Primitive lead in deep crustal xenoliths from the Snake River Plain, Idaho

Abstract

The petrology and geochemistry of deep crustal rocks are poorly understood due to the inaccessibility of such materials. Most inferences of the lower crust are based on studies of ancient high-grade metamorphic terranes now exposed after a history of deep burial. However, direct sampling of deep crustal rocks is feasible when they are carried to the surface as accidental fragments (or xenoliths) in lava flows. Ferro-basalt to ferrolatite lavas from Craters of the Moon lava field (COM) in the Snake River Plain, southern Idaho are characterised by variable and high ⁸⁷Sr/⁸⁶Sr ratios (0.708–0.712) interpreted as reflecting variable degrees of crustal contamination^1,2. Some of these lavas contain xenoliths of crustal derivation (silicic charnockites and pyroxene granulites) that are found nowhere in the vicinity in surface outcrops. I present here initial isotopic and trace element results for a suite of these crustal xenoliths. As part of a detailed Pb isotopic study of the lavas (in preparation), three granulites were analysed to test the contamination hypothesis. The results indicate an affinity between the xenoliths and many exposed granulite-facies metamorphic terranes, and the unusually primitive lead isotopic ratios recorded establish the presence of early Archean crust beneath south-central Idaho.