The Spinel‐Garnet Phase Transition in the Earth's Upper Mantle: A First‐Principles Study

Abstract In this study using a combination of first‐principles density functional theory and quasiharmonic approximation (QHA) we have investigated the influence of various relevant dopants, namely, Fe, Cr and Ca on the stability of spinel with respect to garnet under upper mantle pressure and temperature conditions. Our calculations are specifically directed toward establishing the phase boundary at lower temperature conditions (<900°C), a temperature range which despite being critical for accurate geodynamical modeling of the lithospheric mantle is found to remain insufficiently explored. The Clapeyron slope at low temperatures, irrespective of the chemical composition, is always found to be negative. We find that Fe 2+ supresses, whereas Fe 3+ enhances the phase transformation pressure, at a given temperature. Incorporation of Cr on the other hand is found to “significantly” enhance the stability field of spinel. By combining our results with previous experimental studies, we conclude that the Clapeyron slope of the spinel‐garnet phase transformation changes sign at high temperatures in the absence of Cr, whereas it stays negative in the presence of Cr at all temperatures of interest. The contrasting behavior of the Clapeyron slope of the phase transition boundary in the presence/absence of Cr may initiate delamination of the lower parts of the mantle lithosphere, causing enhanced domal uplift during continental rifting. Our analysis indicates that the Lehmann discontinuity observed in deeper parts of the upper mantle can be satisfactorily explained by the transformation of Cr‐doped spinel to garnet.