A new olivine-melt thermometer based on the partitioning of Ni (DNiOl/liq), with a form similar to the Beattie (1993),DMgOl/liq thermometer, is presented in this study. It is calibrated on a data set of 123 olivine-melt equilibrium experiments from 16 studies in the literature that pass the following five filters: (1) 1 bar only, (2) analyzed totals between 99.0–101.0 wt% for olivine and 98.5–101.0 wt% for quenched glasses, (3) olivine is the only silicate phase in equilibrium with the melt, (4) the NiO concentration is ≥0.1 wt% in olivine and ≥0.01 wt% in quenched glass, and (5) no metallic phase is present other than the capsule. The final data set spans a wide range of temperatures (1170–1650 °C), liquid compositions (37–66 wt% SiO2; 4–40 wt% MgO; 107–11 087 ppm Ni), and olivine compositions (Fo36–100; 0.10–15.7 wt% NiO). The Ni-thermometer recovers the 123 experimental temperatures within ±29 °C (1σ), with an average residual of 0 °C. A re-fitted version of the Mg-thermometer of Beattie (1993), calibrated on the same 123 experiments as for the Ni-thermometer, results in an average residual of 1 ± 26 °C (1σ). When both thermometers are applied to the same 123 experiments, the average ΔT (TMgTNi) is 1 ± 29 °C (1σ), which confirms that the Mg- and Ni-thermometers perform equally well over a wide range of anhydrous melt composition and temperature at 1 bar. The pressure dependence of the Ni-thermometer under crustal conditions (≤1 GPa) is shown to be negligible through comparison with experimental results from Matzen et al. (2013), whereas the pressure dependence of the Mg-thermometer is up to 52 °C at ≤1 GPa (Herzberg and O’Hara 2002). Therefore, neglecting the effect of pressure when applying both thermometers to basalts that crystallized olivine at crustal depths (≤1 GPa) is expected to lead to negative ΔT (TMgTNi) values (≤ −52 °C). Application of the two thermometers to nine mid-ocean ridge basalts results in an average ΔT of −3°, consistent with shallow crystallization of olivine under nearly anhydrous conditions. In contrast, application of the two thermometers to 18 subduction-zone basalts leads to an average ΔT of +112°; this large positive ΔT value cannot be explained by the effect of pressure, temperature or anhydrous melt composition. It is well documented in the literature that DMgOl/liq is affected by dissolved water in the melt and that Mg-thermometers overestimate the temperature of hydrous basalts if an H2O correction is not applied (e.g., Putirka et al. 2007). Therefore, the reason why hydrous arc basalts have higher ΔT (TMgTNi) values than MORBs may be because DNiOl/liq is less sensitivite to water in the melt, which is supported by new Ni-partitioning results on three olivine-melt equilibrium experiments on a basaltic andesite with up to 5 wt% H2O. More hydrous experiments are needed to confirm that the Ni-thermometer can be applied to hydrous melts without a correction for H2O in the melt.

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