PHENOCRYST OR XENOLITH? GROWING OLIVINE FROM HIGH-MG# BASALTIC ANDESITES FROM MT. BAKER

Mount Baker is an andesitic stratovolcano in the northern Cascade Range. The volcano’s last major eruption occurred around 6 Ka, with the more mafic magmatism occurring ~10 Ka and older. The Tarn Plateau lavas erupted at ~203 Ka and produced magma of high-Mg# (Mg#~70) basaltic andesite (HMBA). These HMBAs likely form during the subduction of oceanic plates into continental margins with the subducting slab melting and sending its hydrous material into the mantle creating partial melts from the garnet lherzolite source rock (Moore & DeBari, 2011), though exact formation of HMBAs and similar high magnesium andesites (HMA) is debated. This has led to questions about whether the olivine found in the Mount Baker magmas is a phenocryst or a xenocryst. In other words, is the source of the HMBA the product of direct mantle melting and subsequent fractional crystallization, or has the magma interacted with multiple magma chambers/or picked up country material? Mount Shasta, a similarly petrologic volcano in northern California, has HMAs with a controversial origin (Streck et al., 2007; Barr et al., 2007). These HMAs have both xenocrystic olivine and skeletal phenocrystic olivine, which have been interpreted as both the product of magma mixing and primitive slab melting. The varying nature of olivine and HMA petrogenesis in Mt. Shasta provides an analogy to Mt. Baker and highlights the need to confirm the origin of olivine in the Mt. Baker Tarn Plateau HMBA.

To explore the nature of the olivine found in the Tarn Plateau HMBA, we are conducting piston cylinder melting experiments. By varying water contents and crystallization temperatures, we are trying to determine if olivine can naturally crystallize in this lithology. The experiments are run at a uniform pressure of 5 kbar with varying amounts of water and crystallization temperatures. We start by bringing the sample to a temperature above the liquidus (1260°C) and then lowering the temperature to ideally just below the liquidus. Our preliminary results show that we have not successfully grown olivine. SEM-EDS analysis of the experiments shows orthopyroxene, clinopyroxene, plagioclase feldspar, and glass. Additional tests will be run at higher crystallization temperatures to see if we can avoid pyroxene and plagioclase crystallization.