Kosuke Onuma

Element partitioning between silicate perovskites and calcic ultrabasic melt

Abstract Element partitioning between Mg- and Ca-perovskites and anhydrous calcic ultrabasic melt was studied experimentally at 25 GPa and 2200°C. The observed partitioning relationships in this system are similar to the results of previous studies in magnesian ultrabasic and basaltic systems, and are consistent with predictions based on crystal chemical considerations. These results provide new insights into early global mantle differentiation, and kimberlite petrogenesis from the lower-mantle source regions.

Melting and crystal–liquid partitioning in the system Mg2SiO4–Fe2SiO4 to 25 GPa

The melting relation of the Mg2SiO4–Fe2SiO4 system was studied at 8.5 GPa. Olivine and spinel melts incongruently into pyroxene and the liquid in the Fe2SiO4 rich portion of the system. Olivine melts congruently at least in the compositional range from (Mg0.5Fe0.5)2SiO4 to (Mg0.9Fe0.1)2SiO4, although the end member of Mg2SiO4 melts incongruently into periclase and the liquid at pressures above 8 GPa. The melting relation of (Mg0.9Fe0.1)2SiO4 was also studied up to 25 GPa. Olivine and modified spinel of this composition melt incongruently into anhydrous phase B and liquid, and into magnesiowustite and liquid at higher temperatures in the pressure range from 18 GPa to 23 GPa. Magnesiowustite is the liquidus phase at higher pressures at least up to 25 GPa in this composition. The partition coefficients between crystal and liquid are; DMg=1.19(±0.06), DSi=0.88(±0.04), DV=0.63(±0.16), DCr=1.04(±0.21), DMn=0.42(±0.08), DFe=0.76(±0.07), DCo=1.24(±0.44), DNi=1.46(±0.81) for modified spinel; DMg=1.41(±0.06), DSi=0.72(±0.03), DV=0.44(±0.11), DCr=0.94(±0.12), DMn=0.30(±0.07), DFe=0.56(±0.06), DCo=1.19(±0.30), DNi=1.65(±0.87) for anhydrous phase B; DMg=2.09(±0.11), DV=0.64(±0.06), DCr=1.01(±0.05), DMn=0.7(±0.04), DFe=1.45(±0.08), DCo=2.73(±0.15), DNi=4.72(±0.25) for magnesiowustite. The present results of melting relations and partitioning are applied for the fractionation in the mantles of the terrestrial planets, such as Mars and Mercury.

The join CaMgSi2O6CaAl2SiO6CaCrAlSiO6 with special reference to chrome clinopyroxene and chrome spinel

Abstract In the Cr-poor part of the join CaMgSi 2 O 6 CaAl 2 SiO 6 CaCrAlSiO 6 (less than 0.24 wt.% Cr 2 O 3 ), studied in air at 1 atm. and at 8–12 kbar, clinopyroxene (Cpx), spinel (Sp), anorthite (An) and melilite (Mel) are encountered, among which clinopyroxene and spinel contain Cr. A wide spinel primary field is present at 1 atm, but spinel disappears by the reaction with liquid at lower temperatures and the subsolidus region consists of single-phase clinopyroxene and Cpx + An + Mel, indicating that chrome clinopyroxene melts incongruently into chrome spinel and liquid. The same phenomenon is presumed to occur at high pressure. In the assemblage Cpx + Sp + liquid the Cr 2 O 3 contents of the clinopyroxenes and spinels increase with increasing Cr 2 O 3 in the bulk compositions and attain 3.2 and 49 wt.%, respectively, at 1 atm. The Cr 2 O 3 content of clinopyroxene seems to decrease with increasing pressure when excess water is present, while no change was observed under conditions undersaturated with water. The Cr 2 O 3 content of spinel increases considerably (up to 70%) with increasing pressure when excess water is present, but seems to decrease with the decreasing water content of the host liquid at constant pressure and temperature.

Melting experiments on the forsterite–pyrope system at 8 and 13.5 GPa

The melting phase relations (melting curve of forsterite and binary eutectic composition) of the forsterite (Mg2SiO4)–pyrope (Mg3Al2Si3O12) system have been studied at 8 and 13.5 GPa. Forsterite is observed to melt incongruently into periclase+melt above about 8 GPa and the liquidus field of periclase gradually expands at higher pressures. The eutectic temperature is about 1850°C at 8 GPa and about 2100°C at 13.5 GPa, whereas the eutectic composition of the system is about 75 mol% forsterite at both pressures.