Supercritical Fluids And Their Geochemical Effects In Continental Subduction Zones:constraints From A UHP Eclogitic Vein And Its Host Eclogites In The Dabie Orogen

Fluid activity in subduction zones is one of the frontiers and hotspots of Solid Earth Sciences.Fluids released from the subducting slab would transfer not only fluidmobile elements but also fluid-immobile elements and multivalent elements into the mantle wedge,leading to the chemical heterogeneity in the mantle wedge that is eventually recorded by mafic arc magmas.Supercritical fluids are ideal media for mass transfer from the subducting slab into the mantle wedge because of their high solubility of elements and low viscosity.The study of supercritical fluids is important to understand the processes and mechanisms of crust-mantle interaction,the compositions of arc magma,and the recycling of crustal materials at convergent plate margins.The Dabie-Sulu orogenic belt is one of the largest ultrahigh-pressure(UHP)metamorphic terranes on Earth.It provides an ideal natural laboratory to study the supercritical fluids and their geochemical effects in subduction zones.This PhD thesis presents a combined study of petrology and geochemistry for a coesite-bearing eclogitic vein and its surrounding eclogites from the Bixiling complex in the Dabie orogen.The results are used to decipher the composition and source of supercritical fluids,the P-T conditions and timing of their formation,and their geochemical effects in the continental subduction zone.A combined study of petrology,geochronology and element geochemistry was carried for the eclogitic vein and surrounding eclogites.The results provide new insights into the property of supercritical fluid and their Nb-Ta effect in the subduction zone.The eclogitic vein is composed of garnet,omphacite,quartz,amphibole,rutile and apatite,with minor amounts of epidote,plagioclase and zircon.Coesite was identified in omphacite in the vein,demonstrating its formation under UHP metamorphic conditions that correspond to subarc depths in oceanic subduction zones.Zircons from the vein give concordant U-Pb ages of 225±5 Ma and flat HREE patterns for newly grown rims,consistent with their crystallization at the UHP eclogite facies.The vein shows similar Hf-O isotope compositions to the host UHP eclogites,indicating that the UHP vein-forming fluid is internally buffered within the UHP eclogites.Minerals in the vein contain not only higher contents of Cr,Ni,Sr,REE and HFSE,but also multiphase crystal inclusions(such as omphacite,quartz,epidote,apatite,amphibole,plagioclase,mica,rutile,calcite,and anhydrite)as well as liquid and gas phases of H2O.These observations indicate that the eclogitic vein was formed by supercritical silicate-rich fluids rather than aqueous solutions and hydrous melts.The recovered major element compositions for the supercritical fluids are 31 wt.%SiO2,20 wt.%CaO,10 wt.%(SO4)2-,8 wt.%(CO3)2-,8 wt.%FeO,7 wt.%Al2O3,and 9 wt.%H2O,with traces of Na2O,K2O,MgO and TiO2.The P-T-t path of the UHP eclogitevein system indicates that the second critical endpoint of the basalt-H2O system is located close to 3.4 GPa and 770℃.Quantitative modelling shows that the Nb/Ta ratios of supercritical fluids in equilibrium with UHP eclogites are 22.8-40.3.These results,in line with the rutile crystals with high Nb/Ta ratios of 21.2-29.5 in the vein,indicate that dehydration of subducting eclogites at subarc depths can produce supercritical fluids with suprachondritic Nb/Ta ratios.The UHP eclogites and mantle wedge peridotites metasomatized by such supercritical fluids can acquire suprachondritic Nb/Ta ratios and thus provide a complementary reservoir to balance the subchondritic reservoirs on Earth.Based on statistics analyses of Nb-Ta data,two indices are proposed to identify the possible existence of supercritical fluids in fossil subduction zones:(1)rutile crystallized from supercritical fluids shows lower Nb contents than that from aqueous solutions and hydrous melts;(2)UHP eclogites associated with supercritical fluids exhibit a distance>0.1 in their Nb-Ta compositions to the line defined by basalts in the plot of log[Nb]vs.log[Ta].Therefore,the existence of supercritical fluids during UHP metamorphism at subarc depths can be recognized from their effect on the mobility of fluid-immobile trace elements in UHP metamorphic rocks.A comprehensive study of petrology and geochemistry was carried out for the eclogitic vein and surrounding eclogites.The results reveal the redox state of supercritical fluid in the subduction zone.Multiphase crystal inclusions trapped within minerals in the vein contain highly oxidized daughter minerals including calcite,anhydrite and magnetite,suggesting that the supercritical fluids contain abundant oxidizing components including S6+,C4+ and Fe3+.Garnet-clinopyroxene oxybaroneter,mineral V portioning,apatite Eu and Ce anomalies as well as whole-rock compositions indicate that the metasomatism