Abstract

Recent analyses of fluid inclusions in peridotite minerals suggest that CO2 is a dominant volatile species in the upper mantle. In a CO2-bearing oceanic mantle, the low-velocity zone (LVZ) can be explained by a large decrease in the peridotite solidus temperature at a depth of about 90 km, causing melting by intersection with a geotherm. This decrease in the solidus temperature has been found in the system CaO-MgO-SiO2-CO2 and results from a change in partial melt composition along the solidus from enstatite-normative at pressures less than 26 kb to larnite-normative (melilititic) at greater pressure. Although these liquids dissolve up to 20 wt percent CO2, they are silicate liquids containing at least 30 percent SiO2. These silica levels are appropriate for kimberlitic liquids, but the liquids are more calcic than typical kimberlites.

At depths of less than 90 km in suboceanic mantle, CO2 may be present in carbonate minerals or in vapor, depending upon the geotherm, but cannot be in solution in silicate peridotite minerals. Beneath continents, CO2 will be present in carbonate minerals, and the mantle will not melt at least to depths of 120 km.

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