Silicon isotopic fractionation during metamorphic fluid activities: constraints from eclogites and ultrahigh-pressure veins in the Dabie orogen, China

Abstract

To understand Si isotope fractionation during metamorphic fluid activities in the subduction zone, this study presents the Si isotopic compositions of two high- to ultrahigh-pressure (HP–UHP) eclogite–vein systems from the Ganghe and Hualiangting areas in the Dabie orogen, eastern China. The results show that Si isotopes are significantly fractionated between the veins and their host eclogites in both areas. The δ30Si values of Ganghe eclogites range from −0.50‰ to −0.39‰, higher than that of the studied omphacite−epidote vein (−0.63 ± 0.07‰). The Hualiangting eclogites have δ30Si values of −0.36‰ to −0.29‰, whereas the δ30Si values of the Hualiangting multi-stage veins show greater variation, from −0.45‰ to +0.05‰, revealing significant Si isotope fractionation during metamorphic fluid evolution and vein formation.For the Hualiangting and Ganghe samples, the enrichment of heavy Si isotopes in minerals follows the order of δ30Siphengite (−0.01 to +0.13‰) ≈ δ30Siquartz (−0.14 to +0.10‰) > δ30Siomphacite (−0.63 to −0.33‰) ≈ δ30Siepidote (−0.60 to −0.30‰) ≈ δ30Sikyanite (−0.42 to −0.28‰) > δ30Sigarnet (−0.92 to −0.44‰). The Si isotope fractionation between metamorphic minerals was generally consistent with the equilibrium Si isotope fractionation factors calculated by the first-principles methods. These results suggest that the vein minerals are likely in Si isotopic equilibrium with each other.The δ30Si of the Hualiangting veins are linearly correlated to SiO2 contents with a steeper slope than that of the magma differentiation trend. By also considering the mineralogy of the veins, we conclude that this linear relationship reflects the sequential variation of mineral composition on the Si isotope signature of the veins. The SiO2 content and δ30Si of veins increased with continuing crystallization from the fluid, and the δ30Si of the fluid also increased. The results suggest that Si isotope data can be used to constrain element recycling and metamorphic fluids activities in the subduction zones.