Natural evidence for garnet-spinel transition (GST) in the Earth's mantle

This study based on compiled world-wide garnet peridotite data confirms the GST in high P and T conditions, which was obtained from Al-free experimental system (MgO-Cr~2~O~3~-SiO~2~) by Klemme (2004). A new spinel-phase peridotite zone, garnet peridotite discontinuous zone, is defined, and another GST, although no experimental data, is principally and presumably proposed to exist. The garnet inclusion-bearing spinel harzburgite from Lashaine (Tanzania) provides the first evidence for the existence of ultra-high spinel zone and is explained as recrystallized minerals hosting the interacted residue of ancient oceanic lithosphere subducted into the great depth of more than 220km. These previously unexpected findings are generating great challenges to phase transition in extreme conditions and to our understanding of layered-structure of the Earth. In addition, coexistent five-phase peridotite is suggested to be naturally useful tool to constrain the GST.