Melting and storage conditions of rhyolites in the Snake River Plain Province , USA

The Snake River Plain-Yellowstone province preserves a unique record of bimodal magmatism (basalt-rhyolite) resulting from basaltic magma underplating and subsequent crustal recycling. The proposed project focuses on the generation and storage of rhyolites in the Snake River Plain (SRP) and is designed to clarify the “crustal recycling” processes occurring at depth as a result of basaltic magma underplating. A variety of generation models are currently proposed (fractionation, assimilation, partial melting) but they are not tested experimentally and, as a consequence tend to remain qualitative. Starting from one end-member hypothesis and assuming that rhyolites are formed from partial melting of crustal material, high pressure experiments will be conducted to determine the geochemical signature (major and trace elements) of partial melts produced from typical protoliths. Two main source types which are under debate will be tested: hydrothermally altered rhyolites and mafic basaltic rocks. The experimental results and the composition of the experimental melts will be compared with the composition of glasses in natural samples from the ICDP cores and from selected eruptive units covering the 16 Ma activity of the SRP. The geochemical compositions of the natural glasses will also be used to apply a new barometer recently calibrated by the proponents. Selected samples from a few eruptive units (e.g., Kimberley and Sugar City core) will be used to check if large eruptions result from different magma batches stored at various P-T conditions. The results will contribute to understand the nature of the source rocks of the rhyolites, the conditions of melting and of magma storage prior to eruption (depth, temperature, volatile contents), and the evolution of magma storage conditions in space and time.