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An olivine-melt thermometer based on the partitioning of NiDN   iO1/liq$\left(D_{\mathrm{Ni}}^{\mathrm{O} 1 / \mathrm{liq}}\right)$   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 onDM   gO   l/l   i   q.$D_{\mathrm{Mg}}^{\mathrm{Ol} / \mathrm{liq}}.$   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 H 2 O in the melt onDM   gO   l/l   i   qandDN   iO1/liq$D_{\mathrm{Mg}}^{\mathrm{Ol} / \mathrm{liq}} \,\text{and}\, D_{\mathrm{Ni}}^{\mathrm{O} 1 / \mathrm{liq}}$   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 calculateDM   gO   l/l   i   qandDN   iO1/liq$D_{\mathrm{Mg}}^{\mathrm{Ol} / \mathrm{liq}} \,\text{and}\, D_{\mathrm{Ni}}^{\mathrm{O} 1 / \mathrm{liq}}$   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% H 2 O 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 H 2 O (≤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 thatDN   iO   1/liq$D_{\mathrm{Ni}}^{\text{O}1 / \mathrm{liq}}$   has a negligible dependence on dissolved water in the melt (≤4.3 wt% H 2 O), which is in marked contrast to the strong dependence ofDM   gO   1/liq$D_{\mathrm{Mg}}^{\text{O}1 / \mathrm{liq}}$   on water in the melt. An understanding of whyDN   iO   1/liq$D_{\mathrm{Ni}}^{\text{O}1 / \mathrm{liq}}$   is insensitive to dissolved water, unlikeDM   gO   1/liq,$D_{\mathrm{Mg}}^{\text{O}1 / \mathrm{liq}},$   is obtained from spectroscopic evidence in the literature, which shows that Ni 2+  (transition metal) and Mg 2+  (alkaline earth metal) have distinctly different average coordination numbers (predominantly fourfold and sixfold, respectively) in silicate melts and that fourfold-coordinated Ni 2+  is unaffected by the presence of dissolved water in the melt. This difference in coordination number explains whyDN   iO1/l   i   qandDM   gO1/l   i   q$D_{\mathrm{Ni}}^{\mathrm{O} 1 / \mathrm{liq} \mathrm{}} \,\text{and}\, D_{\mathrm{Mg}}^{\mathrm{O} 1 / \mathrm{liq}}$   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 H 2 O 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 H 2 O contents. This study demonstrates that a thermometer based onDN   iO   l/l   i   q$D_{\mathrm{Ni}}^{\mathrm{Ol} / \mathrm{liq}}$   can be applied to hydrous basalts at crustal depths without the need to correct for dissolved water content or pressure.

Experimental evaluation of a new H 2 O-independent thermometer based on olivine-melt Ni partitioning at crustal pressure