Geochemical and isotopic study of peridotite-bearing lavas in eastern California.

Abstract

Isotopic and geochemical variations in basalts have commonly been used to map out and characterize mantle source regions in the western United States. It has been suggested that some of this variability is a product of cryptic crustal contamination and not mantle heterogeneity. To test this hypothesis, mantle xenolithic lavas, which presumably travel through the crust too quickly to be contaminated, were studied in detail. Cones from two eastern California alkali basaltic fields, Big Pine in Owens Valley and Cima in the Mojave Desert, were chosen for study. In the two study areas, there is little geochemical and isotopic variation within individual cones, but there is significant variation between cones. Lavas from a peridotite-bearing Waucoba cone of the Big Pine field have 46-48 wt% SiO sub 2 and 9-9.5 wt% MgO. An adjacent nonxenolithic cone has much higher SiO sub 2 51 wt% and lower MgO 5.5-7 wt%. Both cones have indistinguishable super 87 Sr/ super 86 Sr ratios of 0.7059-0.7060 and epsilon sub Nd values of -2.4 to -1.6. The nonxenolithic cone can be related to the peridotite-bearing cone by 4 wt% fractionation of olivine and 14 wt% addition of granodiorite. The Cima lavas have 48-49 wt% SiO sub 2 , 5-6 wt% MgO, and abundant peridotite and gabbro xenoliths. The Mgof these lavas are too low to be in equilibrium with mantle olivine in spite of the abundance of peridotites. The low Mgcould be explained by high pressure fractionation before picking up the xenoliths. These lavas have constant but more depleted super 87 Sr/ super 86 Sr = 0.7029; epsilon sub Nd = 8.8 to 9.7 isotopic values. Lavas from both areas show minor evidence of crustal contamination in the form of sparse quartz xenocrysts. Cryptic crustal contamination did not play a role in the chemical characteristics of any of the lavas studied, and may be limited to nonxenolithic lavas.

30 samples from this reference