Effect of variable carbonate concentration on the solidus of mantle peridotite

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American Mineralogist
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To explore the effect of variable CO2 concentrations on the solidus of natural carbonated peridotite, we determined near-solidus phase relations of three different nominally anhydrous, carbonated lherzolite bulk compositions at 6.6 GPa. Starting mixes (PERC, PERC2, and PERC3) were prepared by adding variable proportions of a carbonate mixture that has the same Ca:Mg:Fe:Na:K ratio as the base silicate peridotite [MixKLB-1: Mg no. = 89.7; Ca no. = molar Ca/(Ca + Mg + Fe*) = 0.05]. For all three bulk compositions, the subsolidus assemblage includes olivine, orthopyroxene, clino-pyroxene, garnet, and magnesite solid solutions. Above the solidus, crystalline carbonate disappears and quenched Fe, Na-bearing dolomitic carbonatite melts were observed. For PERC3 (1.0 wt% bulk CO2; Na2O/CO2 weight ratio = 0.30), the observed solidus is between 1190 and 1220 degrees C; for PERC (2.5 wt% bulk CO2; NaO/CO2 = 0.12), it is between 1250 and 1275 degrees C; and for PERC2 (5.0 wt% bulk CO2; NaO/CO2 = 0.06), it is between 1300 and 1330 degrees C. At 6.6 GPa, experimental solidi of natural magnesite peridotites are 100-200 degrees C lower than the CMAS-CO2 solidus, chiefly owing to the fluxing effect of alkalis, and solidus temperatures increase with increasing bulk CO2 (i.e., decreasing bulk Na2O/CO2), owing to dilution of Na2O in near-solidus melt. The effects of Mg no. and Ca no. on carbonated peridotite solidi appear to be less significant than that of Na2O/CO2. Trends of decreasing solidus temperature with increasing Na2O/CO2 and with decreasing CO2 indicate that natural mantle peridotite with similar to 100-1000 ppm bulk CO2 will have solidus temperatures similar to 20 degrees to similar to 100 degrees lower than that determined experimentally. The solidus of peridotite drops discontinuously by similar to 600 degrees C (at 6.6 GPa) at the CO2 bulk concentration (similar to 5 ppm) at which carbonate is stabilized, but then varies little with increasing bulk CO2. This result contrasts with the effect of H2O, which lowers the solidus continuously with increasing concentration.


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Doi 10.2138/Am.2007.2201