橄榄石微量元素原位分析的现状及其应用

随着高精度EMPA和LA-ICP-MS分析技术的发展和矿物微量元素测试精度的提高,利用橄榄石中的微量元素示踪地幔部分熔融、地幔交代作用、岩浆早期结晶过程等地质问题成为近年来一个新兴的研究方向。一系列开拓性的研究发现也被陆续的发表,主要涉及橄榄石中Ni、Co、Al、Cr、Zn、Ti、Li、V、Sc、Mn、Ca和P等元素的示踪使用。一些卓有成效的示踪方法为:Ca、Al、Ti、Ni及Mn能够很好的用于区分橄榄石捕掳晶和斑晶;橄榄石-尖晶石地幔演化趋势线(OSMA:olivine-spinel mantle array)图解可以用于表征岩浆源区的亏损程度;玄武岩中橄榄石斑晶的Li同位素及Li含量可以很有效地指示岩浆源区是否存在地壳物质再循环及地幔交代作用;橄榄石斑晶中Ni、Ca、Mn、Cr和Al协变关系图解可以识别岩浆的辉石岩源区;利用橄榄石捕掳晶中Zr和Sc的含量差异特征可将橄榄岩中三种最主要的类型(尖晶石橄榄岩、石榴石橄榄岩以及尖晶石-石榴石橄榄岩)区分开来;一些元素的比值或组合(例如Ni/Co、Fe/Mn、V/Sc、Zr和Sc、Ca和Ti)可以指示源区交代作用、岩浆作用过程及氧化状态;基于橄榄石中Al、Cr及Ca的地质温度计可以为推算地幔热状态提供新方法;基于橄榄石分离结晶Fo-NiO演化线的原始岩浆计算模型可以较好的推算原始岩浆成分;利用橄榄石的环带及微量元素的扩散机制可以判别更多岩石成因信息,如识别交代介质、熔体类型以及地质构造背景等。基于上述最新研究的相关资料和已有成果,本文对橄榄石微量元素的地球化学示踪方法做系统性的归纳整理,并对橄榄石微量元素赋存状况、橄榄石微量元素测试方法、橄榄石微量元素的使用条件及需注意的问题等进行讨论,为读者在做相关研究时提供参考。

The research status of olivine trace elements in-situ analysis and perspectives of its application

With the development of high-precision electron microprobe and Laser-ICP-MS in-situ analyses, the measurement of low concentration trace elements in olivine is nicely accessible. The trace elements in olivine are used as a new geochemical indicator of mantle partition melting, mantle metasomatism and magma fractionation in recent years. A series of pioneering research papers is released continually to refer to Ni, Co, Al, Cr, Zn, Ti, Li, V, Sc, Mn, Ca and P in olivine. Several striking feasible approaches of the indication by trace elements above are listed as follows: 1) Ca, Al, Ti, Ni and Mn can be helpful to differentiate mantle and volcanic olivine in the ambiguous situation. 2) The compositional relationships between olivine Mg^# and Cr^# (Cr/(Cr+Al)) of the spinel (olivine-spinel mantle array) can be used to indicate the melt extraction of the mantle source. 3) Li concentration and isotopic composition in olivine are distinctive parameters for identifying continental crustal recycling and mantle metasomatism. 4) Ni, Ca, Mn, Cr and Al are fit monitors for the melting of olivine-free pyroxenite magmatic source, which is interpreted as being derived from recycled ocean crust. 5) The Zr and Sc concentration characteristic in mantle olivine can be used to distinguish from three types of peridotite (spinel peridotite, spinel-garnet peridotite, and garnet peridotite). 6) Some ratios and assembly between the trace elements, such as Ni/Co, Fe/Mn and V/Sc, Zr and Sc, Ca and Ti, are informative indicators for metasomatism, magmatic process and oxidation state in source region. 7) The new geothermometers based solely on Al, Cr, Ca in olivine can be widely applicable. 8) The olivine fractionation model, basing on the Fo-NiO covariation relationship, can be used to calculate the primitive magma composition, etc. More and more petrogenesis functions related to the zonation and diffusion of trace elements in olivine can be expected in coming years, such as distinguishing between different metasomatic agents, identifying different infiltrating melt types and recognizing the locality and geotectonic setting. At present, we summarize the latest achievements in this area and try to introduce the latest research progress and developing tendency of the issues as follows: the occurrence state of trace elements in olivine; the measurement, data reliability and the key point of measurement process of trace element in olivine; the tradition tracing indicator and the latest applications; the precautions for using the trace elements as indicator.