Assimilation and mixing of crystal-bearing magmas in the Snake River Plain, NW United States: rheological timescales for magmatism of large silicic provinces

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The Snake River Plain (SRP) represents a unique opportunity to study the impact of a thermal anomaly on continental crust and the resulting dynamics in magma reservoirs. Deep, mantle-derived mafic melts ascend through the crust, assimilate adjacent country rock, mix and evolve. These processes, along with the composition of the primitive magma, govern the cooling and crystallisation dynamics and thus constitute a fundamental contribution to magmatic differentiation. The diversity of compositions along the SRP demonstrates the physico-chemical complexities and thus provides a keyhole to the evolution of these magmas. The textural evidence for magma mixing in core rhyolite and the stratigraphic alternation of sediments with igneous rocks in the SRP drill log indicate the occurrence of magma mixing of rhyolite (also crustally derived) and basalt melts. The continuation of a series of experiments is proposed to investigate, for the first time, the influence of crystals on magma mixing. The present proposal aims to (1) characterize the composition of recognized end-members (e.g. interstitial melt compositions, volatile and crystal content), (2) constrain the nature and timescales of mixing under contrasting crystal and/or volatile contents, and (3) compare the results to SRP units, especially those found at the interface between contrasting end-members (basalt-rhyolite; basalt-sediment), such as at the Kamama and Mountain Home drillholes.