Experimental evaluation of a new H2O-independent thermometer based on olivine-melt Ni partitioning at crustal pressure

Abstract

An olivine-melt thermometer based on the partitioning of Ni (DNiOl/liq) 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 DMgOl/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 H2O in the melt on DMgOl/liq and DNiOl/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 calculate DMgOl/liq and DNiOl/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% 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 DNiOl/liq has a negligible dependence on dissolved water in the melt (≤4.3 wt% H2O), which is in marked contrast to the strong dependence of DMgOl/liq on water in the melt. An understanding of why DNiOl/liq is insensitive to dissolved water, unlike DMgOl/liq, 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 DNiOl/liq and DMgOl/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 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 DNiOl/liq can be applied to hydrous basalts at crustal depths without the need to correct for dissolved water content or pressure.

Origin of alkali olivine basalts and hawaiites in the western Mexican arc: Evidence of rapid phenocryst growth and magma mixing during ascent along fractures

“…15; Table 4A). This result also shows internal consistency with application of the Ni thermometer of Pu et al (2017Pu et al ( , 2021. All the remaining samples fail the Mn-Mg K D (oliv-melt) test, with differences of >0.04 between analyzed values (this study; Table 4B) and those calculated from the model of Blundy et al (2020) and analyzed values (this study; Table 4B).…”

“…For example, Albert et al (2020) showed that the texture of olivine phenocrysts changes in a stratigraphic sequence of erupted tephra from a monogenetic cone, corresponding to variations in rapid ascent rates via fractures to the surface. However, there is no evidence that textural disequilibrium corresponds to chemical disequilibrium, as demonstrated by the olivine-melt equilibrium experiments conducted by Pu et al (2021). In that experimental study, samples held above the liquidus and then rapidly dropped below the liquidus and held for 8-24 h, grew large, sparse olivine crystals with diffusion-limited growth textures.…”

“…Because information on melt-water contents is not always known for all basalt samples, Pu et al (2017) ), which shows no dependence on dissolved water in the melt or pressure under crustal (<1 GPa) conditions (Pu et al, 2021). Thus, the Ni thermometer provides the actual temperature at the onset of olivine growth because it requires no correction for H 2 O content.…”

“…In other words, one of our key objectives is to constrain the temperatures and water contents of the intraplate SB utilizing the new olivine-melt thermometer/hygrometer of Pu et al (2017Pu et al ( , 2021, which is based on the partitioning of Ni (D Ni ol/liq ) and is independent of dissolved water content. This overcomes the major limitation of all other thermometers applicable to basalts, including olivine-melt thermometers based on D Mg ol/liq (e.g., Putirka et al, 2007) and plagioclase-liquid thermometers (e.g., Waters and Lange, 2015), both of which require that water contents be known in advance before accurate thermometry can be conducted.…”

A new high-pressure experimental apparatus to study magmatic processes at precisely controlled redox conditions

“…Inferring the Fe 3+ /Fe 2+ ratios in the melt from the composition of silicate melt -olivine pairs require the accurate estimation of the value of , − ⁄ . The partitioning of Mg between olivine and melt was shown to be influenced by H 2 O (Putirka et al 2007;Pu et al 2017Pu et al , 2021, whereas the dissolved H 2 O content has been shown to have no significant effect on the ferric/ferrous ratio in silicate melts (Moore et al 1995;Gaillard et al 2001 concentration in the silicate melt based on the measured olivine compositions and the measured Mg concentrations in the melt. The concentration of Fe 3+ in the melt was obtained by calculating the difference between the measured total Fe in the melt and the estimated Fe 2+ concentration.…”