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Effect of variable carbonate concentration on the solidus of mantle peridotite

Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, Minnesota 55455, U.S.A.

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, clinopyroxene, 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 °C; for PERC (2.5 wt% bulk CO₂; Na₂O/CO₂ = 0.12), it is between 1250 and 1275 °C; and for PERC2 (5.0 wt% bulk CO₂; Na₂O/CO₂ = 0.06), it is between 1300 and 1330 °C. At 6.6 GPa, experimental solidi of natural magnesite peridotites are 100–200 °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 ~100–1000 ppm bulk CO₂ will have solidus temperatures ~20° to ~100° lower than that determined experimentally. The solidus of peridotite drops discontinuously by ~600 °C (at 6.6 GPa) at the CO₂ bulk concentration (~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.

Key Words: Carbonated peridotite • carbonatite • mantle solidus • experimental petrology • partial melting

This article has been cited by other articles:

				R. Dasgupta, M. M. Hirschmann, and N. D. Smith Partial Melting Experiments of Peridotite + CO2 at 3 GPa and Genesis of Alkalic Ocean Island Basalts J. Petrology, November 1, 2007; 48(11): 2093 - 2124. [Abstract] [Full Text] [PDF]