Rhenium-Osmium Isotope Investigations of the Taitao Ophiolite

准确地限制上地幔的平均187Os/188Os比值(187Re衰变到187Os，半衰期约为420亿年)，应该有助于确定俯冲洋壳与大洋中脊玄武岩的上地幔来源隔离的时间。当洋壳通过地幔熔融形成时，Re-Os同位素体系的母元素Re优先从地幔中移出，并集中在洋壳中。子元素Os主要留在地幔残留物中。由于残馀地幔中母子比的降低，洋壳与上地幔的长期搬运和隔离(可能是通过搬运到下地幔)将导致上地幔的187Os/188Os比没有洋壳搬走时的增长缓慢。因此，测定上地幔的平均187Os/188Os比值可以揭示有多少洋壳已经从上地幔移走，并尚未混合回到上地幔。以前的一些研究以蛇绿岩(侵位在长寿陆壳中的大洋上地幔和地壳的薄片)为目标，以揭示上地幔中Os同位素随时间的逐渐演化。然而，由于蛇绿岩不同岩石中Os的来源存在不确定性，对蛇绿岩中Os同位素数据的解释已被证明是有争议的。本文拟对智利西南部太陶蛇绿岩中不同岩石矿物的187Os/188Os比值和Re、Os含量进行研究。太陶蛇绿岩可能是最年轻的蛇绿岩(16 Ma)，与智利三联结共生。研究非常年轻的蛇绿岩对于了解蛇绿岩中Os同位素的来源很重要，因为蛇绿岩的年轻减少了由于年龄校正而导致的初始同位素组成的不确定性，这是此类岩石中Re-Os系统的一大不确定源。这种简化将允许重点关注可能使蛇绿岩组合的Re-Os系统学复杂化的其他过程，如熔体-岩石和水-岩石相互作用。这项工作的结果将使大量橄榄岩、铬铁矿、橄榄石、辉石和可能的硫化物之间的同位素比较成为可能。也有可能比较不同类型蛇绿岩的同位素组成。有关太陶蛇绿岩中Re-Os同位素体系行为的知识最终将被用于解释不断增长的全球老蛇绿岩数据库。反过来，这些信息可能被用来对上地幔的187Os/188Os比值的演化施加更可靠的约束。

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.