Geochemistry And Implications Of Li-Mg Isotopic Systematics In Garnet

Garnet is one of the most important rock-forming minerals in silicate earth,essential in a variety of magmatic and metamorphic rocks,which is stable through a wide P-T range.Because of its high refractoriness and low solubility in fluids/melts,garnet is incomparable in its ability to record and preserve compositional zonation related with temperature,pressure and oxygen fugacity.Therefore,garnet is widely used to reveal the evolution process of magma,restore the P-T conditions and formation age of metamorphic rocks,reconstruct the history of fluid evolution during plate subduction and record the information of crust/mantle interaction.Studies have shown that Li was incorporated into garnet via coupled substitution during which trivalent rare earth elements(REEs)provided local charge balance for Li+when substituting for Mg2+.This low diffusivity of Li in garnet makes it likely to preserve the Li isotopic values of growth zones.However,due to the lack of standard samples for in situ Li isotope analysis of garnet,the application and development of Li isotope in micro scale are greatly limited.Considering the special substitution between Li and Mg,this doctoral thesis selects the garnet porphyroblast from Yardoi schists to study the micro-area Li-Mg isotopic geochemistry and discusses whether the garnet can record the Li-Mg isotope growth information and their geological significance during metamorphism.At the same time,the micro-area Li isotope geochemistry of garnet in pegmatite from Argentina and Li isotopic composition of different types of garnets in Jingshan granite are also studied to understand the implications in tracing the process of magmatic differentiation.Finally,the standard sample for in situ analysis of Li isotope in garnet is also preliminarily developed.The above work is rarely involved in the field of international geosciences and has high research value and originality.One garnet porphyroblast of 6 cm diameter was sampled from the Yardoi schists of Tibet,and the major-,trace-,and Li-Mg isotopic compositions of distinct growth zones were determined in situ combined with petrographic observation and mapping in elements.The results show that Li and Mg isotopes have compositional zonings in garnet.The δ7 Li values range from+6.0‰ to+4.1‰ and follow ’S-shape’ patterns on both sides of the garnet’s core,revealing a two-stage growth process corresponding to the fluid-assisted sequential recrystallization of chlorite and micas during prograde metamorphism.By contrast,once corrected for the overprinting by retrograde metamorphism,the δ26 Mg values vary monotonously from-1.73 ‰ in the core to-1.32 ‰in the outer rim,reflecting a single-step process interpreted to result from increasing temperature and the solid-state recrystallization of chlorite-biotite during prograde metamorphism,and the dehydration process has little effect on it.This different behavior of Li and Mg isotopes is interpreted to result from the fact that Li is fluidmobile and present in garnet in trace concentrations,whereas Mg is relatively less mobile and present as a major element.The garnet(about 5cm in diameter)was collected in pegmatite from Argentina was studied by petrographic observation,surface scanning analysis and mappings of elements,in situ major and trace elements with LA-ICP-MS,cutting and sampling along the garnet profile with diamond wire saw,and a total of 24 Li isotope samples were analyzed.The results show that the Li isotope composition within garnet is relatively flat,but δ7Li value increased significantly at the edge of garnet within a distance of about 5mm,resulting in a fractionation value of～1.6 ‰.Based on the comprehensive analysis of the existing geochemical data,it can be recognized that there are two growth stages of garnet.The interior is metamorphic garnet formed in the process of early progressive metamorphism,and the edge is magmatic garnet formed by crystallization in the process of later magmatism.By simulation of Rayleigh fractionation model of Li content vs.δ7Li in the process of magmatic differentiation,the results show that the observed Li isotope fractionation value can be produced when the magmatic evolution is about 60%.Although the Li isotope fractionation value is little during magmatic evolution,garnet can still well preserve its growth information.At the same time,based on the detailed research work of predecessors on Jingshan granite,the whole rocks and garnets in restite,main body granite and aplite are selected under the microscope,and Li isotopes are analyzed.The results show that δ7Li values of three different types of garnets and whole rocks increases gradually along with the magmatic evolution,which has similar variation characteristics with the micro-area Li isotopic composition of garnet in Argentine pegmatite.The comparative analysis of the two can better define the geochemical behavior of Li isotope in garnet during the magmatic process.The comparative study of Li isotopic composition of Jingshan garnet with which in Argentine pegmatite can better define the geochemical behavior of Li isotope in garnet during the magmatic process.Finally,the initial development of standard sample for in situ measurement of Li isotopes in garnet by fs-LA-MC-ICP-MS is carried out in this doctoral thesis.At present,the accuracy of instrument without matrix matching(NIST 610)is evaluated by using existing garnet samples.The results are far from the Li isotope data measured by SNMC-ICP-MS,showing severe matrix effect.According to the average chemical composition of garnet in Yardoi shcist,the optimum ratio of oxide powder is selected,and the single mineral reference material is obtained by quenching after hightemperature melting.Later,the standard sample will be tested in situ by fs-LA-MCICP-MS,and matched with the Li isotope date measured by SN-MC-ICP-MS.