Insight Into the Evolving Composition of Augustine
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1 INSIGHT INTO THE EVOLVING COMPOSITION OF AUGUSTINE VOLCANO’S SOURCE MAGMA FROM A LOW-K DACITE A Thesis Presented to the Honors Tutorial College, Ohio University In Partial Fulfillment of the Requirements for Graduation from the Honors Tutorial College with the degree of Bachelor of Science in Geological Sciences By Christian Thomas 2018 Dedicated to Robert J. Thomas 1965-2017 Table of Contents: Introduction 1 Background 3 Eruptive History 4 Methods 9 Sample acquisition and preparation 9 Microprobe analysis and modeling processes 11 Data and Results 14 Mineralogical relationships 18 Model Results 21 Fe-Ti oxide thermometry 21 Amphibole barometry 22 Plagioclase-amphibole thermobarometry 25 Plagioclase-melt hygrometry 25 Suitability of models vs. potential error 27 Discussion 29 Interpretation of Model Results 30 Patterns and strength of data-trend fit 30 Implications 32 Model data and petrologic comparison 32 Possibilities for unit relations 36 Implications for evolution 37 Conclusions 48 References 50 Appendices 57 1 Introduction: Augustine Volcano is a volcanic edifice in southern Alaska that has erupted intermittently from the Pleistocene epoch to as recently as 2006 (Waitt and Begét, 2009; Coombs and Vasquez, 2014). The 1.25 km-tall volcanic cone is part of the Aleutian arc, and is exposed as a 90 km2 island in Cook Inlet, shown in Figure 1. This location has posed problems in the recent past, as the 2006 eruption forced air and sea traffic to be rerouted or closed entirely (Neal et al., 2010). Given that Cook Inlet is host to significant oil industry activity and all sea shipping routes to Anchorage and ash from Augustine is capable of disrupting significant volumes of air traffic, knowledge of how Augustine Volcano could potentially erupt in the future will be beneficial for the physical safety and economic security of the local population and air traffic in the area (Blong, 1984; Neal et al., 2010). The Augustine volcanic center has been active for at least 25 ka, and 238U-230Th ages for zircons found in dioritic inclusions from the 2006 eruption indicate cooling within a pre-Augustine magmatic region at ~200 ka (Coombs and Vasquez, 2014). The ideal data set for petrologically assessing a long-lived system like this would contain an uninterrupted sequence of rock units, allowing us to observe the volcano’s changing compositional and eruptive dynamics through time. Unfortunately, since large volcanic eruptions only occur intermittently at Augustine, volcanic rocks like tephras and pumices are often poorly indurated, and the island has been subject to many environmental factors detrimental to preservation (e.g., glacial and volcanic erosion; mass wasting). Accordingly, significant gaps exist in the geologic record of Augustine’s eruptions. These time gaps typically appear as unconformities, usually as pyroclastic flow deposits or landslide surfaces in outcrop (Wallace et al., 2013; Waitt and Begét, 2009). Moreover, these gaps may correspond to 2 Figure 1: Location of Augustine Volcano within Alaska. Note the position of Anchorage NE of Augustine. Figure after Nadeau et al. (2015). actual gaps in eruptive activity, to units lost to the aforementioned processes, or both. The resulting limited temporal resolution in the rock record is not ideal, but the units discovered and described on Augustine Island still serve as a base for the elucidation of the volcano’s evolution. Research to describe and characterize the presently-exposed stratigraphy is valuable for creating a model of Augustine’s past and future activity. Small-to-moderately-sized eruptions from the past ~2 ka have been extensively studied (Waitt and Begét, 2009; Tappen et al., 2009; Wallace et al., 2010; Webster et al., 2010; Coombs and Vasquez, 2014), as have the oldest (by radiometric age dating) known volcanic units, some of which were generated by significantly larger and more explosive eruptions than their modern counterparts (Coombs and Vasquez, 2014; Nadeau et al., 2016, Nadeau et al. 2015, Zimmer et al., 2010). However, units from the period ~26 to 2.1 ka 3 have not been the subject of in-depth research, with the exception of the high-P2O5 dacite (Nadeau et al., 2016). The corresponding research gap has made interpretation of Augustine’s compositional evolution difficult, as much of the volcano’s eruptive history is not known in detail. Although the only eruption that produced particularly voluminous deposits was the ~26 ka rhyolite, material related genetically to this rhyolite was brought to the surface in the 2006 eruption (Coombs and Vasquez, 2014). This lends credence to the possibility that modern eruptions can still draw from ancient magma sources, and more explosive eruptions producing large volumes of ejecta may be possible in the future. Therefore, this project investigated the petrology