Lithium Isotope Experimental Method And Its Application In The Qulong Granite

A high-yield lithium separation technique for roch samples has been established together with precise Li isotope analysis based on MC-ICP-MS.Three separate stages of ion-exchange chromatography were carried out using organic cation exchange resin(AG50W-X8,200-400 mesh). Eluants used for lithium separation in different stages were 2.8mol/L HCl,0.5 mol/L HCl and 0.5 mol/L HCl in 30% ethamol, and the volume of the eluants collected for each column were 6ml,23ml and 9ml, respectively.The recovery is more than 97.6%. International standard materials were analyzed, including AGV-2,BHVO-2,NTK and IRMM016, the in-run precise and reproducibility of measured standard materials were 7.08±0.6‰,4.11±0.6‰, 9.80±0.7%o and+0.34±0.7%o, respectively.The results are consistent with other laboratory abroad.Researches have been carried out to solve the origin of the Miocene igneous rock in Qulong, but it is still controvertible. In order to constrain the problem using lithium isotope,27 samples were analyzed, including 14 granodiorite (whole roch),8 diorite enclaves (whole roch) and 5 amphibole minerals. Neither 57Li nor Li concentration in granodiorite varies largely,-2.68%o～+6.55‰and 4.5×10-6～8.1×10-6, correspondingly. Li in diorite enclaves varies from 4.9×10-6 to 9.4×10-6, andδ7Li varies dramaticly from-8.89‰to+1.73‰. As a Li enriched mineral, Li in amphibole is up to 6.8×10-6, and the avrage is 4.5±2×10-6 (1σ), the range ofδ7Li is -5.03‰～+1.02‰. Considering the influence of mineralization and Li fractionation induced by diffusion between diorite enclaves and wall rock, thus the lithium isotope compositions for granodiorite and diorite enclaves are +2.51‰～+6.55‰and -3.69‰～+1.02‰, respectively.Based on the Li isotope data, combined Sr-Nd system, we consider granodiorite derives from partial melting of mantle wedge caused by fluid released from Tethys slab subduction.87Li of diorite is lower than that of granodiorite, we interpret that:(1) fractionation of Li duffusion was happened when enclavse were migrated to the surface; (2) duffusion fractionation was induced by under plating of magmas,which can lead to the source region where the enclaves are from has a lighterδ7Li; (3) if the data can represent the truth, we infer that diorite enclaves derives from partial melting of residual subducted-slab.