Lithosphere versus asthenosphere mantle sources at the Big Pine Volcanic Field, California

作者：Gazel Esteban^*; Plank Terry; Forsyth Donald W; Bendersky Claire; Lee Cin Ty A; Hauri Erik H

来源：Geochemistry Geophysics Geosystems, 2012, 13(6): Q0AK06.

DOI：10.1029/2012GC004060

• Basin and Range
• lithosphere-asthenosphere boundary
• melt inclusions
• shear wave tomography

Here we report the first measurements of the H2O content of magmas and mantle xenoliths from the Big Pine Volcanic Field (BPVF), California, in order to constrain the melting process in the mantle, and the role of asthenospheric and lithospheric sources in this westernmost region of the Basin and Range Province, western USA. Melt inclusions trapped in primitive olivines (Fo(82-90)) record surprisingly high H2O contents (1.5 to 3.0 wt.%), while lithospheric mantle xenoliths record low H2O concentrations (whole rock %26lt;75 ppm). Estimates of the oxidation state of BPVF magmas, based on V partitioning in olivine, are also high (FMQ + 1.0 to + 1.5). Pressures and temperatures of equilibration of the BPVF melts indicate a shift over time, from higher melting temperatures (similar to 1320 degrees C) and pressures (similar to 2 GPa) for magmas that are %26gt;500 ka, to cooler (similar to 1220 degrees C) and shallower melting (similar to 1 GPa) conditions in younger magmas. The estimated depth of melting correlates strongly with some trace element ratios in the magmas (e. g., Ce/Pb, Ba/La), with deeper melts having values closer to upper mantle asthenosphere values, and shallower melts having values more typical of subduction zone magmas. This geochemical stratification is consistent with seismic observations of a shallow lithosphere-asthenosphere boundary (similar to 55 km depth). Combined trace element and cryoscopic melting models yield self-consistent estimates for the degree of melting (similar to 5%) and source H2O concentration (similar to 1000 ppm). We suggest two possible geodynamic models to explain small-scale convection necessary for magma generation. The first is related to the Isabella seismic anomaly, either a remnant of the Farallon Plate or foundered lithosphere. The second scenario is related to slow extension of the lithosphere.

• 出版日期2012-6-27