PT Phase Relations of Silicic, Alkaline, Aluminous Glasses Trapped in Mantle Xenoliths

David S. Draper and Trevor H. Green (School of Earth Sciences, Macquarie University, Sydney NSW 2109 Australia, +61-2-9850-8347, email david.draper@mq.edu.au)

Silicic, aluminous, alkaline glasses are found in many mantle xenoliths, as patches, veins, thin films, and inclusions in minerals and have become the subjects of increasing recent interest in relation to debate over the nature of low-degree mantle melts and mantle metasomatism. Our piston-cylinder liquidus experiments were conducted on three such compositions at 1.0 to 3.0 GPa and 825 to 1350°C under anydrous conditions as well as in the presence of excess COH fluids ranging in composition from XH2O = 0.5 to 1.0. The nepheline-normative part of this compositional range is saturated with a harzburgitic assemblage (magnesian ol + opx), and is close to saturation with a lherzolite assemblage (i.e. cpx is a near-liquidus phase) under anhydrous conditions. We also demonstrate that the compositions of liquids like those of our starting materials are not due to sluggish plagioclase nucleation: plagioclase-addition experiments do not result in crystallization of that mineral. Under fluid-saturated conditions with XH2O = 0.5, phlogopite mica is present as the hydrous phase along with anhydrous phases similar to those found under dry conditions. Phlogopite is the sole liquidus phase at nearly all the studied conditions in fluid saturated runs where XH2O = 1.0. At XH2O = 0.5 and 3.0 GPa pressure, carbonate minerals (Mg-rich magnesite-siderite and ferroan dolomite) appear as near-liquidus phases and the liquidus surface is depressed to lower temperatures than at lower pressures, giving it a shape reminiscent of that of the solidus of carbonated peridotite. Also under these conditions, garnet and kyanite appear as liquidus or near-liquidus phases.

We conclude that the saturation of these liquids with harzburgite, over pressures and temperatures characteristic of the upper mantle, suggests that these liquids would face no chemical or thermal obstacles to circulating amongst and coexisting with harzburgitic mantle. This view is upheld by comparisons of our experimental results with calculated silica activities as buffered by olivine and orthopyroxene; also, textural evidence indicates that such melts should be mobile. Accordingly, we conclude that these kinds of liquids could in fact act as agents of mantle metasomatism. We suggest a two-stage model for the formation of these liquids in which mantle material at depths corresponding to pressures ranging from ~1.5 to 3.0 GPa undergoes a process of pre-enrichment in low-melting-temperature components (and probably volatiles) via the ascent and percolation of alkaline, mafic liquids along geotherms, steeper than those typical of subcratonic regions, that cross inflections in the solidus of CO2-bearing peridotite (Bailey 1987). This pre-enriched mantle then undergoes (probably low-degree) partial melting to give rise to the liquids that are ultimately trapped as xenolith glasses.