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

Abstract

The oxygen isotope composition of minerals from quartz-veins and host-eclogites in Dabieshan, China was measured in order to place geochemical constraints on the origin and transport of high-pressure metamorphic fluids. The results, along with structural and petrological relationships between vein and wallrock, show that the quartz veins are the high-pressure metamorphic and thus formed prior to eclogite-facies recrystallization when they were exhumated from mantle depths to deep crustal levels. Not only is the oxygen isotope composition of the vein-quartz identical to that of the host-eclogite, but in addition the oxygen isotope geothermometry of mineral-pairs from the quartz-veins yield temperatures that are close to the eclogite-facies temperatures. Therefore, the vein-forming fluid was likely derived from the local host-eclogites by the exsolution of dissolved hydroxyls within eclogite minerals due to significant pressure decrease. Local advective transport of fluid is the predominant mechanism in the processes of vein precipitation. Fluid flow prior to the eclogite-facies recrystalization may occur mainly along pressure gradients. The loss of the UHP or HP fluid at the different depths during exhumation may be the potential cause for concordant and discordant isotope temperatures between different mineral-pairs in the eclogites. © 2001 Elsevier Science Ltd. All rights reserved.