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The application of the olivine–spinel aluminum exchange thermometer to natural samples is limited by the restricted experimental dataset on which it was calibrated. Here, we present a new dataset of 46 high-temperature crystallization experiments and 21 reanalyzed published experiments, which we used to extend the calibration to higher and lower temperatures. The final calibration dataset spans a range of conditions relevant to crustal and upper mantle processes: 1174–1606 °C, 0.1–1350 MPa, QFM−2.5 to QFM+7.2 (oxygen fugacity, fO2, reported in log units relative to the quartz–fayalite–magnetite buffer, QFM), and 0–7.4 wt.% H2Omelt. We propose three new models. The first is thermodynamically self-consistent, based on spinel Fe, Mg, Al, and Cr compositions and Al exchange between olivine and spinel. The second and third are empirical models that consider fewer elemental exchanges: the second uses only Al exchange and spinel compositions, whereas the third considers olivine–spinel Al and Cr exchange. All models include the modest effect of pressure on olivine-spinel equilibrium chemistry, whereas fO2 and water content have negligible effects. In general, as fewer elements are considered in the olivine–spinel exchange, the fit to experimental data worsens. Conversely, the associated decrease in model complexity improves their robustness against systematic errors when applied to natural crystal pairs: the thermodynamic model may underestimate crystallization temperatures in natural samples due to spinel subsolidus re-equilibration, whereas the empirical models (independent of Fe and Mg in spinel) are less sensitive to re-equilibration but yield temperatures with larger uncertainties. We applied a statistical test to select the most appropriate model for application to natural samples. When applied to lavas from mid-ocean ridges, Iceland, Skye, Emeishan, Etendeka, and Tortugal, our new temperature estimates are 30–100 °C lower than previously proposed. The lower temperature estimates cause a lower mantle melting temperature and significant impacts on the mantle lithology constraints.
The application of the olivine–spinel aluminum exchange thermometer to natural samples is limited by the restricted experimental dataset on which it was calibrated. Here, we present a new dataset of 46 high-temperature crystallization experiments and 21 reanalyzed published experiments, which we used to extend the calibration to higher and lower temperatures. The final calibration dataset spans a range of conditions relevant to crustal and upper mantle processes: 1174–1606 °C, 0.1–1350 MPa, QFM−2.5 to QFM+7.2 (oxygen fugacity, fO2, reported in log units relative to the quartz–fayalite–magnetite buffer, QFM), and 0–7.4 wt.% H2Omelt. We propose three new models. The first is thermodynamically self-consistent, based on spinel Fe, Mg, Al, and Cr compositions and Al exchange between olivine and spinel. The second and third are empirical models that consider fewer elemental exchanges: the second uses only Al exchange and spinel compositions, whereas the third considers olivine–spinel Al and Cr exchange. All models include the modest effect of pressure on olivine-spinel equilibrium chemistry, whereas fO2 and water content have negligible effects. In general, as fewer elements are considered in the olivine–spinel exchange, the fit to experimental data worsens. Conversely, the associated decrease in model complexity improves their robustness against systematic errors when applied to natural crystal pairs: the thermodynamic model may underestimate crystallization temperatures in natural samples due to spinel subsolidus re-equilibration, whereas the empirical models (independent of Fe and Mg in spinel) are less sensitive to re-equilibration but yield temperatures with larger uncertainties. We applied a statistical test to select the most appropriate model for application to natural samples. When applied to lavas from mid-ocean ridges, Iceland, Skye, Emeishan, Etendeka, and Tortugal, our new temperature estimates are 30–100 °C lower than previously proposed. The lower temperature estimates cause a lower mantle melting temperature and significant impacts on the mantle lithology constraints.
The Nyiragongo volcano is one of the most alkali-rich volcanic centres on the planet (Na2O + K2O generally >10 wt.%, agpaitic index up to 1.34), characterized by a semi-permanently active lava lake which hosts silica-undersaturated (SiO2 <40 wt.%), low viscosity lavas. To improve our understanding of this unique magmatic system, we present a set of 291 samples, acquired during new field excursions between 2017 and 2021. The major and trace element composition of all samples was measured, revealing a lithological range extending from primitive picrites (Mg# 82) erupted from parasitic cones to a variety of highly evolved nephelinites, leucitites, and melilitites erupted from the main edifice as recently as 2002, 2016, and 2021. We measured major and trace element compositions from the full spectrum of minerals present in all sampled lithologies. From these we calculated that the main magma reservoirs feeding Nyiragongo are at approximately 9 – 15 and 21 – 33 km depth, in agreement with recent seismic observations. Fractional crystallization modelling using observed mineral compositions and proportions was performed to quantitatively link the lithologies to specific residual liquid fractions assuming evolution from an olivine-melilite parental melt. Our modelling indicates that fractionation cumulate formation in deep chambers reduces the melt fraction remaining to ~60%, after which melts are injected into upper, liquid dominated magma chambers where fractionation and accumulation of clinopyroxene, melilite, and feldspathoids dominate. Characterisation of mineral textures and geochemistry reveals high crystal mobility in a repeatedly recharging plumbing system split between liquid-dominated, evolved magma chambers and more solid-dominated, primitive mushes, decreasing in liquid fraction with depth.
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