P-T phase relations of silicic, alkaline, aluminous mantle-xenolith glasses under anhydrous and C-O-H fluid-saturated conditions

High-pressure liquidus experiments on three silicic, aluminous, alkaline melts, modelled on glasses found in many mantle xenoliths, show that part of this compositional range is saturated with harzburgite (or possibly lherzolite) under anhydrous conditions. Under C-O-H fluid-saturated conditions with X-H2O = 0.5, phlogopite mica is present along with anhydrous phases similar to those found under dry conditions. Phlogopite is the sole liquidus phase when X-H2O = 1.0. At X-H2O = 0.5 and 3.0 GPa, garnet, kyanite and carbonate minerals appear as near-liquidus phases and the shape of the liquidus surface is reminiscent of that of the carbonated peridotite solidus. Saturation of these liquids with harzburgite, and comparisons with calculated melt silica activities, suggests that these liquids would face no chemical or thermal obstacles to circulating amongst and coexisting with harzburgite mantle. Also, there is textural evidence that these melts may be mobile. Accordingly, these kinds of liquids could act as cryptic metasomatic agents. If mantle at similar to 45-90 km depth is pre-enriched in low-melting-temperature components, and probably volatiles, via the ascent and percolation of alkaline, mafic liquids (along geotherms that cross inflections in the solidus of CO2-bearing peridotite), then subsequent low-degree partial melts could yield the liquids that are ultimately trapped as xenolith glasses.