POWRE: Synchrotron Micro-XANES Study of Fe Redox in Mantle Phases

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9806182DyarThe goal of this research is to measure in situ partition coefficients for Fe3+ and Fe2+ among all the Fe-bearing phases and for H among hydrous phases in mantle xenoliths and mantle-analog experimental runs. The analyses of experimental runs will provide quantitative data on the kinetics of hydrogen transport via redox exchange. In the natural samples, the proposed research will generate the first ever in situ, truly microscale measurements of Fe3+ /EFe in mantle minerals. Use of naturally occurring samples from previously well-studied rocks will allow results to be considered within a tightly constrained petrologic framework in which the effects of pressure, temperature, and mineral assemblages will be considered. These measurements will be made on standard 1" round petrographic thin sections using beam sizes of less than 10 x 20 um and the relatively rapid, non-destructive techniques of SmX for Fe3+/EFe, ion microprobe (SIMS) for h, and the electron microprobe (EPMA) for other major elements. The P.I.s will devote their efforts to understanding zoning of Fe3+/EFe within individual mineral grains, variation of Fe3+/EFe among grains of the same mineral in the same rock, the temperature and pressure dependence of Fe3+/EFe (and implication for geothermometry and geobarometry), and substitution mechanisms that charge balance variations in H+, Fe3+, and Fe2+ contents. With these results, the P.I.s can address the questions of 1) How do chemical analyses compare to electron microprobe estimates of Fe3+ and H+?; 2) How do these differences affect distribution of Fe2+/Mg, Fe3+/ Al, Fe3+/ Cr3+ etc. between coexisting minerals?; 3) How do these distribution coefficients affect petrologic interpretation of intensive variables?; 4) Do complete analyses affect cation site assignments (relative to those obtained only by EPMA only)?; 5) Do the distribution coefficients offer new insights into ways to interpret intensive parameters from mineral compositions?; 6) Do complete analyses show chemical zonation that cannot be seen with EPMA? Ca zonation be used to infer oxidation/dehydrogenation history?; and 7) How are Fe3+ and H+ related, and what are the implications of this relationship for estimation of mantle fO2 and fH2 ? Once completed, the proposed program of study will enable formulation of expressions for estimating compositional variables as Fe3+, H+, etc. for rocks in which such detailed characterizations are not available. Funding will be used to support the P.I. part-time during each of two summers, and to support her during the second academic year so that she can take advantage of the exciting development of the new SmX tool.