Are the glasses in mantle xenoliths witness of the metasomatic agent composition?

Glass veins and pockets in mantle xenoliths are often considered as indicators of the composition of metasomatic agents affecting the mantle. Here we demonstrate that infiltration of, and reaction with, the host basalt may produce glassy veins and pockets whose composition encompasses that of different potential metasomatic agents. The xenoliths studied are 4-19 cm large, equigranular, spinel-facies harzburgites and lherzolites from the Patagonia lithospheric mantle. A reaction rim occurs at the contact with the basalt. Peridotite orthopyroxene is in reaction with the basalt and a glassy pocket (up to 600 μm in diameter) is thereby formed. New crystals of euhedral olivine (Fo = 84) and Ti-oxides crystallise in the glass pocket close to the basalt, while euhedral crystals of clinopyroxene (mg# = 85-89) and olivine crystallise close to orthopyroxene. The reaction-crystallisation processes induce dramatic compositional variations in theglass pocket from phonotephryte to trachyte. Trace element concentration and patterns and the element anomalies are controlled by the reaction-crystallisation process. Orthopyroxene dissolution has mainly a dilution effect, whereas clinopyroxene crystallisation and the crystallization of Ti oxides (and apatite) largely controls the trace element fractionation and element anomalies. The largest trace element variations are documented in the veins. The studied glasses obviously do not represent metasomatic agents that affected the lithospheric mantle, but the large compositional variations they document encompass those of glasses quoted in literature (Wulff-Pedersen et al., 1996; Coltorti et al., 2000) and believed to represent metasomatic agents, thus suggesting caution in drawing inferences on mantle processes and components from glass pockets and veins in xenoliths.