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锂同位素实验方法及其在西藏驱龙花岗岩中的应用
【作者】 李真真；
【作者基本信息】 昆明理工大学 ， 地球化学， 2011， 硕士
【摘要】 本文采用MC-ICP-MS分析测试技术,在国内实验室建立了有效的Li同位素实验方法。条件实验的工作溶液由纯碳酸锂和标准岩石样品(AGV-2、BHVO-2和NKT)混合而成,采用三根阳离子交换树脂(AG 50W-X8)填充的聚丙烯交换柱和石英交换柱对Li进行分离提纯,淋洗液分别为2.8 M HCl、0.15 M HCl以及0.5 M HCl和30%乙醇,接收区间分别是6ml、23ml和9ml。整个实验流程中Li的回收率大于97.6%。利用本文建立的Li同位素实验方法,测定了国际标样AGV-2、BHVO-2、NTK(相对于IRMM016)和IRMM016(相对于L-SVEC)的δ7Li值,分别为7.08±0.6%o、4.11±0.6‰、9.80±0.7‰和+0.34±0.7‰,与前人分析结果吻合,实现了标准样品的稳定重现。本文尝试将锂同位素技术运用于岩浆成因研究中,首次获得了西藏驱龙巨型斑岩铜矿区一批岩石和矿物的锂同位素分析数据,进一步限定了岩浆来源。本次共分析了27个样品的Li同位素组成,其中14个花岗闪长岩全岩样品,8个闪长质包体的全岩样品,以及5个包体中角闪石样品。CIA(chemical index of alteration)指数显示,花岗闪长岩和闪长质包体都没有受到蚀变的影响。花岗闪长岩的Li含量和δ7Li的变化都不大,Li含量为4.5×10-6-8.1×10-6,δ7Li的变化范围是-2.68%o-+6.55%o。闪长质包体的Li含量变化大,最低为4.9×10-6,最高为9.4×10-6。8个闪长质包体样品中,δ7Li最低为-8.89%o,最高位+1.73%o。角闪石是主要的富锂矿物之一,最高Li含量为6.8×10-6,平均为4.5±2×10-6(1σ),δ7Li为-5.03‰～+1.02‰。由于部分闪长质包体不仅存在矿化现象,而且有可能在被携带至地表的过程中发生了动力分馏,因此分析认为花岗闪长岩和闪长质包体真实有效的Li同位素组成范围分别是十2.51‰～+6.55‰和-3.69‰～+1.02‰。根据Li同位素研究结果,结合Sr-Nd同位素体系,本文认为驱龙铜矿区花岗闪长岩可能是由俯冲板片脱出的流体交代地幔楔形成。而闪长质包体的δ7Li明显较低,有3种可能：一是包体在被携带运移的过程中与围岩之间发生了扩散分馏,使包体中富集6Li；二是底侵的岩浆与包体所代表的源区之间发生了同位素的动力分馏,使其源区的Li同位素变轻；再者就是包体反应的是真实的Li同位素组成,推测这种低δ7Li的特征可能源自残余板片的部分熔融。
【Abstract】 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.