PYROXENE-CARBONATE REACTIONS IN THE UPPER MANTLE

Abstract

TheP-T location of the reaction diopside + magnesiteaienstatite + dolomite has been determined by experimental reversals from 800 to 1300°C, 20–50 kbar. Up to ∼ 1150°C this reaction has a slope of ∼ 52 bar/deg, and above this temperature a slope of ∼ 80 bar/deg. The change in slope is attributed to the presence of disordered dolomite at high temperature.

It had previously been suggested that dolomite would be the stable carbonate mineral at the mantle solidus to depths of ∼ 150–200 km and magnesite only at greater depths, whereas our data indicate that magnesite will occur at the solidus at depths of< 100–120 km for a H2O + CO2-bearing mantle. Dolomite will only occur at the solidus between ∼ 80 and 110 km depth. The intersection of a shield geotherm with the solidus within the stability field of magnesite produces liquids low in CaO and Al2O3—inferred to be kimberlitic magmas. Oceanic geotherms intersect the solidus in the dolomite stability field, producing liquids more enriched in CaO and of olivine melilitite composition.