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The magma mushes and pre-eruptive magma bodies of the Bishop Tuff

Guilherme A. R. Gualda , Calvin F. Miller

Abstract

The Bishop Tuff (California, USA) is a zoned rhyolite supereruption deposit, which has been the focus of recent controversy about the architecture of the magmatic system. The deposit can be subdivided into three main portions: early-erupted units (EBT), which only appear in the East and South, are pyroxene-free, and are the first erupted units; eastern and southern late-erupted units (LBT-East), which are pyroxene-bearing and overlie EBT; and northern late-erupted units (LBT-North), which are not in direct contact with the other portions, but are pyroxene-bearing and inferred to be synchronous with LBT-East. We present 73 paired whole-rock and matrix-glass compositions for individual pumice clasts from all three portions, with the goal of assessing magmatic processes and the architecture of the magmatic system. We are particularly interested in pressures of magma extraction from crystal-rich mush and pressures of pre-eruptive storage derived using rhyolite-MELTS. EBT and LBT-East whole-rock compositions are relatively evolved and strikingly similar to each other; they contrast with LBT-North compositions, which are less evolved (e.g., lower SiO2, higher Ba, Sr, LREE). LBT-East extraction pressures are the shallowest (~100-180 MPa), followed by EBT (~100-240 MPa), while LBT-North are deeper (~100-300 MPa). LBT-East and LBT-North cannot have tapped the same magma body; instead, EBT and LBT-East magmas are similar to each other. While magma storage pressures are similar for all magmas, the magma mushes from which they derived are distinct: EBT and LBT-East storage is contiguous with extraction, while LBT-North was extracted from a deeper and probably more extensive mush system.