A Geochemical Study Of Apatite In Metamorphic Rocks From Subduction Zones

Apatite is an important host for phosphorus,halogen and hydrogen in subducting crust.Studying the behavior of apatite during subduction-zone metamorphism is of great significance for understanding the distribution and cycling of related elements in the interior of Earth.In this dissertation,a combined analytical method was developed for the determination of U-Pb age,trace elements and Cl contents of apatite based on laser ablation-inductively coupled plasmamass spectrometry(LA-ICP-MS).The combined analytical method is able to obtain multiple in-situ geochemical information simultaneously,which is of great significance for the geochemical analysis of apatite.On this basis,a systematic study of apatite in different metamorphic grades of metabasic rocks,veins formed by aqueous fluid and hydrous melt and related rocks was carried out.As a result,the chlorine content and trace composition of different metamorphic origin of apatites were investigated to identify and distinguish different metamorphic origin of apatites in the metamorphic rocks in subduction zones.Furthermore,the behavior of apatite during subduction-zone metamorphism is discussed by the combination of the U-Pb dating results and the other geochemical characteristics.Based on LA-ICP-MS analytical technology and Iolite data processing software,the U-Pb isotopic ages,trace element and chlorine contents of six apatite standards were determined under different beam spot conditions.In combination with micrographic analysis and mineral major element analysis,the analytical results were found to be consistent with the recommended values of the reference standards at both 44 μm and 32 μm conditions.In particular,comparing different calibration methods for Cl content,it was found that a standard calibration curve using multiple apatite standard samples and internal standard element 43Ca could overcome the influence of the "apparent halogen signal" of apatite,and the obtain results are consistent with those obtained by EPMA.Baased on a series of experiments,an analytical method was developed for the simultaneous in-situ determination of apatite U-Pb ages,trace elements and Cl contents.The comparative study of apatites in different metamorphic grades of metabasic rocks in subduction zones was carried out.In combination with the whole-rock major and trace element compositions,the results show that apatites have different metamorphic origin and compositions in different metamorphic grades of metabasic rocks.Apatites in the high pressure(HP)to ultrahigh pressure(UHP)eclogites were mainly formed at the HP-UHP metamorphism stages.Part of them were reworked by retrograde metamorphism during their exhumation to surface,and apatites before HP-UHP metamorphism were hardly preserved.In the amphibolite formed by the retrograde metamorphism of UHP eclogite,there are mainly apatites of amphibolite-facies metamorphic origin,peak eclogite-facies metamorphic origin as well as later retrograde metamorphic origin.Apatites formed at different metamorphic stages have different characteristics in trace element compositions,mainly in the contents of LREE and Sr and Eu negative anomalies.Apatites of HP metamorphic origin usually have low LREE contents,low Sr/Y ratios,low Cl contents and lack of negative Eu anomalies,whereas those of UHP metamorphic origin usually have high LREE contents,high Sr/Y ratios,high Cl contents,and weak or no negative Eu anomalies.Apatites of amphibolite-facies metamorphic origin usually have high LREE contents,low Sr/Y ratios,variable Cl contents and marked negative Eu anomalies.Trace elements and Cl in apatite are controlled by a combination of mineral assemblage,P-T conditions of the system,and fluid metasomatism.Apatites in HP eclogite were not suitable for U-Pb dating due to its extremely low U content.U-Pb dating for apatites of UHP metamorphic origin in the UHP eclogite records the amphibolite-facies retrograde metamorhism of the eclogite during exhumation due to the low close temperature of U-Pb isotopic system in apatite.Apatites of amphibolite-facies metamorphic origin can provide timing constraints on the amphibolite-facies metamorphism.A comparative study was carried out on apatite formed by aqueous fluid and hydrous melt in subduction zones.The in-situ trace element analysis was carried out for apatites in aqueous fluid-forming veins and their host eclogites in southwestern Tianshan orogen,wherease the insitu U-Pb dating,trace elements and Cl content analysis was carriedd out on apatite in hydrous melt-forming veins and their host rocks in the northern Qaidam orogen.In combination with the other geochemical analyses,it is found that apatites of fluid origin have lower LREE,U and Th contents and higher Sr contents than those of melt origin.The differences are mainly controlled by the compositions of fluid and melt.The variation in REE,Sr,Mn,U and Cl contents and the degree of Eu negative anomalies in apatites of melt origin reflect the difference in the composition of melt with which apatite was in equilibrium.Apatites formed by partial melting of gneiss and metabasite rocks exhibit differences in trace element compositions,which can be identified by the contents and ratios of characteristic elements.The relatively low U content of fluid-forming apatites makes it difficult to obtain reliable U-Pb dating results.Meltforming apatites can yield more accurate U-Pb ages due to their high U contents,which can be used to timing the anatexis.