of a 6.81 (+/- 0.03) ka dacite from Augustine Volcano, denoted as “low K2O dacite” (LKD) in the stratigraphy produced by Wallace et al. (2013). Since this unit’s position in the Augustine stratigraphy is directly above other pyroclastic deposits that have been recently and thoroughly examined, and which correlate to more dangerous eruptions, studying the low-K dacite with the context of earlier eruptive units will advance the scientific community’s understanding of Augustine’s eruptive history and magmatic evolution. Background: The first eruption of Augustine to be observed and described in a scientific manner as it took place occurred in 1883 (Waitt and Begét, 2009); since then, events have been increasingly thoroughly described as they occurred, due at least partially to advances in observational technology. Eruptive events in Augustine’s history prior to 1883 were long known only by their effects on the rock record, including eruptive deposits and unconformities. However, in recent years, work on prehistoric eruptions has expanded our 4 knowledge of Augustine’s history to deposits approximately 2.1 ka in age (Tephra “G”, described in Tappen et al., 2009). Radiocarbon dates for the low-K dacite indicate an age of approximately 6.8 ka, with the next youngest (unnamed) unit at 6.4 ka, and the remaining four known units between the low-K dacite and Tephra “G” are undated (Wallace et al., 2013). As a result, a span of ~4.5 ka remains effectively uncharacterized, and prior to this study, only one Augustine unit between the ages of 26 and 2.1 ka had been researched in-depth. Studying the low-K dacite in detail is a step toward closing this gap. Eruptive History: The oldest materials known to have been erupted from Augustine are coeval rhyolite and basalt deposits dated at ~26 ka (Waitt and Begét, 2009; Wallace et al., 2013). The rhyolite occurs above the basalt, except for a 20 cm interval where the two are interbedded (Waitt and Begét, 2009). Waitt and Begét (2009) hypothesized that the rhyolite and basalt could have been erupted simultaneously from a peak vent and a submarine flank vent, respectively. The rhyolite has variable exposure on Augustine Island. The irregular distribution of pyroclastic material around Augustine means that outcrops containing material originating from the same event and of similar compositions can have very different physical characteristics. As such, it is likely that the many (occasionally conflicting) reports of the basal rhyolite on Augustine Island describe the same unit(s) (unpublished pers. comm.). For example, Waitt and Begét (2009) described the rhyolite as a 10 m thick white 5 Tephra Age (y.a.) Juvenile clast types (D = Thickness Whole-rock Name dominant type) (m) SiO2 (wt%) Unnamed Unknown WHT pumice (D); rare MG 0.6 66.0, 66.2 microvesicular pumice Unnamed 6430 ± 30 LG microvesicular pumice 0.7 64.5 (D), WHT pumice Low K2O 6810 ± 30 Bone-WHT friable pumice (D) 0.3-1.5 66.5-66.9 dacite High P2O5 Unknown LG-MG pumice (D) 0.5-1.6 61.9, 62.7, dacite 62.9, 63.2 Rhyolite ~26,000 WHT-CRM pumice (D), LG 2-6 72.0-73.8 finely vesicular pumice, CRM/LG banded pumice Table 1: Excerpt of the Augustine tephrostratigraphy column published in Wallace et al. (2013) showing the units immediately above and below the low-K dacite (the subject of this work). pumice with airfall and pyroclastic flow components. More recent work using other exposures of the rhyolite gives a thickness of 2-6 m for the rhyolite, which is more precisely described as consisting of two light-toned pumices that occur both separately and as alternating bands (Wallace et al., 2013). A 238U-230Th age of 26 ka from zircons included in the rhyolite was interpreted as the crystallization age of the unit by Coombs and Vazquez (2012). Nadeau et al. (2015) did more detailed work on the rhyolite using samples from new exposures generated by landslides associated with the 2006 eruption. That work noted a unit thickness closer to 30 m and identified multiple subtypes of the rhyolite, designating them the white, yellow, and flow-banded pumices. The identifiable phenocryst assemblage comprised plagioclase, orthopyroxene, quartz, Fe-Ti oxides, and amphiboles (Nadeau et 6 al., 2015). Melt inclusions were common in all phenocryst types. Oxide geothermobarometry produced temperatures of 800-830 ºC and 759-887 ºC by the QUILF and Ghiorso and Evans (2008) programs (Andersen et al., 1993). The Holland and Blundy (1994) thermometer returned values of 845 ºC and 137 MPa for the white pumice and 858 ºC and 192 MPa for the flow-banded pumice (Nadeau et al., 2015). The unit overlying the rhyolite in the Wallace et al. (2013) tephrostratigraphy is described as a high-P2O5 dacite (hereafter “high-P dacite”). That work described the high- P dacite as a light- to medium-gray pumice of 0.5-1.6 m thickness containing multiple populations of foreign lithic fragments, many of which carry an orange stain (Table 1).