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Carbonatite melt compositions in the Earth's mantle



Carbonatite melt compositions in the Earth's mantle



Earth and Planetary Science Letters 128(3-4): 259-270



Carbonate-rich melts in the mantle have been recognized as primary melts and important agents for altering mantle geochemistry [1-5]. Experimental methods used to characterize these melts have produced different estimates of composition [5-7]. To resolve these differences experiments were conducted on two carbonate-dominated bulk compositions (fertile and refractory with respect to Al content and Fe/Mg ratio) to approximate carbonatite melt compositions in equilibrium with oceanic (fertile) and subcontinental lithospheric (refractory) peridotites. The K/Na ratio was also varied to correspond to the more sodic character of oceanic peridotites and the more potassic character of lithospheric peridotites [8,9]. Carbonatite melt with Na as the dominant alkali (NaCM) is in equilibrium with phlogopite lherzolite at ca. 25 kbar and 1170 degrees C, and carbonatite melt with K as the dominant alkali (KCM) is in equilibrium with phlogopite lherzolite at ca. 32 kbar and 1120 degrees C. Mineral compositions in equilibrium with KCM are similar to those found in phlogopite peridotites from the Kaapvaal lithosphere. In contrast, the NaCM residual mineral compositions are consistent with those from a model fertile oceanic peridotite (Hawaiian pyrolite). The relative stabilities of garnet and diopside with CO (sub 2) define limits (20-50 kbar, 950-1200 degrees C) within which a primary dolomitic carbonatite melt may be in equilibrium with a peridotitic mantle. The K/Na ratio of the mantle component would control this ratio in the resulting carbonatite.

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Accession: 018514891

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DOI: 10.1016/0012-821x(94)90149-x


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