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 i s a controversial topic. At the center of the debate is whether the di fferent reservoirs - continental crust, asthenosphere or sub-continent al lithosphere - contribute to the formation and evolution of these pr ovinces. The isotopic systematics of Re and Os offer significant new i nformation to constrain this debate. Because Os is compatible and Re i ncompatible during mantle melting, the different possible melt reservo irs will develop distinct Os isotopic signatures over time. Therefore, the source of magmas and the different possible contaminants should b e readily distinguishable using the Os isotopic system alone. We have focused our study on the Re and Os composition of the Columbia River b asalt Group (CRBG), which is one of the youngest and best studied cont inental 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 Amer ican Craton and Mesozoic accreted terrane were analyzed. The initial O s-187/Os-188 ranges from the lowest value of similar to 0.13 in the mo st primitive Imnaha Formation and increases to values as high as 0.4 i n flows from formations emplaced in the accreted terrane. Samples loca ted on the craton have initial Os-187/Os-188 isotopic ratios that vary from similar to 1 to similar to 3. The data conclusively demonstrate that continental crust, most likely mafic lower crust, played an impor tant role in the evolution of this flood basalt province. These data a lso show that the sub-continental lithospheric mantle (SCLM) was not s ignificantly involved in the formation or modification of these contin ental flood basalts. (C) 1998 Elsevier Science B.V.