EARLY PARTIAL MELTING IN PYROXENES

Diopside annealed beyond critical conditions of temperature (T(c)) and f(O2), exsolved tiny precipitates of a molten phase enriched in silic a. T(c) was markedly lower than the melting temperature T(M) (T(c) alm ost-equal-to 1100-degrees-C, whereas T(M) almost-equal-to 1350-degrees -C for the composition studied). The size of the precipitates increase d dramatically with temperature. The first detected nuclei had diamete rs of a few tens of nanometers and represented a volume proportion alm ost-equal-to 0.01%. At 1250-degrees-C, the mean precipitate size reach ed 300 nm, and the volume proportion was almost-equal-to 0.2%. After a long annealing time at 1280-degrees-C, the volume proportion was almo st-equal-to 5%, and the largest precipitates reached 1 mum. Furthermor e, elongated melt veins extending over 50 mum also formed. The smaller precipitates were almost pure silica, then as they grew, their compos ition became progressively enriched in sodium, aluminum, calcium, and iron oxides. This early partial melting (EPM) phenomenon still occurre d for annealings under confining pressure up to P almost-equal-to 1.6 GPa at T almost-equal-to 1300-degrees-C. Similarly, EPM began at almos t-equal-to 1300-degrees-C in experimentally annealed enstatite. Natura lly annealed pyroxenes (clino- and ortho-) from xenoliths brought to t he surface by volcanic explosions also exhibit similar precipitates en riched in silica.