碎屑电气石的LA-MC-ICPMS硼同位素原位微区分析及其源区示踪:以哀牢山构造带为例  

In-situ boron isotopic analyses of tourmalines by LA-MC-ICPMS and its application to provenance analyses: A case study for the Ailaoshan Belt

作　　者：聂小松[1,2] 夏小平[1] 张乐[1] 任钟元[1] 李玲[1,2] 

机构地区：[1]中国科学院广州地球化学研究所同位素地球化学国家重点实验室,广东广州510640 [2]中国科学院大学,北京100049

出　　处：《地球化学》2015年第5期438-449,共12页Geochimica

基　　金：国家自然科学基金(41173007);中国科学院135项目(GIGCAS-135-Y234151001)

摘　　要：硼同位素在自然界中变化范围很大，可以用于研究宇宙星云形成过程、地球壳幔演化、板块俯冲以及判别沉积环境、矿床成因，水环境地球化学和重建古海洋环境等。电气石是自然界中最重要的含硼矿物，是理想的硼同位素分析对象。本文利用中国科学院广州地球化学研究所开发的电气石LA-MC-ICPMS硼同位素分析方法，以哀牢山构造带志留系．泥盆系碎屑岩为例，阐述了碎屑电气石的硼同位素对源区示踪的意义。利用一台Neptune Plus多接收等离子体质谱仪连接一台RESOlution M-50准分子（193nm）激光剥蚀进样系统，采用标样．样品．标样交叉法校正仪器分馏，以国际电气石标样IAEA B4为外部标准，中国地质科学院矿产资源研究所IMR RB1实验室标样为未知样，检验了该方法的可靠性和准确度。IMR RB1的δ^11B测试结果为（-13．34±0．20）‰（10，n=25），与之前报道的结果（-12．96±0．49）‰（10）在误差范围一致。哀牢山构造带古特提斯缝合线两侧的志留系．泥盆系沉积岩中的碎屑电气石硼同位素分析结果表明缝合线两侧碎屑电气石δ^11B值具有明显的差异，思茅一侧的碎屑电气石来源比较复杂，主要峰值在-13‰～-18‰之间，部分电气石的硼同位素体系可能受到了非海相蒸发岩、变质流体或者岩浆演化后期强烈的岩浆去气作用的影响。扬子一侧碎屑电气石主要峰值在-13‰--11‰，来源相对比较单一，源区可能受到俯冲流体的影响。这些结果与我们之前的碎屑锆石U-Pb年龄数据一致，指示哀牢山古特提斯缝合线两侧物源在志留纪泥盆纪时代存在显著差异。Boron isotope compositions of nature materials generally show large variations which can be used to study nebula formation, crust and mantle interaction of the Earth, slab subduction process, sedimentary environment discrimination, genesis of ore deposit, water environmental geochemistry and paleo-environmental reconstruction of ocean. Tourmaline is the most important boron-bearing mineral, and ideal for boron isotope analyses. In this study we utilized a Neptune Plus multi-collector ICPMS coupled with a RESOlution M-50 excimer laser ablation system to analyse boron isotope of tourmalines. The IAEA B4 standard tourmaline was used as an external standard to calibrate instrument isotope fractionation. An in-house standard IMR RB 1 was used as an unknown sample to test the reliability of the method. Twenty-five spot analytical data obtained for IMR RB 1 yield a weighted mean of （-13.34±0.20）‰ （1σ）, which is well consistent with the previous result （（-12.96 ±0.49）‰, 1σ）. The method was applied to the detrital tourmalines separated from the Silurian to Devonian sedimentary samples collected from the Ailaoshan Belt. Our results show a remarkable difference in δ^11B values between samples collected from different sides of Paleo-Tethys suture. Detrital tourmalines from the Simao side are originated from a complicated source, whose δ^11B has peak values of-13‰ - -18‰, indicating affection of nonmarine evaporates and metamorphic fluid or highly evolved magma. Meanwhile, detrital tourmalines from the Yangtze side has dnB peak values of-13‰ - -11‰, implying a relatively simple source and affection of fluid from subduction slabs. These results indicate different provenances for the two sides of Paleo-Tethys suture in the Silurian-Devonian, which are consistent with our previous detrital zircon U-Pb data.

关 键 词：LA-MC-ICPMS 硼同位素 碎屑电气石 哀牢山 古特提斯 

分 类 号：P597[天文地球—地球化学]