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Using Major, Minor, and Trace Element Geochemistry of Melt Inclusions to Study 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
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