HR: 0830h
AN: V41C-0305    [PDF]
TI: Water contents in olivines from spinel peridotites from the sub-arc mantle wedge
AU: * Peslier, A H
EM: apeslier@mail.uh.edu
AF: University of Houston, Texas Center for Superconductivity and Advanced Materials (TCSAM), Houston, TX 77204 United States
AU: Luhr, J F
EM: jluhr@volcano.si.edu
AF: Smithsonian Institution, Department of Mineral Sciences, NHB-119, Washington, DC 20560 United States
AB: Water contents of olivines in spinel-peridotite xenoliths from Mexico and Simcoe (WA, USA) were measured by FTIR. The xenoliths come from mantle domains that were likely affected by subduction, with the host volcanic centers located at various distances from the trench. O-H vibration peaks for most of the high-forsterite content olivines are in the 3400-3600 cm$^{-1}$ range only. One Mexican sample, however, has its main O-H peaks in the lower wavenumber range of 3100-3400 cm$^{-1}$, a feature so far only reported in experimentally annealed olivines. The two ranges of O-H peaks indicate that hydrogen can enter 2 different sites (defects) in the olivine structure, probably depending on the environment (fluids, depth, temperature) of crystallization and on any later re-equilibration of the mineral. Calculated water contents in olivine range from 12 to 111 ppm H$_{2}$O, and correlate positively with those of clinopyroxenes and orthopyroxenes from the same samples. The water content of the sample with low wavenumber peaks lies on the correlation if only its high wavenumber O-H peaks are taken into account. The calculated water contents of the whole-rock peridotites range from 34 to 195 ppm H$_{2}$O. As is the case for pyroxene water contents, those of olivine correlate positively, though roughly, with indices of melting (e.g. bulk-rock aluminum content), and correlate negatively with Fe$^{3+}$/(Fe total) ratios in spinel and oxygen fugacities calculated from them. These correlations indicate that although H behaves as an incompatible element in these minerals during partial melting, the main control of H incorporation in their structure is the oxidation state of the peridotite. The anhydrous phases of the mantle wedge, a typically oxidized environment, thus appear to incorporate very little water from fluids or melts from the slab that may transfer through it to feed volatile-rich highly explosive volcanoes lying above.
DE: 1025 Composition of the mantle
DE: 1030 Geochemical cycles (0330)
DE: 1749 Volcanology, geochemistry, and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting