Experimental study on the production of rhyolites from basaltic sources in the bimodal Snake River Plain-Yellowstone province

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Tholeiitic basalts from many tectonic environments are commonly associated with silica-rich eruptive products. This association is commonly bimodal, with a noticeable dearth of intermediate compositions between basalts and A-type rhyolites. Three processes are usually invoked to explain the origin of A-type rhyolites: (1) protracted fractional crystallization of parental basalts, (2) partial melting of a mafic source in the middle or upper crust, (3) silicate liquid immiscibility. However, evidence for large-scale separation of immiscible melts in volcanic setting has yet to be shown. The Snake River Plain (SRP) magmatic province, with a clear record of bimodal magmatism, and the ICDP Kimama and Kimberley drillcores offers a unique opportunity to test these three hypotheses proposed for the origin of A-type magmas.In this project, we combine investigations of natural core samples with experimental work to determine whether the origin of A-type rhyolites in SRPY is best explained by fractional crystallization, silicate liquid immiscibility or partial melting in the crust. A detailed mineralogical and petrographical investigation of evolved basalts is conducted to determine magma storage conditions (thermobarometry) and time scales of magmatic processes. Crystallization experiments simulating the formation of rhyolitic liquids from four possible basaltic sources are conducted at 300 and 600 MPa to identify the liquid line of descent of these basalts and possible conditions at which immiscibility may occur. Complementary partial melting experiments of gabbro and basalt are conducted. The results are used to evaluate which of these three processes best explains the major and trace element composition of SRP rhyolites. Preliminary results show that basalts have a complex history. Olivine phenocrysts often records replenishment events shortly prior to eruption (the determination time scales using compositional profiles is in progress). Crystallization experiments indicate that formation of SRP rhyolites by fractional crystallization is probably only realistic from evolved ferrobasaltic compositions. In such systems, melt immiscibility processes, both at 300 and 600 MPa, are expected to play a role in the geochemical signature of the magmas, as demonstrated experimentally for the first time. Partial melting experiments will be conducted in the next 18 months. This project will also add to the current understanding of phase equilibria and immiscibility in tholeiitic ferrobasalts. It will also put new constraints on melting reactions and the liquids produced by partial melting of dry mafic rocks.