic volcanism of the Chikurachki Volcanoe (Kurile arc, Russia): Insights on pre-eruptive magmatic conditions and volatile budget revealed from phenocryst- hosted melt inclusions and groundmass glasses: Journal of Volcanology and Geothermal Research, v. 147, p. 203-232.
Hanson, G. N., 1978, The application of trace elements to the petrogenesis of igneous rocks of granitic composition: Earth and Planetary Science Letters, v. 38, p. 26- 43.
Kent, A. J. R., Blundy, J., Cashman, K. V., Cooper, K. M., Donnelly, C., Pallister, J. S., Reagan, M., Rowe, M. C., and Thornber, C. R., 2007, Vapor transfer prior to the October 2004 eruption of Mount St. Helens, Washington: Geology, v. 35, p. 231- 234.
LeBas, M. J., LeMaitre, R. W., Streckeisen, A., and Zanettin, B., 1986, A Chemical Classification of Volcanic Rocks Based on the Total Alkali-Silica Diagram: Journal of Petrology, v. 27, p. 745-750.
Lowenstern, J. B., 1995, Applications of silicate-melt inclusions to the study of magmatic volatiles, in Thompson, J. F. H., ed., Magmas, Fluids and Ore Deposits, Mineralogical Association of Canada Short Course, p. 71-99.
Lukács, R., Harangi, S., Ntaflos, T., and Mason, P. R. D., 2005, Silicate melt inclusions in the phenocrysts of the Szomolya Ignimbrite, Bükkalja Volcanic Field (Northern Hungary): Implications for magma chamber processes: Chemical Geology, v. 223, p. 46-67.
Norman, M., Garcia, M. O., and Pietruszka, A. J., 2005, Trace-element distribution coefficients for pyroxenes, plagioclase, and olivine in evolved tholeiites from the 1955 eruption of Kilauea Volcano, Hawai'i, and petrogenesis of differentiated rift- zone lavas: American Mineralogist, v. 90, p. 888-899.
Roman, D. C., Cashman, K. V., Gardner, C. A., Wallace, P. J., and Donovan, J. J., 2006, Storage and interaction of compositionally heterogeneous magmas from the 1986 eruption of Augustine Volcano, Alaska: Bulletin of Volcanology, v. 68, p. 240- 254.
Stix, J., Layne, G. D., and Williams, S. N., 2003, Mechanisms of degassing at Nevado del Ruiz volcano, Colombia: Journal o the Geological Society of London, v. 160, p. 507-521.
27
USGS, 2009, Alaska Volcano Observatory: Augustine description and details: http://www.avo.alaska.edu/volcanoes/volcinfo.php?volcname=Augustine (last accessed June 20, 2008).
Wade, J. A., Plank, T., Melson, W. G., Soto, G. J., and Hauri, E. H., 2006, The volatile content of magmas from Arenal volcano, Costa Rica: Journal of Volcanology and Geothermal Research, v. 157, p. 94-120.
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Webster, J. D., 1992a, Fluid-melt interactions involving Cl-rich granites: experimental study from 2 to 8 kbar: Geochimica et cosmochimica Acta, v. 56, p. 659-678.
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Witter, J. B., Kress, V. C., and Newhall, C. G., 2005, Volcán Popcatéptl, Mexico. Petrology, Magma Mixing, and Immediate Sources of Volatiles for the 1994- Present Eruption: Journal of Petrology, v. 46, no. 11, p. 2337-2366.
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APPENDIX. GEOLOGY-INSPIRED POEM
Introduction: I challenged myself to write this poem in a creative writing class. It is a pseudo- ode to the sounds of mineral names and to my mineralogy class (Winter 2007 with Bereket Haileab). I thought, since this comps project dabbles a bit in the realm of minerals, that I would include it here, with Cam Davidson’s go-ahead. I hope you enjoy the fluency, exuberance, and humor of these words and this poem.
Mineralogy
Lepidolite actinolite Magnetite malachite Siderite antigorite Sphalerite kyanite
A mineral is not a rock (Like a rock is not a stone). Mineral is individual, With personality of its own.
Titanite tremolite Epidote ilmenite Rhodonite dolomite Enstatite esperite
Corundum is colored in a dusty rouge. Fayalite is dainty, lively, and rare. Staurolite grows in a crystalline cross. Azurite allures into a blue affair.
Sanidine serpentine Travertine tourmaline Spessartine nepheline Almandine olivine
One hundred to learn in four short weeks, One hundred formulas, spirits, and streaks. One hundred times I owe an apology, But Reader, this—is Mineralogy.
Katie Marks 11 March 2009 Introduction to Creative Writing