Silicon Isotopes Study Of Yimaguan Loess-paleosol And Metaperidotites From Franciscan Complex

As the major element of the crust and the mantle,silicon participates in most geological processes.Therefore,silicon isotope has been widely studied,covering lithosphere,hydrosphere,biosphere,etc.,and has made a lot of research achievements in the field of high temperature and low temperature geochemistry.In this paper,I have carried out two investigations using silicon isotopes,namely,the study of the Yimaguan loess-paleosol profile and the metaperidotites from the Franciscan Complex of California.To investigate the sedimentary process of loess-paleosol sequences and constrain the silicon isotope compositions of the upper continental crust(UCC),this study analyzed the silicon isotopes of loess and paleosol samples from Yimaguan,Gansu Province,China.With the increase of depth,the SiO2/TiO2 and SiO2/Al2O3 values of Yimaguan loess-paleosol profile are negatively correlated with Xfd and CIA.The whole profile has relatively homogeneous silicon isotope compositions,with ?30Si values range from-0.28‰ to-0.16‰.This limited variation of silicon isotopes indicates that the chemical weathering degree of the profile is not significant.The negative correlations between SiO2/TiO2,SiO2/Al2O3 and ?fd,CIA are mainly controlled by the grain size sorting effect,which was dominated by the variation of winter monsoon intensity.Based on silicon isotope analysis results of loess-paleosol samples from Yimaguan,combined with the silicon isotope data of loess from other regions of the world,the average ?30Si value of the UCC is constrained at-0.22±0.06‰(2SD),lightly heavier than the lower continental crust.In order to study the silicon isotopic fractionation behavior during fluid metasomatism in subduction zones,and to constrain silicon isotope compositions of dehydrated residual slab,we selected a set of well-characterized metaperidotites from the Franciscan complex of California for silicon isotopic analysis.Due to multistage fluid metasomatism,these mantle wedge peridotites suffered a series of fluid-mineral reactions,such as serpentinization,talcification,and tremolitization,and formed partially and completely serpentinized peridotites,talc-dominated metaperidotites and tremolite-dominated ones successively.All of the studied samples have relatively homogeneous ?30Si values ranging from-0.39‰ to-0.26‰,with an average of-0.32±0.08‰(2SD,n=15).This demonstrates that subduction-zone fluid metasomatism may not significantly alter the silicon isotope composition of mantle peridotites.Mass balance calculations indicate that the slab-derived fluids in the Franciscan Complex have similar silicon isotope signature compared to that of mantle peridotites.Metamorphic dehydration processes may also not change silicon isotope composition of the residual slab.