Mg3Al2Si3O12 jeffbenite inclusion in super-deep diamonds is thermodynamically stable at very shallow Earth's depths

Abstract

Jeffbenite (having the same chemical composition of pyrope, ~ Mg₃Al₂Si₃O₁₂, and also known as TAPP phase) is a mineral inclusion only found in diamonds formed between about 300 and 1000 km depth) and is considered a stable phase in the transition zone (410-660 km depth) and/or in the shallowest regions of the lower mantle (around 660-700 km depth). This rare and enigmatic mineral is considered to be a pressure marker for super-deep diamonds and therefore it has a key role in super-deep diamond research. However, the pressure-temperature stability fields for Mg₃Al₂Si₃O₁₂ jeffbenite is unknown and its actual formation conditions remain unexplored. Here we have determined the thermodynamic pressure-temperature stability field for the jeffbenite Mg-end member and surprisingly discovered that it is stable at low pressure-temperature conditions, i.e., 2-4 GPa at 800 and 500 °C. Thus, Mg₃Al₂Si₃O₁₂ jeffbenite is not the high-pressure polymorph of pyrope and is likely a retrogressed phase formed during the late ascent stages of super-deep diamonds to the surface.