The Behavior And Property Of Metamorphic Fluids In A Paleotethyan Oceanic Subduction Zone:Constraints From Eclogites In The Hong’an Orogen

Fluid activity in subduction zones is one of the frontiers and hotspots of Solid Earth Sciences.Water released from the subducting slab would not only transfer fluid-incompatible trace elements from the slab to the mantle wedge but also reduce the mantle solidus and catalyze melting,leading to arc magmatism and crustal growth.In this classical model,the crucial problems that remain unresolved are the location and mechanism of the fluid released by the subducting plate and the amount of water that can be recycled into the deep mantle.Trace amount of water can have a significant effect on many physical and chemical properties of minerals(even rocks and melts).Therefore,understanding the release of water(fluid)at different depths of the slab and its geochemical effects is of great significance to the study of volatile cycling and mass transfer in subduction zones.The geochemical behavior of variable valence elements such as carbon,nitrogen,sulfur,hydrogen,and iron will significantly affect the redox properties of the mantle wedge during slab-mantle interactions,and the behavior of these variable valence elements is obviously controlled by oxygen fugacity.Therefore,understanding the oxygen fugacity of fluid released during plate subduction is crucial to understanding the element mobility and crust-mantle interaction in subduction zones.Metamorphic rocks in subduction zones underwent compicated metamorphic processes.The record the recycling of subducted crustal materials,and thus provide us an excellent oppertunity to study the fluid activity and element mobolity in subduction zones.As one of the most common minerals in metamorphic rocks in subduction zones,garnet has stable physical and chemical properties.The major elements,trace elements and water contents as well as the inclusions of garnet can effectively record the changes of physical and chemical conditions during subduction zone metamorphism.Therefore,a detailed study of the geochemical characteristics of garnet can provide key information on the metamorphic evolution of its host rocks and even the metamorphic processes of subduction zones.The key is to select typical garnet samples recording multistage metamorphic processes and accurately determine its water content as well as element compositions.In doing so,the accurate determination of garnet water content is dictated by the reliable infrared absorption coefficient of hydroxyl group in garnet.In this PhD thesis,14 natural gem-grade garnets with different composition were studied to obtain the infrared absorption coeficient,and a systematic study was further carried out on granets of multistage growth in eclogite from the Hong’an orogeny.The results provide robust constraints on the behavior and property of metamorphic fluids in the Paleo-Tethyan oceanic subduction zone.Accurate determination of water content in garnet is critical to quantify the transport of water to the deep mantle by the subducted oceanic crust beyond the breakdown of hydrous phases.Fourier transform infrared spectroscopy(FTIR)is the most widely used approach to determine the species and contents of water in garnet.Accurate quantification of OH in garnet requires independent calibration using an external method,as OH absorbance is mineral and composition specific.To obtain the infrared absorption coefficients of structural hydroxyl in garnet,a combined study of spectrometric analyses by FTIR and a method combining a thermal conversion elemental analyzer with isotope ratio mass spectrometry(TC/EA-MS)was carried out for 14 natural gem-quality garnet crystals with variable compositions.The obtained molar absorption coefficients are 9322±338 and 240±26 l·mol-1·cm-2 for grossularand spessartine-rich garnet and pyrope-almandine garnet,respectively.These results are within the range of previous studies.A new molar absorption coefficient of 689±177 l·mol1·cm-2 is obtained for pyrope-spessartine garnet.The large variation in the absorption coefficient indicates its control by both garnet composition and OH-absorption bands.The obtained absorption coefficients can only be appropriate for certain types of eclogitic garnet.More studies should be carried out on eclogitic garnet.The metamorphic fluid release from the subducting crust is of great importance to understand recycling of volatiles at convergent plate boundaries.Garnet in metamorphic rocks can retain chemical zonings and crystal inclusions produced during subduction,providing insights into the geochemical recycling in subduction zones.This PhD thesis presents a combined study of mineralogy and geochemistry for garnets in oceanic-type eclogite from the Paleotethyan subduction zone in the Hong’an orogen,east-central China.The results show distinct zones of major and trace elements,crystal inclusions,water contents and oxygen fugacity.Five stages of garnet growth were recognized based on the garnet element compositions and phase equilibrium modelling.Stage Ⅰ to Ⅳ garnet growth occurs during prograde subduction of the Paleotethyan oceanic crust from blueschist-to eclogite-facies along low geothermal gradients of 7.8-9.7℃/km.Stage Ⅴ garnet was formed through dissolution and reprecipitation during mineral-fluid reaction in the early exhumation stage.Garnet exhibits a decrease of water contents from core to rim,indicating that the bulk water content of its host eclogite is gradually decreased during the growth of garnet.Combining with the theoretical calculation based on the mineral mode and mineral water contents,it is suggested that the subducting oceanic crust would release decreased amounts of fluid from blueschist-to eclogite-facies metamorphism.All of the different garnet growth zones have fO2 values ranging from ΔFMQ+1.77 to ΔFMQ+3.26,suggesting that the fluid released from the subducting oceanic crust is oxidizing during subduction to the peak eclogite-facies metamorphism and the early exhumation stage.These garnet fO2 values are correlated with water contents for water-saturated garnet,implying that garnet water content is a potential tool to recover the redox state of its host eclogite.The increase of fO2 values during prograde metamorphism of the eclogite indicates the breakdown of chlorite and possible fluid influx from the slab serpentinite,whereas the significant drop of fO2 values during the early exhumation stage indicates the breakdown of lawsonite to release the oxidized fluid.This study demonstrates that a combined study of water content,major and trace elements and mineral inclusions in garnet can potentially provide a new perspective for intensity and redox state of fluid released from the subducting oceanic crust in the fossil oceanic subduction zone.