STABLE ISOTOPE SYSTEMATICS OF MANTLE PERIDOTITES IN THE CONTEXT OF THEIR TECTONIC HISTORY

普遍认为，流体在地幔富集过程中起着关键作用，地幔富集事件经常以复杂的方式叠加在橄榄岩地幔上，而橄榄岩地幔由于先前的部分熔融而贫化了不相容的微量元素和轻稀土元素。化学和放射性同位素数据表明，这种交代流体可能部分来自再循环的地壳物质，某种形式的流体交代作用可能是板内碱性岩浆作用的必要前兆。稳定同位素研究提供了解决地幔不均质性和富/贫化过程的独特手段，因为1)氧是其行为方面的主要元素，而氢是一种集中在水相流体中的痕量元素，以及2)从氧和氢同位素组成来看，地幔都是不均匀的。该项目将对晚第三纪至现代碱性玄武岩喷发出具有明显不同岩石学、化学和放射性同位素特征的地幔橄榄岩的四个区域进行详细的稳定同位素研究。这四个Ge离子还具有不同构造历史的次大陆岩石圈地幔的特征。研究的重点是了解每个区域内不同类型橄榄岩稳定同位素变化的原因，并比较四个区域不同构造史背景下的橄榄岩的同位素体系。

It is generally agreed that fluids play a critical role in mantle enrichment processes and that mantle enrichment events frequently are superimposed in a complex manner on peridotitic mantle that has been depleted in incompatible trace elements and light rare- earth elements through prior partial melting. Chemical and radiogenic isotope data suggest that such metasomatic fluids may be derived, in part, from recycled crustal materials and that some form of fluid metasomatism may be a necessary precursor to intraplate alkalic magmatism. Stable isotope studies provide a unique means of addressing mantle heterogeneity and enrichment/depletion processes because, 1) oxygen is a major element in terms of its behavior, whereas hydrogen is a trace element that is concentrated in aqueous fluid phases, and 2) the mantle has been shown to be heterogeneous in terms of both its O- and H-isotope compositions. This project will undertake a detailed stable isotope study of four regions in which mantle peridotites of distinctly different petrological, chemical, and radiogenic isotope character have been erupted in Late Tertiary to Recent alkalic basalts. The four gerions also are characterized by having sub-continental lithospheric mantle with differing tectonic histories. The focus of the research is an understanding of the causes of stable isotope variations in the different types of peridotites present within each region, and a comparison of the isotope systematics of peridotites from the four gegions in the context of their different tectonic histories.