微区-微量样品Rb-Sr同位素分析技术及其应用前景

利用微钻取样技术和微量样品Rb-Sr同位素分析方法,本文对出露在东秦岭造山带的中生代合峪花岗岩的自形钾长石巨晶进行微区-微量样品Rb-Sr同位素组成分析。分析结果表明,钾长石斑晶具有显著的Rb/Sr比值和Sr同位素组成变化,斑晶和基质钾长石均构成年龄为132~133Ma的Rb-Sr等时线,代表岩浆的后期冷却时代。钾长石晶体的初始⁸⁷Sr/⁸⁶Sr比值由边缘相到中心相没有明显的变化,代表花岗质岩浆结晶阶段的Sr同位素组成,暗示合峪花岗岩的钾长石巨晶为原生成因。以高空间分辨率为特征,微区取样技术已经广泛地应用在变质岩和深成岩浆岩的同位素年代学和成因研究。结合微量样品同位素分析技术,微区-微量样品Rb-Sr同位素方法有望在火山岩的成因和年代学方面得到应用。

Micro-sample Rb-Sr analysis using the microsampling technique and its applications on geochronology and petrogenesis

This study presents analytical data of Rb-Sr isotopic composition of K-feldspar megacrysts from the Mesozoic Heyu granite pluton exposed in the eastern Qinling orogenic belt, by using the micro-drilling technique and the micro-sample Rb-Sr isotopic analytical method. Analytical results show significant variations in Rb/Sr ratio and Sr isotopic composition within a single K-feldspar crystal. Rb-Sr isochron ages of about 132 Ma to 133 Ma can be obtained from both megacryst and matrix K-feldspars. No significant variations of ⁸⁷Sr/⁸⁶Sr initial ratio can be detected in the centre and marginal parts of K-feldspar crystals, suggesting homogeneous Sr isotopic composition during the crystallization of the magma and implying primary origin for the K-feldspar megacrysts of the Heyu granite. Owing the feature of high spatial resolution, a combination of the micro-drilling technique and the micro-sample Rb-Sr isotopic analytical method increases the applications in studies on isotopic geochronology and petrogenesis of metamorphic and plutonic rocks as well as volcanic rocks.