Genesis of carbonate in pyrope from ultramafic diatremes on the Colorado Plateau,southwestern United States |
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| Authors: | Douglas Smith |
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| Institution: | (1) Department of Geological Sciences, University of Texas, 78713 Austin, TX, USA |
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| Abstract: | Mineral associations and compositions of carbonates within pyrope crystals are clues to the genesis of mantle carbonate and to the character of metasomatic melts in depleted peridotite. The pyrope crystals are in ultramafic diatremes of the Navajo field on the Colorado Plateau. Although inclusions of olivine and pyroxene are typically monomineralic, 4 of 6 inclusions of carbonates and hydrates are polymineralic. Polymineralic assemblages include: pargasite-magnesite-dolomite-apatite-spinel; pargasite-dolomite-Ba phlogopite (with 10% BaO); olivine-dolomite-spinel; edenite-chlorite; and olivine-ilmenite-spinel. Magnesite and chlorite are present also as monomineralic inclusions. The two inclusions with pargasite plus carbonate are in the same garnet; the association of carbonates plus hydrates and the enrichment in Ba are evidence that the included minerals originated from melt trapped in pyrope. The pargasite and mica are F-poor and contain about 0.4 and 1.1 wt% Cl, respectively, more than any other analyzed mantle amphibole or mica. If the parent melts of such inclusions are similar to those responsible for trace-element metasomatism of continental lithosphere, then these melts have higher Cl/F ratios than those inferred from typical xenolith minerals. Amphibole-garnet and olivine-spinel equilibration temperatures are in the range 500–700° C, so the garnets cooled to low temperatures within the mantle following inclusion of melt. All the hydrates and carbonates may have formed from trapped melt, but evidence is strong only for the complex pargasite-carbonate-mica inclusions. Two garnets containing chlorite are more Cr-rich and Fe-poor than most other inclusion-bearing pyropes, and the chlorite may have been included during prograde metamorphism of subducted lithosphere. |
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