Assessing Crustal Assimilation In The Petrogenesis Of Columbia River Basalt: Plagioclase 87Sr/86Sr Stratigraphy

Washington State University, Pullman WA

Investigators

Abstract

Wolff EAR-0125986 Assessing the relative contributions of enriched mantle and continental crust to continental flood basalt lavas is a long-standing problem, because essentially the same whole-rock geochemical data can be used to argue for either case. We propose a test using the Columbia River Basalt Group, the best-studied flood basalt province in the world. We shall use laser ablation sampling in combination with multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) to establish whether or not plagioclase phenocrysts in CRBG lavas exhibit zoning in 87 Sr/ 86 Sr. Because plagioclase crystallizes from basaltic liquids at crustal pressures, this will establish whether or not the magmas experienced changes in Sr isotope ratio during residence in (or transport through) the crust. While laser ablation MC-ICP-MS for in-situ isotope ratio analysis is not yet a fully developed technique, Sr in plagioclase has been demonstrated as feasible, and presents relatively few analytical challenges. This research has the potential to place unequivocal spatial constaints on the depth at which lithosperic contamination has affected the compositions of the Columbia River lavas. The results will affect our understanding of mantle plume-lithosphere interactions during genesis of this flood basalt province.