Boron Isotopic Composition of the Troodos Ophiolite

洋中脊的热液反应对海水和海洋地壳的化学和同位素组成有深远的影响，其中许多元素的化学通量超过了全球河流排放的化学通量。尽管它很重要，但这些热液过程的基本方面，如在不同温度下的反应程度和流体流动路径，并没有得到很好的约束。此外，蚀变地壳的俯冲作用导致来自水圈的元素重新注入地幔。这些回收材料的大量化学转移速率和化学特征受到很差的限制。最近已经证明，存在与海洋地壳蚀变有关的大量硼同位素分馏（Spivack和Edmond， 1987）。这种交换对海水的硼同位素组成起着主要的控制作用（与大陆和海洋火成岩相比，硼同位素的比例为11B/10B，每毫升约为40），并为地壳提供了硼同位素特征，该特征可作为俯冲挥发物的示踪剂。在评价热液过程的性质，包括流体反应历史，反应温度和流动路径的诊断。本文拟开展Troodos蛇绿岩的硼同位素研究，主要有两个目的：1)建立蚀变洋壳的一级硼同位素剖面；2)为今后热液系统研究中硼同位素的开发提供必要的背景数据。为此，我们打算在矿物学和化学特征良好的整个岩石和脉分离物中确定(B)和11B。蚀变温度和流体演化受氧和锶同位素数据的限制。

Hydrothermal reactions at mid-ocean ridges have a profound influence on the chemical and isotopic composition of seawater and the oceanic crust, involving chemical fluxes of many elements exceeding those from global river discharge. Despite its importance, fundamental aspects of these hydrothermal processes such as the extent of reaction at various temperatures and fluid flow paths are not well constrained. In addition, subduction of the altered crust results in the reinjection of elements derived from the hydrosphere back into the mantle. The bulk chemical transfer rates and the chemical signature of these recycled materials is poorly constrained. Recently it has been demonstrated that there are large boron isotopic fractionations associated with the alteration of the oceanic crust (Spivack and Edmond, 1987). This exchange exerts a primary control on the boron isotopic composition of seawater (~40 per mil enriched in the 11B/10B ratio relative to continental and oceanic igneous rocks|) and imparts a boron isotopic signature to the crust that should be useful as a tracer of subducted volatiles, and diagnostic in evaluating the nature of hydrothermal processes including fluid reaction histories, temperatures of reaction and flow paths. We propose to conduct a boron isotope study of the Troodos ophiolite that is aimed at two goals: 1) establishing a first order boron isotope profile for the altered oceanic crust and 2) developing the background data required for the future exploitation of boron isotopes in the study of hydrothermal systems. To these ends we intend to determine (B) and 11B in whole rocks and vein separates that are mineralogically and chemically well characterized. Alteration temperatures and fluid evolution wii be constrained by oxygen and strontium isotope data.