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A Novel Thermobarometer to Infer Mantle Melting Conditions

A Novel Thermobarometer to Infer Mantle Melting Conditions

Basaltic melts erupting on the Earth’s surface preserve information on the temperature and pressure of the Earth’s upper mantle in the region where they form at several tens of kilometers mantle depth. The retrieval of the mantle temperatures – a key parameter to understanding mantle flow – is complicated, however, because the basaltic melts cool en route to surface and modify through crystallization of several mineral phases, such as olivine ± plagioclase ± clinopyroxene in variable extent and proportions. The accuracy of upper mantle temperatures thus critically depends on how well the individual crystallization paths can be reversed.

Krein et al. [2021] present a sophisticated automated algorithm named ‘ReversePetrogen’ (RevPET) that return erupted basaltic melts to ‘primary’ melts (that is melts in equilibrium with mantle prior to crystallization) by back-adding the most plausible mineral assemblage lost. Application of RevPET to existing large data sets of basaltic melts (n=13,589) from the global mid-ocean ridge system retrieves viable primary melts and associated mantle temperatures and pressures, for 72 percent of the data. The robustness of RevPET is validated through general consistency with earlier studies that predict comparable ranges of apparent mantle melting temperatures (Tp* = 1322°C ± 56°C) beneath the mid-ocean ridges.

While this result is remarkable, the true asset of the RevPET algorithm is that it allows to explore the interplay between basalt composition and the inherent mantle heterogeneity, melting mode and temperature. Thus, the RevPET tool is valuable for designing future experimental and observational studies that explore the thermal state of an inaccessible region of Earth.

Citation: Krein, S. B., Molitor, Z. J., & Grove, T. L. [2021]. ReversePetrogen: A Multiphase dry reverse fractional crystallization-mantle melting thermobarometer applied to 13,589 mid-ocean ridge basalt glasses. Journal of Geophysical Research: Solid Earth, 126, e2020JB021292.

—Susanne Straub, Associate Editor, JGR: Solid Earth

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