Oxygen isotope composition of quartz-vein in ultrahigh-pressure eclogite from Dabieshan and implications for transport of high-pressure metamorphic fluid

The oxygen isotope composition of minerals from quartz-veins and host-eclog ites in Dabieshan, China was measured in order to place geochemical constra ints on the origin and transport of high-pressure metamorphic fluids. The r esults, along with structural and petrological relationships between vein a nd wallrock, show that the quartz veins are the high-pressure metamorphic a nd thus formed prior to eclogite-facies recrystallization when they were ex humated from mantle depths to deep crustal levels. Not only is the oxygen i sotope composition of the vein-quartz identical to that of the host-eclogit e, but in addition the oxygen isotope geothermometry of mineral-pairs from the quartz-veins yield temperatures that are close to the eclogite-facies t emperatures. Therefore, the vein-forming fluid was likely derived from the local host-eclogites by the exsolution of dissolved hydroxyls within eclogi te minerals due to significant pressure decrease. Local advective transport of fluid is the predominant mechanism in the processes of vein precipitati on. Fluid flow prior to the eclogite-facies recrystallization may occur mai nly along pressure gradients. The loss of the UHP or HP fluid at the differ ent depths during exhumation may be the potential cause for concordant and discordant isotope temperatures between different mineral-pairs in the eclo gites. (C) 2001 Elsevier Science Ltd All rights reserved