EVIDENCE OF AN ASTHENOSPHERIC MAGMA SOURCE OF THE CIMA VOLCANIC FIELD, MOJAVE DESERT, CALIFORNIA, USING U-SERIES ISOTOPE DATA

The Cima Volcanic Field, located in the eastern Mojave Desert, California, has undergone two distinct eruptive periods ranging from 7.6-3.0 Ma and <1.0 Ma. The field is located near the Cenozoic continental margin in which thermal and tectonic activity here would have greatly affected the lithosphere. The more than 50 cinder cones are primarily basaltic in composition, and contain both mantle and crustal xenoliths. We have completed major and trace element analyses, as well as U-series isotopic data on samples from three lava flows, including the two youngest lava flows. The ranges of some of these major and trace element analyses include values of MgO (0.17-0.18), Fe₂O₃ (10.86-11.64), Ni (105-137), Cr (102-148), Sr (609-655), Zr (334-369), and Rb (40-44). The major and trace element compositions of these samples suggest an ocean island basalt (OIB) type source. In addition, certain trace element ratios, in particular Zr/Nb (<10), Ba/La (8.14-8.27), and La/Yb (15.7-16.2), for the three youngest samples are also consistent with the ranges for an alkali OIB source. Other authors have postulated a Pacific MORB source which rose during the transition from convergent to transform plate margin.

The U-series data provide an insight to magma generation and source region for these samples, in that Th-U fractionation will occur in the presence of garnet, creating ²³⁰Th enrichment, indicating a deep mantle source. A lithospheric source would create a ²³⁰Th/²³⁸U ratio that is at equilibrium. Our U-series data shows an enrichment of ²³⁰Th over ²³⁸U (²³⁰Th/²³⁸U=1.21-1.25), which is similar to data from other regions (e.g. Pinacate Volcanic Field, Southern Rio Grande Rift) characterized by deep asthenospheric sources. Coupling this with the major and trace element data, we can postulate a possible garnet peridotite magma source for these flows, with little or no lithospheric involvement; the lithosphere being extensively thinned, or completely pinched out due to regional extension during the Miocene.