Early partial melting in the upper mantle: an A.E.M. study of a lherzolite experimentally annealed at hypersolidus conditions

Abstract A natural spinel lherzolite (60% olivine, 25% enstatite, 13% Cr-diopside, and 2% Cr-spinel) was annealed and deformed at 1 GPa and 900–1000°C in H 2 O-saturated conditions and at f o 2 roughly corresponding to the fayalite-magnetite-quartz buffer. The topology and textural development of the glass (i.e. quenched melt) was investigated by analytical transmission electron microscopy. In addition to the large glass slots (> 10 μm) previously observed using scanning electron microscopy (Bussod, G.Y. and Christie, J.M., 1991, Textural development and melt topology in spinel lherzolite experimentally deformed at hypersolidus conditions, J. Petrol., special lherzolite issue, pp. 17–39), we detected intracrystalline glass droplets of 0.1–0.2 μm within pyroxenes and olivine grains and intergranular isolated glass pockets 1 to 3 μm wide at grain boundaries. X-ray microanalysis shows that the intracrystalline glass droplets are highly enriched in silica (≈70 wt% SiO 2 ), and depleted in MgO. Their shape and composition are similar in both pyroxenes and olivine grains. These droplets are characteristic of the phenomenon of early partial melting (EPM) previously observed in pyroxenes. The intergranular pockets are also SiO 2 -rich (54 to 66 wt%); their Mg content increases with the size of the pocket (from 0 to 7.6 wt% MgO). These observations provide a plausible scenario for the very first stage of melt formation in the upper mantle.