In Situ Study On Platinum Group Mineral In Podiform Chromitites

Platinum-group elements are mainly occurring in mantle rocks and chromitites,can trace the petrogenesis and evolutions of its host rocks.Diamond,boninite spinel and a coesite with stishovite pseudomorp shape discovered in Os-Ir alloy and Fe-Ti alloy respectively,suggested that PGM can link the chromitite and deep UHP and extremely reduce mantle environment.Mineral associations and in situ microstructure of platinum-group minerals(PGM)can provide an efficacious clue to understand the physical and chemical conditions during the crystallization of chromitite.Therefore,it is significant for us to understand the formation of unusual mineral preserved in chromitite and the crystallization of chromitite,be carried out in situ PGM study.The occurrence of PGM has been carried out by the microscope and EPMA for those chromitites sampling from Tibet,Turkey,Albania and Urals.In this thesis,we investigate the PGM assemblage and microstructure for understanding the physical and chemical conditions of the formation and evolution of PGM.Analysis of the genetic relationship between the PGMs and chromitites,in order to provide mineralogical clues for discussion of the deep genesis of chromitite.The main results of this thesis are summarized as follows:(1)The in situ occurrence status of PGM in each study chromitites has been identified.The occurrence and microstructure of PGM have been investigated for those chromitites sampling from Tibet,Turkey,Albania and Urals,sixteen kinds of PGMs include PGE sulfides,alloys,sulfarsenides have been identified.The PGM occurrence includes single phase and composite phase inclusions,and the composite inclusions can be divided into PGM ± silicate,PGM±BMM(base metal minerals)±silicate and BMM±silicate combination of three types.Those composite inclusions are considered controlling by melt/rock reactions,crystallization and melt mixing processes.PGM assemblages and compositions are different in each research area,suggesting PGM are closely related to their formation environment.(2)Platinum group minerals have crystallised directly from fluid/melts and controlled by temperature and sulfur fugacity.In the early stage,Os-Ir alloys crystallized under high temperature and low sulfur fugacity environment,then formed high Ru laurite,followed by sulphides of lauriteerlichmanite solid-solution series,sulfarsenides,BMS upon cooling and increasing sulfur fugacity.Os-Ir alloys formation in the early stage under temperature more than 1200? and sulfur fugacity logf(S2)<-2.Laurite formation under the temperature below 1200? and sulfur fugacity logf(S2)?-2 to-1.Erlichmanite and sulfarsenides crystallization from a temperature close to 1100?and sulfur fugacity logf(S2)?0.During BM sulfide formation the temperature decreased to 1000?and f(S2)increased to logf(S2)?+2.(3)The PGMs show that parental melts of Luobusa chromitite are high temperature and low sulfur fugacity,and the chromit forms during melt rock reaction and crystallization differentiation.The parental melts of Kangjinla chromite origin from the melt mingling process.The PGMs from Xiangkashan may be subjected to the process of serpentinisation and hydrothermal metamorphism after the chromite crystallization.The PGMs from Zedang,Purang,Dongbo Tibet show that parental melts related to melt rock reaction and crystallization differentiation.The PGMs from PozantiKarsanti shows that parental melts under high temperature and low sulfur fugacity environment during crystallization differentiation.However,Albanian and Urals suggesting PGM formed from the melt mingling process.(4)The relationship between Os-Ir alloy minerals and unusual UHP minerals has been discussed.Os-Ir alloy minerals formed first under high temperature and low sulfur fugacity environment(T>1200? and logf(S2)<-2)from shallow lithosphere to the core-mantle boundary.The Os-Ir alloy contain unusual UHP minerals may direct crystallization from UHP and extremely reduced environment,in this scenario,Os-Ir alloy minerals capture the UHP unusual minerals from melts and overall included into chromite,then transported to the shallow lithosphere by uplift melts under mantle convection.(5)The study on PGM combining in situ examination and mechanical separation can obtain both their abundance and textural information.In the future,the study on PGM should strengthen itself microstructure,including Os isotopes,S isotopes and trace elements in micro area.In addition,it is worthwhile to point out that the geological interpretation of whole rock Re-Os isotopic must have a detailed knowledge of the distribution of PGM and in situ textural information.