Abstract
An Adirondack marble contains the assemblage calcite–diopside–forsterite–monticellite, with trace amounts of zincian spinel, titanian andradite, idocrase, and sphalerite. Microprobe analyses indicate that the monticellite and forsterite both have Mg/(Mg+Fe) greater than 0.9, and that monticellite is more iron-rich than coexisting forsterite. Monticellite host grains contain rare microscopic exsolution lamellae of forsterite, a feature noted in only one other terrestrial monticellite occurrence. Diopside is strongly zoned and contains up to 6 weight percent A12O3 and substantial ferric iron, indicating a fassaitic character typical of calc-silicate clinopyroxenes.
Distribution of Fe and Mg among the major silicate minerals indicates that the monlicellite-producing reaction must be an Fe–Mg continuous reaction combining the end-member reaction, with two Fe–Mg exchange equilibria: Calculations suggest that Fe does not lower the reaction temperature significantly. The occurrence of monticellite-free marble in the same outcrops and variable mineral compositions in the calcite-diopside-forsterite-monticellite assemblage implies that these rocks formed in an aCO2 gradient with aCO2 probably decreasing from center to edge of the marble layer. P–T–XCO2 relations for the iron-free reaction have been calculated, and suggest that formation of monticellite in the above reaction, under presumed conditions of Adirondack regional metamorphism of 8–10 kbar and about 700°–800°C, requires either great dilution of CO2 by H2O in metamorphic fluid, or low aCO2 under fluid-absent conditions. A possible alternative explanation suggests earlier contact metamorphism at low pressures followed by a later Grenville-age high-pressure regional metamorphism in which the pre-existing high-grade mineral assemblages were not substantially affected.
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