Model System Controls on Conditions for Formation of Magnesiocarbonatite and Calciocarbonatite Magmas from the Mantle

Abstract

Experimental data indicate that carbonate-rich magmas may be generated at depths greate than ∼70 km by partial melting of carbonated peridotite. The near-solidus magmas lie on the liquidus field boundary between silicates and carbonates. Liquid compositions are dominated by th system CaCO_3–MgCO_3, and precise compositions (e.g. Ca/Mg) are define by the peridotite mineralogy (e.g. harzburgite, lherzolite, wehrlite); alkali contents reflect directly the peridotite composition. These liquids are dolomitic, 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 silicate–carbonate liquid immiscibility volume, the silicate–carbonate liquidus field boundary, and probable liquid paths. The formation of carbonate-rich liquids immiscible with silicate magmas in the mantle is therefore unlikely, which denies the generation of immiscible CaCO_3 ocelli and primary natrocarbonatite magmas. Rising carbonate-rich magmas retaining equilibrium with mantle lherzolite will react, crystallize and release CO_2 vapor at depths of ∼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 wehrlite by this decarbonation reaction. At shallower depths, wehrlite (but no other peridotite) can coexist with carbonatite magma relatively enriched in Ca/Mg. If metasomatism of lherzolite to wehrlite can occur through depth of tens of kilometers, our new data at 1 GPa confirm an earlier proposal that primary calciocarbonatite magmas can be generated at some depth between 70 km and 40 km, but indicate considerably higher silicate components. The shallowest magmas contain a maximum of 73 wt % CaCO_3 (equivalent to 89% CaCO_3 in the carbonate components of the liquid) with 18% silicate components at 1 GPa. Phase relations in the system CaO–MgO–CO_2–H_2O show that magnesiocarbonatite magmas can precipitate sovites (calciocarbonatite rocks).