Electron microprobe measurement of pyroxenes coexisting with H_2O-undersaturated liquid in the join CaMgSi_2O_6-Mg_2Si_2O_6-H_2O at 30 kilobars, with applications to geothermometry

Abstract

Mixtures of synthetic crystalline enstatite and diopside were reacted with small water contents in sealed capsules in piston-cylinder apparatus at 30 kb between 1000° C and 1700° C. The compositions of coexisting enstatite and diopside solid solutions were measured with an ARL-EMX electron microprobe between 1000° C and 1500° C. Between 1100° C and 1500° C the pyroxenes coexisted with H_2O-undersaturated liquid which quenched to inhomogeneous pyroxene crystals. The presence of liquid facilitated growth of pyroxene crystals suitable for microprobe determinations. The solvus of Davis and Boyd (1966) is generally used in geothermometry; our enstatite solvus limb is a few mol-% richer in Mg_2Si_2O_6 in the temperature range 1000–1400° C; our diopside solvus limb is a few mol-% richer in Mg_2Si_2O_6 below 1100°C, in close agreement between 1100° C and 1200° C, but richer in CaMgSi_2O_6 between 1200° C and 1500° C. Estimated equilibration temperatures for a diopside with composition 78.7% Di is 1300° C according to our results compared with 1210° C for the Davis and Boyd solvus.

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