Experimental Evaluation of a New H2O-Independent Thermometer Based on Olivine-Melt Ni Partitioning at Crustal Pressure

An olivine-melt thermometer based on the partitioning of Ni (⁠DOl/liqNi⁠) was hypothesized by Pu et al. (2017) to have a negligible dependence on dissolved water in the melt (and pressure variations from 0–1 GPa), in marked contrast to thermometers based on DOl/liqMg⁠. In this study, 15 olivine-melt equilibrium experiments were conducted on a basaltic glass starting material (9.6 wt% MgO; 353 ppm Ni) to test this hypothesis by comparing the effect of dissolved H₂O in the melt on DOl/liqMg and DOl/liqNi on the same set of experiments. Results are presented for six anhydrous experiments at 1 bar, two anhydrous experiments at 0.5 GPa, and seven hydrous experiments at 0.5 GPa. Analyzed olivine and glass compositions in the quenched run products were used to calculate DOl/liqMg and DOl/liqNi values for each experiment, which in turn permit temperature to be calculated with the Mg- and Ni-thermometers calibrated in Pu et al. (2017) on anhydrous, 1-bar experiments from the literature. The Ni-thermometer recovers the temperatures of all 15 experiments from this study with an average deviation of –3 °C, including those with up to 4.3 wt% H₂O dissolved in the melt. In contrast, the Mg-thermometer recovers the anhydrous, 1-bar experimental temperatures within +14 °C on average, but overestimates the hydrous experimental temperatures by +49 to +127 °C, with an average of +83 °C. When the Mg-thermometer of Putirka et al. (2007) is applied, which includes a correction for analyzed H₂O (≤4.3 wt%) in the quenched melts of the run products, all experimental temperatures are recovered with an average (±1σ) deviation of +7 °C. The combined results show that DOl/liqNi has a negligible dependence on dissolved water in the melt (≤4.3 wt% H₂O), which is in marked contrast to the strong dependence of DOl/liqMg on water in the melt. An understanding of why DOl/liqNi is insensitive to dissolved water, unlike DOl/liqMg⁠, is obtained from spectroscopic evidence in the literature, which shows that Ni²⁺ (transition metal) and Mg²⁺ (alkaline earth metal) have distinctly different average coordination numbers (predominantly fourfold and sixfold, respectively) in silicate melts and that fourfold-coordinated Ni²⁺ is unaffected by the presence of dissolved water in the melt. This difference in coordination number explains why DOl/liqNi and DOl/liqMg each have a different dependence on pressure, anhydrous melt composition, and melt water content. Application of the Ni-thermometer of Pu et al. (2017) to five natural samples from the Mexican arc, for which H₂O contents (3.6–6.7 wt%) in olivine-hosted melt inclusions are reported in the literature, leads to temperatures that match those obtained from the Putirka et al. (2007) Mg-thermometer that corrects for analyzed H₂O contents. This study demonstrates that a thermometer based on DOl/liqNi can be applied to hydrous basalts at crustal depths without the need to correct for dissolved water content or pressure.

Xiaofei Pu, Gordon M. Moore, Rebecca A. Lange, Jack P. Touran, Joel E. Gagnon; Experimental evaluation of a new H2O-independent thermometer based on olivine-melt Ni partitioning at crustal pressure. American Mineralogist 2021; 106 (2): 235–250. doi: abstract