Discovery of clinoenstatite in the Luobusa ophiolitic mantle peridotite recovered from a drill hole, Tibet

Clinoenstatite is an uncommon mineral, and has been documented in Sulu ultrahigh-pressure (UHP) garnet pyroxenite. However, clinoenstatite (CLEN) was rarely reported in the previous studies of ophiolitic mantle peridotites. The studied ophiolitic peridotites are recovered from a drilling hole of the Luobusa ophiolitic massif, Tibet that is unique in occurrences of super reducing UHP minerals in both chromitite and peridotite. Our petrographic and TEM studies reveal abundant CLEN as lamellae associated with diopside exsolution in orthoenstatite (OREN) from the Luobusa mantle peridotite. In addition, termination of CLEN lamellae and Guinier-Preston zones (nanometer-scale precipitates with compositions between enstatite and diopside) are also observed in the OREN. Both CLEN lamellae and host OREN have nearly identical (Mg0.9Fe0.1)2Si2O6 compositions. The CLEN lamellae in host OREN from two core samples have even numbers of 9 Å (1 0 0) fringes without twins, and are oriented parallel to (1 0 0) of the host OREN. The crystallographic data of the CLEN are d100 = 9.06 Å (R1), d020 = 4.34 Å (R2), d120 = 4.03 Å (R3), R1∠R2 = 90°, R1∠R3 = 64° and R2∠R3 = 26°, whereas the host OREN have d100 = 18.1 Å (R1), d020 = 4.48 Å (R2), d120 = 4.34 Å (R3), R1∠R2 = 90°, R1∠R3 = 77° and R2∠R3 = 13°. No antiphase domain boundaries are observed in TEM images; it probably indicates that the clinoenstatite is the low-P polymorph of clinoenstatite (LCLEN) with P21/c symmetry. The exsolution of diopside and the presence of Guinier-Preston zones are attributed by cooling of primary high-T Ca-bearing (very minor) OREN. The transformation of LCLEN from OREN may have occurred mainly by cooling along the P-T boundaries between the OREN and LCLEN stability fields; shear stress only enhances the transition. The occurrence of CLEN lamellae in OREN and the existence of super reducing UHP minerals in peridotite provide an additional constraint for a very deep mantle origin of the Luobusa peridotite.