Garnet-spinel transition in the upper mantle: Review and interpretation.

This study carried out a comprehensive review of mantle-derived garnet peridotites on the basis of their garnet-spinel compositional relationships. The P-T estimates of the garnet peridotites of both orogenic and xenolithic derivations confirm previously established two garnet-spinel transition (GST) zones. Results of natural samples and experimental studies of all the garnet peridotites plot below the first GST at low P (<2 GPa). The second GST at high P (6-10 GPa), obtained from the experimental system (MgO-CrO-SiO), was firstly evidenced from natural samples in this study and may imply a challenge to our understanding of Cr-rich and Al-poor mineral phase transition under ultrahigh P condition. Based on the data and our estimates, we propose the existence of another GST within the garnet peridotite discontinuous zone and a potential spinel-phase peridotite zone in the upper mantle. [ABSTRACT FROM AUTHOR]