The Behavior And Fractionation Mechanism Of Calcium Isotopes During Plate Subduction Process

The subduction zone is an essential way for exchange of materials between the surface and interior of the earth.Recycling of basaltic oceanic crust and carbonates in subduction zone is an important mechanism for mantle heterogeneity.To better understand the Ca isotopic behavior and fractionation mechanism during slab subduction,we made a detailed study of Ca isotopes of meta-basaltic rocks and UHP(ultra-high pressure)marbles from the Dabie-Sulu orogen.The major achievements of this dissertation are summarized below.(1)This study analyzed and compared Ca isotopes of a suit of prograde meta-basaltic rocks with similar protoliths and age from the Dabie-Sulu orogen,including greenschists,amphibolites and eclogites,to investigate the behavior of Ca isotopes in meta-basaltic rocks during subduction.Except for two greenschists which suffered from secondary alteration have heavier ?44/42Ca,most greenschists(0.36 to 0.41‰),amphibolites(0.32 to 0.39‰)and eclogites(0.32 to 0.42‰)have comparable ?44/42Ca values,plotting on the range of terrestrial basalts previously reported(0.36±0.05‰,2SD;N=34).There is no correlation between ?44/42Ca and dehydration indicators(H2O and Rb/TiO2),suggesting that prograde metamorphic dehydration cannot significantly fractionate Ca isotopic compositions of meta-basaltic rocks.Considering that the?44/42Ca values of terrestrial basalts are systematically lower than that of the upper mantle,subduction of mafic lithologies(e.g.,oceanic slabs)could reflux light Ca isotopes back into the mantle and result in mantle heterogeneity(2)In order to reveal the behavior of Ca isotopes in carbonates during subduction,the weak metamorphic Sinian sedimentary carbonates,Rongcheng UHP marbles and carbonated eclogites have been studied.The ?44/42Ca values of weak metamorphic Sinian sedimentary carbonates(0.33-0.47‰)and most carbonated eclogites(0.37-0.45‰;N=6/7)show narrow variations.Rongcheng UHP marbles have a large range of ?44/42Ca,from 0.19 to 0.46‰,with 844/42Ca positively correlated with mole Mg/Ca and negatively correlated with CaO and ?26Mg.The Ca isotopic compositions of Rongcheng UHP marbles cannot be explained by protolith inheritance,chemical diffusion or isotopic exchange between silicate-carbonate,but should reflect the selective dissolution of carbonate under high pressure.This process could result in:(i)isotopically lighter Ca isotopic compositions for the undissolved recycled Ca-carbonatites(down to 0.19 ± 0.04‰),while a limited Ca isotope fractionation for recycled Mg-carbonatites,(ii)up to 95%carbon could be released to surface through fluids,(iii)the ?44/42Ca of fluid cannot be distinguished from the mantle.