Geochemistry and Petrology of Alkaline Magmas from Mt. Waesche, Antarctica

High-precision Ar-Ar dates show that Mt. Waesche, a composite volcano in the West Antarctic Rift zone, routinely erupts during interglacial periods. To understand the feedback between ice sheet thinning and volcanic eruption at Mt. Waesche, we are documenting the petrology of the magmas to establish how the magmatic plumbing system changes through time. We present new whole rock compositions, mineral modes and compositions for 16 magmas that erupted from Mt. Waesche between 425.9 ka and 129.1 ka. Whole rock compositions are enriched in Na2O + K2O and span a continuous range in composition from basalts to tephriphonolites. Mineral modes, determined by point count, reveal that the samples contain between 17 to 27% phenocrysts. Total crystallinity (phenocrysts + microphenocrysts) is ≤41% for all samples. The alkaline magmas are multiply saturated with plagioclase + clinopyroxene + oxides ± olivine ± orthopyroxene. A subset of samples also contained inclusions of xenocrystic or antecrystic material. Inclusions are typically rich in plagioclase and olivine, and occasionally contain clinopyroxene. We analyzed minerals in trachybasalts and a tephriphonolite with energy dispersive spectroscopy using the new JEOL FEI SEM at New Mexico Tech. In the trachybasalts, plagioclase, clinopyroxene and olivine span a range of compositions from An46-84, Wo32-48En35-51Fs8-32, Fo67-80, respectively. In the tephriphonolites, anorthoclase is the stable feldspar; anorthoclase, clinopyroxene and olivine span a range of compositions from An10Ab74Or16, Wo45-47En29-45Fs11-25, Fo70-85, respectively. Spinels in all samples are titanomagnetites. We incorporated the clinopyroxene and whole rock compositions into the thermobarometer of Putirka et al. (2008) to estimate the pre-eruptive storage conditions at the time of clinopyroxene crystallization. Clinopyroxenes in the trachybasalts record hotter temperatures (1135±14°C (±1sigma) to 1165±9°C) and deeper pressures (5.7 ±0.9 to 6.4 ±1.4 kb) than the tephriphonolite (1005 ± 13°C; 3.3±1.7kb). The patterns of temperatures and pressures recorded by clinopyroxenes in all samples from Mt. Waesche resemble an adiabat, which suggests that crystallization could have occurred during ascent.