The partitioning of Ni and Mg between olivine and orthopyroxene is described by the exchange equilibrium

½Mg2SiO4+NiSiO3½Ni2SiO4+MgSiO3.

Previous studies on natural ultramafic rocks showed that this equilibrium is very sensitive to crystallization temperature but is reset slowly enough to bear some interest as a geothermometer. On the contrary, the small volume change for the reaction (ΔVexch ≃ 0.08 cm3) suggests only small dependence on pressure.

The temperature dependence of the exchange reaction was studied with Mg-Fe-Ni olivines and orthopyroxenes synthesized at atmospheric pressure, under reducing conditions, and between 1075 and 1225 °C. Fe was introduced in order to obtain Pbca orthopyroxene and to avoid the crystallization of Pbcn protopyroxene, which is the stable low-pressure, high-temperature polymorph of magnesian pyroxenes. In some cases both polymorphs were obtained, but were discriminated by their compositional differences.

The distribution coefficient

kNiMg=(Ni/Mg)Ol:(Ni/Mg)Opx

was found to be independent of composition at constant temperature in the experimental range. The thermometric relation

lnkNiMg=[3801/T]1.815

is in close agreement with a previous calibration on naturally equilibrated samples.

Although Ni does not behave ideally in olivines (strong ordering on M1 sites), the equilibrium relation may be approximated with an ideal disordered model in a compositional range much larger than that encountered in natural mafic and ultramafic rocks.

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