Petrogenetic grid for siliceous dolomites extended to mantle peridotite compositions and to conditions for magma generation

Abstract

Decarbonation reactions in the system CaO-MgO-SiO_2-CO_2 involve calcite, dolomite, magnesite, and quartz, and the products enstatite, forsterite, diopside, and wollastonite, among others. Each decarbonation reaction terminates at an invariarit point involving a liquid, CO_2 vapor, carbonate minerals, and one or more of the silicate minerals. Fusion curves for mantle mineral assemblages involving forsterite, orthopyroxene, and clinopyroxene in the presence of CO_2, extending from higher temperature regions, terminate at these same invariant points. The points are connected by a series of liquidus reactions involving the carbonates and mantle silicates, at temperatures generally lower than the silicate-CO_2 melting reactions. Experimental data and theoretical analysis permit construction of a series of partly schematic phase diagrams. Petrological and geophysical conclusions include the following: (1) Free CO_2 cannot exist in the mantle; it is stored as carbonate. (2) CO_2 appears to be as effective as H_2O in causing incipient melting of mantle peridotite, and this remains our preferred explanation for the seismic low-velocity zone. (3) At depths greater than about 80 km, mantle peridotite with CO_2 (as carbonate) yields carbonatitic magmas with about 40 percent CO_2 and 10 percent silicates in solution; with progressive fusion the liquid becomes kimberlitic. (4) Primary carbonatite or kimberlite magmas rising from the asthenosphere must evolve CO_2 near 80 km depth, which would contribute to their explosive eruption. (5) Through a wide pressure range, SiO_2-undersaturated basic magmas with CO_2 in solution can yield residual kimberlitic or carbonatitic magmas. (6) Deep mantle magmas may include the carbonated alkali ultrabasic magmas that have been proposed as the parents from which continental associations of highly alkalic rocks are derived.

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