Water Contents of the Mantle Lithosphere beneath the Southwestern United States: FTIR Analysis of Peridotite Xenoliths from the Rio Grande Rift, NM, and Dish Hill, CA
Schaffer, Lillian 1987-
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This project aims at understanding the distribution and partitioning behavior of water in the Earth's mantle relative to melting, metasomatism, and oxygen fugacity conditions. For that purpose, the water contents, Fe3+ contents, and major and trace element contents of peridotite xenoliths from the southwest USA (Dish Hill, CA; Kilbourne Hole, NM; and Rio Puerco, NM) were analyzed, a region where rifting and subduction processes influence mantle geochemistry. Three main conclusions resulted from this study. First, unlike most other species in minerals water does not always partition between minerals and melt according to Nernst’s Law of distribution. Because H-bearing molecules in melts (e.g., H2O, H2) dissociate to protons in solids, Sieverts' Law may be more applicable to water partitioning behavior. New melting models using Sieverts’ Law predict that despite water’s incompatible behavior, peridotites are not completely dehydrated during mantle melting, consistent with measured natural peridotite water contents. This results in slightly higher water contents in the upper mantle compared to previous estimates, which may trigger melting where low seismic velocities are observed beneath oceanic lithosphere at 90-200 km depth. Additionally, portions of the Lunar magma ocean where H2 was the dominant water species contained twenty times less water than formerly estimated. Second, this study shows that beneath the southern part of the Rio Grande Rift (Kilbourne Hole), lithosphere water contents are primarily controlled by melting processes, while beneath the northern part of the rift (Rio Puerco), the mantle lithosphere appears to have been enriched in water during subduction-related metasomatism. This contrasting control of water distribution is likely linked to the angle of the Farallon slab beneath the region with flat-slab subduction in the north and a steeper slab-dip angle in the south. Third, the iron oxidation state is a key control on the distribution of water in mantle minerals. Water and Fe3+ are depleted in peridotite during melting but enriched during metasomatism. However, my results show that the mechanism of incorporation of H in pyroxene is not linked to a redox equilibrium, contrary to what is typically assumed from experimental studies.