摘要
通过氧同位素分析可以对岩石的成因进行认识,使之成为岩石学研究的一个强有力工具。根据花岗岩的氧同位素高低可以判断其来源于变沉积岩还是变火成岩;对于一个复式岩体,如果不同期次岩石的氧同位素组成存在明显变化,可以有效地判断该岩浆在演化过程中存在不同来源物质的混染,因为岩浆从镁铁质成分向长英质成分的化学分异过程,不会引起明显的氧同位素分馏,分馏值一般不超过0.3‰。氧同位素分析手段从常规BrF5法,到激光探针BrF5法,再到离子探针分析法,对岩石矿物样品从全熔分析到微区分析的发展,显示了它们的发展历程和各自的优势及应用范围。本文介绍了氧同位素的不同分析方法,以花岗岩(流纹岩)和变质岩的研究为例阐述了氧同位素分析技术的发展。苏州花岗岩利用全岩和造岩矿物常规氧同位素分析方法得出其为低δ18O和正常δ18O岩浆成因的不同认识,近年通过锆石激光氧同位素分析技术,获得岩浆锆石的δ18O平均为4.92‰,证实其来源于壳源的低δ18O岩浆。类似地,利用全岩和造岩矿物常规氧同位素分析方法得出的观点难以解释美国黄石高原低δ18O流纹岩中矿物颗粒间和颗粒内部的氧同位素变化,该氧同位素变化只能通过离子探针矿物微区原位分析得出。在变质岩研究方面,通过激光探针氧同位素分析,人们普遍认为苏鲁造山带变质岩极负的δ18O值是在新元古代原岩形成时获得的,但是最新的离子探针锆石原位氧同位素分析表明苏鲁造山带变质岩极负的δ18O主要是在三叠纪超高压变质作用过程中获得的。今后单颗粒矿物尺度上的氧同位素组成分布规律将是氧同位素研究的发展方向。
Since different types of rocks on the earth have different oxygen isotope compositions,they can be used to discuss the origins of various rocks and it has become a powerful tool for studying petrology.For example,determination on whether the granite was derived from metasedimentary or metavolcanic rocks based on its oxygen isotope compositions can be established.For a complex massif,if rocks of different stages have significantly different oxygen isotope compositions,the deternination that they have been assimilated by other materials during evolution of magmas,because there should be no evident oxygen isotope fractionation （less than O.3‰） during chemical differentiation of magmas from mafic to felsic composition can be made.Analytical techniques of oxygen isotope compositions include traditional BrF5,laser BrF5 and ion microprobe and they reflect the development from bulk analysis to microanalysis.Granites （rhyolites） and metamorphic rocks are used as examples to show that understandings of the origins of rocks have improved with the development of oxygen isotope composition analyses.For both granites （rhyolites） and metamorphic rocks,the ion microprobe in-situ analysis of oxygen isotope composition gain new insights into the origins of rocks.Based on traditional BrF5 method,the Suzhou granite was suggested to have two different origins,a low-δ18O origin and a normal δ18O origin.However,δ18O values of magmatic zircons acquired by laser BrF5 analysis are 4.92‰ ± 0.26‰,confirming that the Suzhou granite has a low-δ18O magma origin that was derived from the crust.Similarly,the proposal drawn from traditional BrF5 analysis on whole rock and rock-forming minerals cannot explain the intergranular and intraparticle oxygen isotope variations in the low-δ18O rhyolites from the Yellowstone plateau which can only be gained by the ion microprobe method.Based on laser BrF5 analysis,the metamorphic rocks of the Sulu orogenic belt were considered to have acquired their extremely negative δ18O values during the formation of their protoliths in the Neoproterozoic.However,recent ion microprobe in-situ analysis of oxygen isotope compositions on zircon demonstrates that the Sulu metamorphic rocks acquired their extremely negative δ18O values during the ultrahigh pressure metamorphism in the Triassic.The study of distribution of oxygen isotopic composition in single gain is a new trend in the future.
出处
《岩矿测试》
CAS
CSCD
北大核心
2013年第6期841-849,共9页
Rock and Mineral Analysis
基金
中国科学院仪器设备功能开发技术创新项目(2011001)
关键词
氧同位素
分析方法进展
花岗岩
变质岩
成因认识
oxygen isotope
progress in analytical method
granite
metamorphic rock
understanding on genesis
