Compositional homogeneity in low-temperature chlorites |
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Authors: | B Velde N El Moutaouakkil A Iijima |
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Institution: | (1) Departement de Géologie, UR 1316 CNRS, Ecole Normale Supérieure, 24, rue Lhomond, F-75230 Paris, France;(2) Geological Institute, University of Tokyo, 7-3-1 Hongo, 113 Tokyo, Japan |
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Abstract: | Electron microprobe analyses of small chlorite grains (10–20 μm width) in diagenetically altered rocks from three deep wells
and a series of outcrop samples ranging in maximum age from 4 to 1300 Ma show an increasingly smaller range in grain-to-grain
composition as a function of the average age of the specimens in a series. The scatter of composition in a sample in each
series changes little with depth despite present day, or estimated maximum temperature differences of 70° C to 100° C from
top to bottom. Comparison of these chlorite compositions with those published for geothermal and metamorphic rocks formed
under conditions of 300–450° C indicates that the homogeneity of chlorite composition is a function of both the temperature
of equilibration and its duration. Apparently total homogenization of Fe/(Fe+Mg), Al/(Al+Mg+Fe) content will occur after 1010 years at low temperatures. Simple calculations indicate that a slow process such as solid state diffusion could be responsible
for the eventual homogenization of mineral grain composition to reach chemical equilibrium after phase equilibrium has been
reached. |
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