Experimental Determination Of Partition Coefficients For Clinopyroxene And Study Of Fe Isotopes In Porphyry Cu Deposits

Elemental and isotopic geochemistry are two of the most fundamental research branches in solid earth science.In recent years,with the development of analytical techniques and improved precisions on elements and isotopes,elemental and isotopic geochemistry have been proved to be of great potential in studies on cosmochemistry,the formation and evolution of the earth,the genesis of mineral deposits,the circulation of nutrients in modern oceans,and the reconstruction of paleoenvironment.This thesis focuses on two scientific issues related to elemental and isotopic geochemistry,namely the experimental determination of trace element partition coefficients between clinopyroxene and silicic melt and the study of Fe isotopes in porphyry Cu deposits.The trace element partition coefficient between minerals and melts is an important parameter for understanding the crust-mantle differentiation and magma evolution.A large amount of works show that mineral/meltD is affected by many factors,including temperature,pressure,oxygen fugacity,mineral and melt composition.Ca-rich clinopyroxene(cpx)is an important rock-forming mineral and plays a key role in controlling trace element behavior during the process of crust-mantle magma differentiation.At present,studies on trace element partitioning between cpx and melt mostly focus on mafic systems,while very few experimental studies were conducted to determine cpx-melt trace element partition coefficients in silicic magma system.As a result,our understanding of trace element partitioning between cpx and silicic melt is incomplete.To solve the problem above,we firstly established a flux-free fusion technique for rapid and accurate determination of major and trace elements in silicate glasses by LA-ICP-MS.This work can further help us to obtain the major and trace element concentrations of the experimental run products(cpx and quenched melts)by LA-ICP-MS,and therefore the cpx/meltD with high quality.Through a series of conditional experiments on the USGS reference materials,we found that we can obtain homogeneous glasses for basalt,andesite,rhyolite,and diabase by melting at 1450 ? for 10 minutes.Instead,higher temperature(1550-1600 ?)and longer time(25-35 min)are required to produce homogeneous glasses for granodiorite.The major and trace element concentrations of these glasses were further measured by LA-ICP-MS.The results of these glasses using our method were generally consistent with the reference values within a discrepancy of 5-10%for most elements.The routine precision of our method was generally better than 5-10%RSD.Following the above LA-ICP-MS method,we further performed experiments at 1070-1100 ? and one atmosphere pressure(i.e.1 bar)to determine the cpx-melt trace element partition coefficients.We obtained a series of diopsidic cpx in equilibrium with silicic(67-69 wt%SiO2),aluminous(14.1-15.5 wt%Al2O3),and alkaline(8.8-9.7 wt%Na2O+K2O)melts.We reported cpx/meltD for Co,Mn,Ni,Cu,Zn,Fe,Sc,Cr,Ti,Zr,Hf,Nb,Ta,V,together with Sr and rare earth elements(REE).We mainly focus on the effect of melt composition on the cpx/meltD for the first row transition element(FRTE)and high field strength element(HFSE)between cpx and silicic melt.Comparing with previous cpx-melt trace element partitioning studies in basaltic and andesitic systems,our results show that the cpx/meltD for most divalent(Mn,Co,and Ni)and trivalent(Sc and Cr)FRTE are elevated significantly,and to a lesser extent for tetravalent(Ti,Zr,and Hf)and pentavalent(Nb and Ta)HFSE.We found that cpx/meltD for Co,Mn and Ni increase significantly with decreasing NBO/T of the melt,and are well correlated with melt Mg#or Mg2+/(M++M2+),indicating a strong influence of melt composition on the partition coefficients.In addition,we observed a negative correlation between cpx/meltDsc and Mg#or Mg2+/(M++M2+)of the melt,while cpx/meltDCr is positively correlated with molar Al2O3/(Na2O+K2O+CaO)ratios(ASI)of the melt.It is worth noting that we observed for the first time that cpx/meltD for Ti,Zr,Hf,Nb and Ta showed a negative correlation with the alkali(Na2O + K2O)contents of the melt.However,cpx/meltDCu and cpx/meltDZn are insensitive to the variations of melt and cpx compositions,which might be due to the crystal field effects for Cu and Zn.In addition,cpx/meltDFe and cpx/meltDv are also not sensitive to changes in melt composition,indicating that the melt composition is not a major factor in controlling cpx/meltDFe and cpx/meltDv.Furthermore,these cpx/meltD together with published partition coefficients for other minerals were used to model the partial melting of the lower continental crust,in order to constrain the generation of the continental adakitic rocks.We show that the modeling results are strongly relied on the values of the cpx/meltD.Compared with the modelling results in previous studies,the modeled melts using cpx/meltD in this study are more deficient in REE,HFSE,Sr,and Y,resulting in lower(La/Yb)N and Sr/Y ratios while higher Zr/Sm and Nb/La rotios.Meanwhile,we found that the FRTE contents and ratios(Mn/Zn,Co/Ni and Cr/Sc)of the modelled melts are insensitive to the partial melting degree of the lower continental crust and the residual mineral composition,indicating that the traditional(La/Yb)N and Sr/Y ratios are more suitable for identifying the source composition of the continental adakites.In addition,in the past two decades,with the emergence of MC-ICP-MS and the improvements of the isotope analysis techniques,more and more non-traditional stable isotope systems(such as Fe,Cu,Zn,Mg,Ba and V isotopes)have been applied to different geological processes.The porphyry copper deposit(PCD)is one of the most important sources of copper,molybdenum and gold in the world.It is of both scientific and economic significance to understand the origin of ore-forming elements and metallogenic process of the PCDs.At present,iron isotopes are increasingly being applied to the study of porphyry-related magmatic hydrothermal deposits.For example,iron isotopes have been used to trace the source of iron,redox states of the magma,and mineralization processes in different ore deposits.Nevertheless,the application of iron isotopes in the PCDs is still in its early stage.One of the key issues is the lack of effective constraints on the iron isotope composition of the ore-forming fluids.To further constrain Fe isotopic behaviors during hydrothermal mineralization as well as to constrain the genesis of PCDs using iron isotopes,this study performed a systematic Fe isotope study on the Tongshankou porphyry-skarn Cu-Mo deposit,Eastern China.The altered ore-bearing granodiorite porphyry rocks display a small?56Fe variation between 0.04‰ and 0.17‰,indistinguishable from the values for global granitic rocks,indicating negligible effect of hydrothermal alteration on the whole-rock ?56Fe of the porphyries.Pyrite and chalcopyrite within the porphyry show a wide range of ?56Fe(-0.60‰ to 0.61‰).The ?56Fe for disseminated pyrites show an overall positive range between 0.14‰ and 0.40‰,likely reflecting Fe isotopic exchange with the fluids during hydrothermal alteration in an open system.By contrast,?56Fe for coexisting pyrite and chalcopyrite in veins display a mirror-image relationship,reflecting ?56Fe for sulfides in veins may be controlled by the reservoir effect in a locally closed system.Based on the weighted average ?56Fe of coexisting pyrite and chalcopyrite in veins,we estimated that Fe isotopic compositions of vein fluids are-0.19 ± 0.20‰(2SD,n = 11).These values are obviously lower than the?56Fe of granite,implying sequestration of isotopically heavy Fe isotopes into the early forming magnetite during porphyry mineralization.It thus supports high fO2 conditions relevant for porphyry Cu mineralization.Additionally,the skarn whole-rocks and minerals show a broader range of ?56Fe(-1.68‰ to 1.12‰),and Fe sulfides along a skarn profile between the porphyry and the carbonate wall rocks show a clear zonation in ?56Fe.This remarkable iron isotope fractionation among skarn minerals can be attributed to the complex hydrothermal alteration and mineralization process of the skarn.In summary,iron isotopes can provide effective constraints for the source nature(such as redox state)and metallogenic processes of the PCDs.