Crust-mantle interaction in large igneous provinces: Implications from the Re-Os isotope systematics of the Columbia River flood basalts

The source and evolution of magmas that form large igneous provinces is a controversial topic. At the center of the debate is whether the different reservoirs - continental crust, asthenosphere or sub-continental lithosphere - contribute to the formation and evolution of these provinces. The isotopic systematics of Re and Os offer significant new information to constrain this debate. Because Os is compatible and Re incompatible during mantle melting, the different possible melt reservoirs will develop distinct Os isotopic signatures over time. Therefore, the source of magmas and the different possible contaminants should be readily distinguishable using the Os isotopic system alone. We have focused our study on the Re and Os composition of the Columbia River basalt Group (CRBG), which is one of the youngest and best studied continental flood basalts provinces in the world. Samples from throughout the well documented stratigraphic column, that represent primitive and contaminated magmas, as well as samples emplaced in stable North American Craton and Mesozoic accreted terrane were analyzed. The initial ¹⁸⁷Os/¹⁸⁸Os ranges from the lowest value of ∼ 0.13 in the most primitive Imnaha Formation and increases to values as high as 0.4 in flows from formations emplaced in the accreted terrane. Samples located on the craton have initial ¹⁸⁷Os/¹⁸⁸Os isotopic ratios that vary from ∼ 1 to ∼ 3. The data conclusively demonstrate that continental crust, most likely mafic lower crust, played an important role in the evolution of this flood basalt province. These data also show that the sub-continental lithospheric mantle (SCLM) was not significantly involved in the formation or modification of these continental flood basalts.