Model system controls on conditions for formation of magnesiocarbonatite and calciocarbonatite magmas from the mantle

Experimental data indicate that carbonate-rich magmas may be generated at d epths greater than similar to 70 km by partial melting of carbonated perido tite. The near-solidus magmas lie on the liquidus field boundary between si licates CaCO3-MgCO3, and precise compositions (e.g. Ca/Mg) are defined by t he peridotite mineralogy (e.g. harzburgite, lherzolite, wehrlite); alkali c ontents reflect directly the peridotite composition. These liquids are dolo mitic, with Ca/(Ca + -Mg) between 0.7 and 0.5 from 2 GPa to at least 7 GPa. At conditions of mantle melting, there is a large separation between the s ilicate-carbonate liquid immiscibility volume, the silicate-carbonate liqui dus field boundary, and probable liquid paths. The formation of carbonate-r ich liquids immiscible with silicate magmas in the mantle is therefore unli kely, which denies the generation of immiscible CaCO3 ocelli and primary na trocarbonatite magmas. Rising carbonate-rich magmas retaining equilibrium w ith mantle lherzolite will react, crystallize and release CO2 vapor at dept hs of similar to 70 km, increasing clinopyroxene/orthopyroxene in the rock. Primary magnesiocarbonatite magmas (dolomitic) can be erupted explosively from this depth. Given sufficient magma, lherzolite can be converted to weh rlite by this decarbonation reaction. At shallower depths, wehrlite (but no other peridotite) can coexist with carbonatite magma relatively enriched i n Ca/Mg. If metasomatism of lherzolite to wehrlite can occur through a dept h of tens of kilometers, our new data at 1 GPa confirm an earlier proposal that primary calciocarbonatite magmas can be generated at some depth betwee n 70 km and 40 km, but indicate considerably higher silicate components. Th e shallowest magmas contain a maximum of 73 wt % CaCO3 (equivalent to 89% C aCO3 in the carbonate components of the liquid), with 18% silicate componen ts at 1 GPa. Phase relations in the system CaO-MgO-CO2-H2O show that magnes iocarbonatite magmas can precipitate sovites (calciocarbonatite rocks).