Zinc Isotope Fractionation During Oceanic Crust Alteration

Plate tectonics-driven oceanic crust recycling has long been considered as the main cause of the mantle heterogeneity.During the processes from the formation of oceanic crust at the mid-ocean ridge to its recycling into the mantle,the chemical compositions of oceanic crust were continuously modified by a series of alteration.The alteration process not only affects the chemical composition of seawater,but also alters the chemical composition of the oceanic crust.The changes in the chemical composition of the oceanic crust caused by the alteration will be transmitted to the mantle through subduction and recirculation,which will influence the understanding of the evolution of mantle chemical dynamics.Therefore,the study of element migration and isotope fractionation during oceanic crust alteration is important to understand crust-mantle material circulation and the evolution of mantle chemical dynamics.Although zinc(Zn)isotope system has been proved to be an indicator for the study of deep carbon cycle and crust-mantle material recirculation,the behavior of Zn isotope fractionation in subducted oceanic crust may be affected by series of processes,including high-temperature magma process,subduction process and alteration process.Previous studies have demonstrated that the role of high temperature magmatic processes and subduction process on the Zn isotope fractionation occurs,however due to the limited size of Zn isotope database and analytical precision,the contribution of alteration to the Zn isotope fractionation is not well constrained yet.Therefore,it is necessary to further study the fractionation behavior of Zn isotopes in this process.This study is the first to investigate zinc isotope composition of altered oceanic crust based on the integrated ocean drilling program IODP1502 drilling in South China Sea.Thirty altered oceanic crust samples were chosen to be analyzed Zn isotope according to their degree of alteration,including fresh basalt,low alteration basalt,carbonate veins,epidote veins and the surrounding rock and surrounding rock,chloritization basalt,epidote veins and the surrounding rock in the pyrite veins and multiphase alteration vein and surrounding rock.The outcomes were applied to constrain the behavior in the process of Zn element and isotope in the oceanic crust alteration.The?⁶⁶Zn isotopic compositions of the altered oceanic crust range from 0.02‰to 0.38‰,with an average of 0.21‰.Carbonate veins have?⁶⁶Zn values ranging from 0.16‰to 0.38‰,which is slightly higher than the average of mid-ocean ridge basalt(MORB)(?⁶⁶Zn=0.28±0.03‰).The?⁶⁶Zn isotopic compositions of porphyrite veins(?⁶⁶Zn=0.20?0.30‰),fresh basalts(?⁶⁶Zn=0.25?0.27‰)and low altered basalts(?⁶⁶Zn=0.23?0.28‰)are similar to those of the mid-ocean ridge basalts,reflecting the limited circulation of sea water and low oxygen fluidity.Chloritization basalts(?⁶⁶Zn=0.11±0.03‰)and multistage alteration veins(?⁶⁶Zn=0.12?0.26‰)have a relative loss of ⁶⁶Zn,which may reflect the contribution from the leaching of multiple high-temperature alteration hydrothermal fluids in the late period.This study demonstrates that Zn isotopes of fresh oceanic crust are fractionated by alteration processes and such a fractionated feature may be impart into convection mantle by deep subduction.