Metamorphism and oxygen isotope geochemistry of the northern Wind River Range, Wyoming.
Sharp, Zachary David
1988
Abstract
This thesis consists of phase equilibrium studies based on newly measured thermochemical/thermophysical data and a thermobarometric and isotopic study of a high-grade metamorphic terrane in the northern Wind River Range, Wyoming. The heat capacity of natural monticellite (Ca$\sb{1.00}$Mg$\sb{0.91}$Fe$\sb{0.09}$Mn$\sb{0.01}$Si$\sb{0.99}$O$\sb{3.99})$ was measured between 13 and 1000 K using low-temperature adiabatic calorimetry and differential scanning calorimetry. The following entropy equation is valid for end-member monticellite between 298-1000K: S$\sbsp{\rm T}{\circ}$ (J$\cdot$mol$\sp{-1.}$K$\sp{-1}$) = S$\sbsp{298}{\circ}$ + 164.79InT + 15.337$\sp{.}10\sp{-3}$T + 22.791$\sp{.}10\sp5$T$\sp{-2} - $968.94, where S$\sbsp{298}{\circ} = 109.4 \pm 0.2$ J$\sp{.}$mol$\sp{-1.}$K$\sp{-1}$. A best fit to the available experiments on monticellite-bearing equilibria is made when S$\sbsp{\rm 289 (merwinite)}{\circ}$ = 250.2 J$\sp{.}$mol$\sp{-1.}$K$\sp{-1}$ and the effects of solid solution between forsterite and monticellite in the original experiments are considered. The reaction sodalite = nepheline + NaCl (s) was reversed at high pressure, (923 K, 7.4-7.6 kbar; 1023 K, 7.8-8.0 kbar; 1123 K, 8.4-8.6 kbar) and the reaction sodalite = carnegieite + NaCl (l) was reversed at 1 bar (1636-1646 K). The calculated slope for the first reaction is $-$9.0 bar/K compared to the experimental slope of +5.3 bar/K. This discrepancy is eliminated if it is assumed that the experimental sodalite has nearly complete Al/Si disorder. Pressure/temperature estimates from the Archean granulite-facies rocks of the northern Wind River Range, Wyoming are 800 $\pm$ 50$\sp\circ$C/8 $\pm$ 1 kbar, 750 $\pm$ 50$\sp\circ$C/6.0 $\pm$.05 kbar and 650 $\pm$ 50$\sp\circ$C/5.5 $\pm$ 0.5 kbar from mineral inclusions within garnet and the core and rim compositions of groundmass phases, respectively. The inferred clockwise P-T path for the terrane is consistent with a tectonic regime of thickened crust followed by early rapid decompression and later slow cooling and uplift. Oxygen isotope fractionations between quartz and magnetite in the iron formations are 8$\perthous$ (600$\sp\circ$C) in the central portion of the iron formation boudins and 10$\perthous$ (500$\sp\circ$C) near the contact with the surrounding granitic orthogneiss. Greater deformation in the marginal portions of the iron formations may explain the lower isotopic temperature estimates. Retrograde exchange by oxygen self diffusion should continue down to 500$\sp\circ$C if the experimental diffusion coefficients are correct. In order to preserve 600$\sp\circ$C, diffusion rates in $\alpha$-quartz must be 2.5 $\times$ 10$\sp{-18}$ cm/sec$\sp2$, two orders of magnitude lower than the experimental value. Water enhances oxygen diffusion rates in silicates. The diffusion experiments were conducted in the presence of water whereas the iron formations cooled under fluid-absent conditions.Subjects
Geochemistry Isotope Metamorphism Northern Oxygen Range River Wind Wyoming
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