P-T Phase Relations of Silicic, Alkaline, Aluminous Mantle-Xenolith Glasses Under Anhydrous and C-O-H Fluid-saturated Conditions

Abstract

High-pressure liquidus experiments on three silicic, aluminous, INTRODUCTION alkaline melts, modelled on glasses found in many mantle xenoliths, The compositions of silicate melts potentially in equishow that part of this compositional range is saturated with librium with the Earth’s mantle have long been of interest harzburgite (or possibly lherzolite) under anhydrous conditions. to petrologists and geochemists. Much useful information Under C–O–H fluid-saturated conditions with XH2O=0·5, phloabout such melts has come from the study of melt gopite mica is present along with anhydrous phases similar to those inclusions in phenocrysts in volcanic rocks and in the found under dry conditions. Phlogopite is the sole liquidus phase minerals of ultramafic mantle xenoliths. In addition, when XH2O=1·0. At XH2O=0·5 and 3·0 GPa, garnet, kyanite many mantle xenoliths contain silicate glasses as a discrete and carbonate minerals appear as near-liquidus phases and the phase, rather than (or in addition to) inclusions (see shape of the liquidus surface is reminiscent of that of the carbonated Appendix A for references). In this paper, we will refer peridotite solidus. Saturation of these liquids with harzburgite, and to discrete-phase xenolith glasses simply as xenolith comparisons with calculated melt silica activities, suggests that these glasses and those found as inclusions in minerals as liquids would face no chemical or thermal obstacles to circulating inclusion glasses. Xenolith glasses have been reported amongst and coexisting with harzburgitic mantle. Also, there is from mantle xenoliths hosted by alkaline, mafic magmas textural evidence that these melts may be mobile. Accordingly, these both from intra-plate and from subduction-related teckinds of liquids could act as cryptic metasomatic agents. If mantle tonic settings. The glasses have a wide range of comat ~45–90 km depth is pre-enriched in low-melting-temperature positions and textural relations: they are found as components, and probably volatiles, via the ascent and percolation intergranular blebs, irregular patches, veins, and thin of alkaline, mafic liquids (along geotherms that cross inflections in films that appear to wet all grain faces. Most glasses the solidus of CO2-bearing peridotite), then subsequent low-degree account for, at most, only a few volume percent of their partial melts could yield the liquids that are ultimately trapped as host xenolith, and some much less than 1 vol. %. xenolith glasses. Xenolith glasses have a very wide range of major