Rhenium-Osmium Isotope Investigations of the Taitao Ophiolite

精确地测定上地幔的平均187Os/188Os比值（187Re衰变为187Os，半衰期约为420亿年）将有助于确定俯冲的海洋地壳与洋中脊玄武岩上地幔源隔离的时间。Re- os同位素系统的母元素Re在地幔熔融形成洋壳时优先从地幔中移出，并集中在洋壳中。子元素o大部分留在地幔残留物中。由于残余地幔中父女比的降低，洋壳与上地幔的长期分离（可能是通过向下地幔的分离）将导致上地幔的187Os/188Os比在洋壳未被分离的情况下增长缓慢。因此，测定上地幔的平均187Os/188Os比值可以揭示有多少海洋地壳已经从上地幔中剥离，而尚未混合回上地幔中。以前的一些研究针对蛇绿岩（上大洋地幔和地壳的碎片嵌入长期存在的大陆地壳中）来揭示上地幔中Os同位素随时间的逐渐演变。然而，由于蛇绿岩不同岩石中存在的Os的起源不确定，对蛇绿岩中Os同位素数据的解释已被证明是有争议的/有问题的。本文拟研究智利西南部太涛蛇绿岩中不同岩石的187Os/188Os比值和Re、Os浓度。太涛蛇绿岩可能是最年轻的蛇绿岩（16ma），与智利三重结有关。研究非常年轻的蛇绿岩对于了解蛇绿岩中Os同位素的起源非常重要，因为蛇绿岩的年轻减少了由年龄校正引起的初始同位素组成的不确定性，而年龄校正是此类岩石中Re-Os系统的主要不确定性来源。这种简化将使人们能够关注可能使蛇绿岩组合的Re-Os系统分类复杂化的其他过程，例如熔融岩和水岩相互作用。这项工作的结果将允许对散装橄榄岩、铬铁矿、橄榄石、辉石和可能的硫化物进行同位素比较。还可以比较不同类型蛇绿岩的同位素组成。关于太涛蛇绿岩Re-Os同位素系统行为的知识将最终应用于解释日益增长的全球古蛇绿岩数据库。这些信息反过来又可能用于对上地幔187Os/188Os比值的演化提供更可靠的约束。

Accurately constraining the average 187Os/188Os ratio of the upper mantle (where 187Re decays to 187Os with a half life of about 42 billion years) should prove useful in determining how long subducted oceanic crust remains isolated from the upper mantle source of mid ocean ridge basalts. The parent element of the Re-Os isotope system, Re, is preferentially removed from the mantle and concentrated in oceanic crust, when oceanic crust is created through mantle melting. The daughter element, Os, largely remains behind in the mantle residue. Because of the reduction in the parent-daughter ratio in the residual mantle, long-term removal and isolation of oceanic crust (perhaps by removal to the lower mantle) from the upper mantle will lead to retarded growth of the 187Os/188Os ratio of the upper mantle compared to that which would have occurred had the oceanic crust not been removed from it. Thus, determination of the average 187Os/188Os ratio of the upper mantle can reveal how much oceanic crust has been removed from the upper mantle, and not yet mixed back into the upper mantle. Some previous studies have targeted ophiolites (slivers of upper oceanic mantle and crust emplaced into long-lived continental crust) to reveal the gradual evolution of Os isotopes in the upper mantle through time. The interpretation of Os isotopic data derived from ophiolite rocks, however, has proven contentious/problematic because of uncertainties in the origin of the Os present in different rocks of an ophiolite. Here, we propose to study the 187Os/188Os ratios and Re and Os concentrations of rocks and minerals from the different rocks present in the Taitao ophiolite of southwestern Chile. The Taitao ophiolite is perhaps the youngest ophiolite (16 Ma) and is associated with the Chile triple junction. Study of a very young ophiolite is important for understanding the origin of Os isotopes in ophiolites because the youth of the ophiolite reduces uncertainties in initial isotopic compositions resulting from age corrections, a large source of uncertainty for the Re-Os system in such rocks. This simplification will permit a focus on other processes that may complicate the Re-Os systematics of ophiolite assemblages, such as melt-rock and water-rock interactions. The results of this work will allow isotopic comparisons between bulk peridotites, chromites, olivine, pyroxene and possibly sulfides. It may also be possible to compare the isotopic compositions of the various types of ophiolitic rocks. Knowledge about the behavior of the Re-Os isotope system in the Taitao ophiolite will ultimately be applied to the interpretation of the growing global database for older ophiolites. This information in turn can potentially be used to place more reliable constraints on the evolution of the 187Os/188Os ratio of the upper mantle.