EARLY PARTIAL MELTING IN THE UPPER-MANTLE - AN AEM STUDY OF A LHERZOLITE EXPERIMENTALLY ANNEALED AT HYPERSOLIDUS CONDITIONS

A natural spinel Iherzolite (60% olivine, 25% enstatite, 13% Cr-diopsi de, and 2% Cr-spinel) was annealed and deformed at 1 CPa and 900-1000 degrees C in H2O-saturated conditions and at f(o2) roughly correspondi ng to the fayalite-magnetite-quartz buffer. The topology and textural development of the glass (i.e. quenched melt) was investigated by anal ytical transmission electron microscopy. In addition to the large glas s slots (>10 mu m) previously observed using scanning electron microsc opy (Bussed, G.Y. and Christie, J.M., 1991, Textural development and m elt topology in spinel Iherzolite experimentally deformed at hypersoli dus conditions, J. Petrol., special Iherzolite issue, pp. 17-39), we d etected intracrystalline glass droplets of 0.1-0.2 mu m within pyroxen es and olivine grains and intergranular isolated glass pockets 1 to 3 mu m wide at grain boundaries. X-ray microanalysis shows that the intr acrystalline glass droplets are highly enriched in silica (approximate to 70 wt% SiO2), and depleted in MgO. Their shape and composition are similar in both pyroxenes and olivine grains. These droplets are char acteristic of the phenomenon of early partial melting (EPM) previously observed in pyroxenes. The intergranular pockets are also SiO2-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 scenar io for the very first stage of melt formation in the upper mantle.