BRIDGING TWO CONTINENTS: EVIDENCE OF CONTINENTAL CRUST NEAR THE MANTLE TRANSITION ZONE FROM THE LUOBUSA PODIFORM CHROMITITES

Podiform chromitites in the Luobusa ophiolite (S. Tibet) receive much attention due to a wide range of unusual mineral separates, including (1) UHP minerals (diamond, coesite, moissanite etc.), (2) highly reduced minerals (native elements, carbide, nitride, alloys), and (3) crustal minerals (quartz, kyanite, zircon etc.). To deduce the long history of upper-mantle convection (mantle-upwelling, horizontal movement and subduction), micro- and/or nano-scale mineral inclusions preserved in refractory minerals potentially provide mineralogical evidence of ancient information. Recent discovery of in-situ diamond in chromite proved a diagnostic UHP evidence and unusual coesite and clinopyroxene exsolutions in chromite are possible evidence of former CF-type high-pressure polymorph of chromite (>380 km deep). However, occurrence of crustal minerals from podiform chromitite still remains a subject of debate.

In this study, we propose that crustal minerals recovered from the Luobusa chromitite were evidence of subducted continental material, probably accumulated near the mantle transition zone. In general, continental crust is predominantly created at a subduction zone and is simultaneously returned to the mantle via sediment subduction, tectonic erosion, and arc subduction. The density calculations under the extreme conditions suggest that there can be two reservoirs of granitic material in the Earth, one on the surface and the other at the base of the mantle transition zone. Therefore, the recovered crustal minerals from the Luobusa chromitites could be an evidence of granitic material accumulated at the mantle transition zone.

This work was supported by the Japan Society for the Promotion of Science.