Partial melt in the oceanic low velocity zone

Parameterizations of experimental data constraining the influence of CO₂ and H₂O on silicate melting, combined with experimental data on the stability of carbonatite in the upper mantle, allow calculation of the stability of partial melts in the seismic low velocity zone (LVZ) beneath oceanic lithosphere. For mantle with volatile contents similar to the sources of mid-ocean ridge basalt (100 ppm H₂O, 60 ppm CO₂), small amounts of melt are thermodynamically stable throughout the LVZ. On a weight basis, CO₂ has less influence than H₂O in stabilizing near-solidus partial melts of peridotite, but stability of volatile-rich near-solidus melts is dominated by CO₂ rather than H₂O because CO₂ is less compatible than H₂O in peridotite. For mantle potential temperatures (MPT) ranging from 1300 to 1400 °C, CO₂-rich silicate melts are stable to depths of 130-180 km and carbonatite stable at greater depths and also in the shallow regions (70-100 km) beneath older lithosphere. Calculated melt fractions of carbonatite do not exceed 0.024% and silicate melt fractions are <0.1% beneath older (>40 Ma) lithosphere, except for a thin region at ∼120 km when the MPT approaches 1400 °C. These melt fractions are maxima if compaction is effective over the ∼100 Ma life of oceanic lithosphere. The regions of high melt fraction correspond broadly with the locus of low shear wave velocity imaged tomographically, though the melt fractions present may be too small to account for the seismic characteristics of the LVZ. Carbonatite cannot be present in large regions of the LVZ, particularly at intermediate depths (60-145 km) beneath young lithosphere (<40 Ma), and therefore cannot be responsible for the high electrical conductivity of this part of the LVZ, as has recently been proposed (Gaillard et al., 2008). Horizontal lenses of melt beneath the lithosphere/asthenosphere boundary may be responsible for the sharp seismic G discontinuity that marks the lithosphere/asthenosphere boundary in oceanic domains, but if so they must consist of melt collected at that horizon by percolation of deeper magmas or melt concentrated in high aspect ratio enriched heterogeneities.