Reaction Lengths and Reaction Rates in Metamorphic Rock Systems Using Isotopic and Age Zoning in Garnet Porphyroblasts

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9805218 DePaolo The proposed research is designed to determine (1) the rates of mineral-fluid exchange reactions in metamorphic rocks at temperatures between 500 and 700 C and (2) the length scales over which chemical "equilibrium" is maintained during metamorphism under these conditions. The approach involves measurements of Sr, Nd and O isotopic compositions of garnet porphyroblasts from metamorphic rocks, combined with similar isotopic measurements of whole rock and matrix samples. The isotopic measurements will be supplemented with electron microprobe studies of major element compositions of minerals. The study will be made on samples from layered metamorphic rocks, the localities chosen specifically because they have characteristics which allow determination of the "equilibration length" or "reaction length" of the fluid-rock system. The required characteristics of the field sites are: (1) interlayered lithologies with different initial Sr isotope ratios and different Rb/Sr rations; (2) abundant large garnet porphyroblasts suitable for microsampling of core, rim, and intermediate zones for isotopic analyses; and (3) a relatively recent age of metamorphism. Appropriate outcrops have been identified in the Alps, Vermont, California and Alaska.