USING CHEMICAL ZONING IN PLAGIOCLASE AND PYROXENE TO UNDERSTAND MAGMATIC PROCESSES AND CHANGES IN ERUPTIVE BEHAVIOR AT AUGUSTINE VOLCANO, ALASKA

Augustine Volcano is a frequently active stratovolcano in the Alaska-Aleutian Arc. Over the past two centuries, Augustine has been characterized by lower explosivity eruptions. The geologic record also preserves evidence of large explosive eruptions, the most recent of which produced the 400 years B.P. “Tephra B” pumice fall deposit. We examined Tephra B at ten different field locations along the southern and western margins of the volcano, recording an average thickness of 5 - 30 cm. Normal grading and reverse grading were both observed. Therefore, to ensure full coverage of compositional variation we sampled from the early, middle, and late parts of the eruption sequence. Due to the lack of historical observations of large explosive eruptions, it is difficult to compare the conditions leading to this type of eruption. We seek to address this issue by using mineral and melt compositions to characterize the storage conditions and pre-eruptive processes leading to the Tephra B eruption. Here, we present a petrological and geochemical analysis of Tephra B, including major, minor, and trace element variations in matrix glasses and core-to-rim variation in individual plagioclase and pyroxene crystals collected by EPMA. Previous studies established that plagioclase and pyroxene can remain stable over a broad range of magmatic conditions, but their compositions are sensitive to key volcanic parameters including temperature, depth of storage, magma composition, and water content. This study will determine whether different crystal populations in Tephra B indicate the mixing of chemically distinct magma components. In addition, different mineral phases with similar zoning patterns are hypothesized to record the same history of magma storage and crystallization. As such we will apply a variety of thermobarometers and hygrometers to reconstruct the magmas’ conditions and ascent paths.