Mafic volcanism on continents provides a critical window to a large portion of the mantle not sampled by plate-boundary magmatism, and is key to deciphering the mantle melting processes of all nonoceanic magmatic provinces. Much of the Earth?s surface is made from this type of continental magmatism. The plume model, which is commonly invoked to explain such magmatism, is increasingly coming under attack as features inconsistent with such an origin are discovered. The Columbia River Plateau (CRP) / Snake River Plain (SRP) large igneous province (LIP) has numerous features that bring the plume model into question. This project is designed to test the hypothesis of whether CRBG-SRP magmatism is entirely plume-related or is the result of multiple processes, including decompression anhydrous partial melting of the upper mantle, and regionally controlled flux melting attributable to proximity to the continental margin where the mantle has been hydrated by subduction.
This three-year research project will involve several key analytical areas: (i) 40Ar/39Ar dating of selected samples from the CRP and SRP suites to establish a chronologic framework for follow-up geochemical work, and for tectonic interpretations; (ii) petrographic and microprobe analysis of thin sections to identify samples best suited for major and trace element modeling to determine the degrees and depth of melting while also being mindful of any fractional crystallization effects; (iii) identify a subset of samples from 'ii' for measurements by Fourier transform infrared (FTIR) spectroscopy, and secondary ionization mass spectrometry (SIMS) of the primary melt volatile concentrations for H2O, CO2, S, F, and Cl in olivine-hosted melt inclusions; (iv) measurement of major elements by electron microprobe and trace elements by laser ablation ICP-MS on the same melt inclusions as those characterized for volatiles to identify the compositions of the most primitive magmas preserved in any form within the CRP/SRP LIP, and (v) measurement of a complete set of Nd, Sr, Pb and Hf isotopic compositions for 40 samples pooled from both subprovinces to assess any relationships there might be with eruption volumes and thickness of the lithospheric mantle. This work will dovetail nicely with EarthScope-sponsored geophysical studies of continental lithosphericevolution in the northwestern U.S. (USArray and the Plate Boundary Observatory), and also with plans currently on the drawing board to drill the SRP under the auspices of the International Continental Scientific Drilling Program (ICDP).