of supercritical fluid results in the elevation of fO2 values in the eclogites from FMQ-3.40～FMQ-3.31 to FMQ-0.08～FMQ+1.86.The eclogites metasomatized by supercritical fluids exhibit lower Fe and S contents and Fe3+/Fetotal ratios as weil as decreased modes of omphacite and pyrite.These observations suggest that the supercritical fluids dissolve omphacite and pyrite accompanied by a Fe-S redox coupling process in eclogites during the fluid-rock reaction.This process further releases both ferric iron and sulfate from eclogite to the supercritical fluids.Profiles of multivalent elements in megacryst minerals in the vein indicate that the supercritical fluids are oxidized with fO2 values ranging from FMQ+1.03 to FMQ+1.30 during their evolution.The preferential precipitation of garnet during the transportation of the supercritical fluids results in the elevation of their Fe3+/Fe ratios and oxygen fugacities.In combination with the occurrence of sulfurbearing minerals in the vein,this observation suggests that sulfur is likely transported farther by the supercritical fluid.Our results demonstrate that the supercritical fluids are oxidizing and can oxidize the metasomatized rocks.The supercritical fluids are thus efficient carriers of oxidized components from the slab to the mantle wedge,eventually contributing to the oxidization of the arc magma source.Whole-rock and mineral Mg-Fe isotope analyses were carried out for the eclogitic vein and its surrounding eclogites.The eclogites metasomatized by the supercritical fluid have lighter Mg isotopic compositions of-0.44 to-0.37‰ but slightly heavier Fe isotopic compositions of 0.09 to 0.13‰ compared to those distant to the vein(δ26Mg values of-0.29 to-0.24‰ and δ56Fe values of 0.01 to 0.11‰).This observation indicates Mg-Fe isotope fractionation during fluid-rock interaction due to preferential dissolution of isotopically heavy omphacite from the eclogites into the vein-forming fluids.The vein has heavier Mg-Fe isotopic compositions compared to the eclogites.The metasomatism of supercritical fluid results in the elevation of δ56Fe and δ26Mg values in garnet and omphacite in eclogites.These observations suggest that the supercritical fluids are enriched in heavy Mg and Fe isotope.Based on the Mg-Fe isotope compositions of garnet and omphacite in the metasomatized eclogites and the Mg-Fe isotopic equilibrium fractionation coefficient between the corresponding minerals and the UHP fluids,the supercritical fluids are recovered to have heavier MgFe isotope compositions with δ26Mg values of 0.30-0.37‰ and δ56Fe values of 0.340.49‰ than aqueous solutions and hydrous melts.Two endmember mixing modelling was further carried out to evaluate the effect of different types of fluids on the Mg-Fe isotope compositions of the mantle wedge peridotites.The simulation results show that the metasomatism of supercritical fluids has great influence on the Fe isotopic composition of the mantle wedge peridotite.Therefore,the supercritical fluid has significant contribution to the mantle source of the arc magma with heavy Fe isotope composition.In-situ zircon Zr isotope analyses were carried out on the eclogites.In combination with the other geochemical analyses,the results reveal that the newly grown zircon rims have significantly elevated Zr isotope compositions of-0.92 to 0.36‰ compared to the relict igneous cores(-7.73 to 0.40‰).A positive correlation is observed in the Zr isotope compositions between the rim and core from the same zircon,suggesting that the Zr isotope compositions of the metamorphic rims are predominantly dependent by the cores.Across the relict igneous cores,δ94Zr values usually show a U-shape profile.This observation,in line with the correlation between the Zr/Hf ratios and the 894Zr values of the relict igneous cores,suggests that the magmatic zircon in the eclogites have a Zr isotopic compositions with light δ94Zr values in the core but heavy δ94Zr values in the rim.Thus,the preferential dissolution of the igneous zircon rim with higher δ94Zr values into the fluids under UHP metamorphism results in the elevated Zr isotope compositions in the newly grown zircon grains.These findings and the simulated results reveal that metamorphic zircons are mainly formed by the dissolution of pre-existing magmatic zircons through fluid activities and the participation of the other Zr-bearing minerals.Diffusion-driven Zr isotope fractionation during fractional crystallization of the protolith magma led to the highly variable and extremely negative Zr isotope composition in the relict magmatic cores.Therefore,zircon Zr isotopes in metamorphic rocks can be used as a geochemical index to identify the magma crystallization process of the protoliths of their host rocks.The Zr isotopic composition of metamorphic zircons are significantly heavier than the protolith magmatic zircons in metamorphic rocks which are formed from magmatic rocks with significant fractionation crystallization.