16-47 Melting Mount Baker: Growing Olivine from High-Mg# Basaltic Andesites in Piston Cylinder Melting Experiments

Session: From Thin Section to Outcrop: Exploration of Undergraduate Research (Posters)

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Abstract:

Mount Baker is an andesitic stratovolcano in the northern Cascade volcanic arc. Lavas from the Tarn Plateau, a relatively understudied area on the volcano’s south flank, erupted ~203 Ka and produced high-Mg# (Mg#~70) basaltic andesites (HMBA) rich in olivine. These HMBAs likely form due to partial melting of garnet lherzolite mantle source rock, influenced by hydrous material from subducting oceanic plates (Moore & DeBari, 2011). Current literature debates the formation of HMBAs and high magnesium andesites (HMAs), leading to questions about the Tarn Plateau olivine. Similarly controversial olivine exists in Mount Shasta, from the southern Cascades (Streck et al., 2007; Barr et al., 2007). The HMAs at Mount Shasta contain both xenocrystic and skeletal phenocrystic olivine from magma mixing and primitive slab melting (Streck et al., 2007). The complex nature of HMA petrogenesis at Mt. Shasta provides an analogue to Mt. Baker, highlighting the need to investigate whether the olivine here is a phenocryst and the result of direct mantle melting or a xenocryst indicating interaction with multiple magma chambers and/or country rock. 

To explore this, we are conducting piston-cylinder melting experiments designed to replicate magma chamber conditions with varying water content, pressure, and temperature. The setup for our experimental runs includes encapsulating powdered rock in platinum tubing and a nickel holder, with a fired pyrophyllite cup between MgO rods and enclosed in graphite, glass, and halite sleeves. Samples are heated above the olivine liquidus (~1260°C) at ~5 kbar, then cooled just below the liquidus for a 24-hour dwell, followed by rapid quenching. This procedure aims to produce a glassy matrix with olivine phenocrysts.

However,  the complexity of the system leads to frequent experiment failures, requiring optimization of assembly materials and salt sleeve production and careful precision of the W-Re thermocouple and associated Eurotherm system. Recent runs have not yet yielded olivine; instead, we have observed the crystallization of orthopyroxene, clinopyroxene, and plagioclase feldspar. To address this, we are incrementally increasing dwell temperatures to refine P-T conditions of olivine stability. A recent successful experimental run from higher crystallization temperatures is currently in preparation for SEM-EDS analysis. These results will help constrain the P-T-H₂O parameters of the HMBA formation at Mount Baker, providing important insight into mantle melting and magmatic evolution in the northern Cascades.

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