Silicon Isotopic Compositions Of Altered Oceanic Crust Of IODP U1365 And U1368

As the third most abundant element in the earth,silicon is involved in most geological process.Therefore,silicon isotope has been widely used in different fields of geochemistry as an available tool,which can be used to trace crust-mantle interactions and continental weathering process as well as the origin and evolution of extraterrestrial objects,etc.Subduction is an important process for material exchange between the earth’s crust and mantle.During subduction process,fluids or melts derived from the subducted slab and the residual subduction plate reacted with the surrounding mantle,which could significantly change the elemental and isotopic composition of the mantle.As the top layer of the subducting slab,the elemental and isotopic composition of the altered oceanic crust could affect the chemical composition and evolution of the mantle.Therefore,it’s important to determine the silicon isotopic compositions of the altered oceanic crust for evaluating the behavior of silicon isotope during subduction and the subsequent interaction process.In this study,we analyzed the isotopic compositions of altered oceanic crust samples from IODP Sites U1365 and U1368,where samples from U1365 have undergone more extensive alteration than U1368.These samples from both drills exhibit distinct alteration features,which makes them an excellent choice for representing material composition of the oceanic crust after alteration.To better evaluate the effect and quantify the magnitude of seawater alteration on the isotopic compositions of the oceanic crust,we analyzed altered-fresh sample pairs in different sections from two sites,which were subsampled from either an individual piece of a fragment or from the identical lithologic unit.The results show that the δ30Si values of both fresh and altered samples from U1368 are in the range of fresh MORB,except for one pair with slightly lower δ30Si(-0.45 ±0.04‰ of fresh sample and-0.39 ± 0.03‰ of altered sample).The δ30Si of fresh oceanic crust samples from U1365(-0.32‰～-0.24‰)are also in the range of fresh MORB(-0.27 ±0.06‰)while the altered samples from the same site display large variation on δ30Si(-0.58‰ to-0.27‰).Although there is no strong correlation between δ30Si and K2O/TiO2,LOI,Rb/TiO2,Cs/TiO2 and Ba/TiO2 ratios,samples with lower δ30Si values generally have high K2O/TiO2,LOI,Rb/TiO2,Cs/TiO2 and Ba/TiO2 ratios,which indicates that the seawater alteration can modify the silicon isotopic composition of a portion of oceanic crust and the silicon isotopic composition of the oceanic crust will become lighter with increasing of the degree of alteration.The results of this study can be used to constrain the silicon isotopic composition of the altered oceanic crust and provide a foundation for studying whether subduction materials can modify the silicon isotopic composition of the mantle.