Rhenium-Osmium Isotope Investigations of the Taitao Ophiolite

精确地限制上地幔的平均187 Os/188 Os比值（187 Re衰变为187 Os，半衰期约为420亿年）应被证明有助于确定俯冲洋壳与洋中脊玄武岩的上地幔源隔离多久。当地幔熔融形成洋壳时，Re-Os同位素体系的母元素Re优先从地幔中移出并富集在洋壳中。子元素Os大部分留在地幔残留物中。由于残留地幔中亲子比的降低，洋壳的长期去除和隔离与大洋地壳未被移出的情况相比，上地幔中的187 Os/188 Os比值的增长将受到阻碍。因此，上地幔187 Os/188 Os平均比值的测定可以揭示有多少洋壳已经从上地幔中移出，而尚未混合回上地幔。 以往的一些研究针对蛇绿岩（上地幔和地壳的薄片进入长寿命大陆地壳），以揭示上地幔中Os同位素随时间的逐渐演化。然而，来自蛇绿岩岩石的Os同位素数据的解释，已被证明是有争议的/有问题的，因为在蛇绿岩的不同岩石中存在的Os的起源的不确定性。在这里，我们建议研究的187 Os/188 Os比值和Re和Os的浓度的岩石和矿物从不同的岩石中存在于智利西南部的Taitao蛇绿岩。台桃蛇绿岩可能是最年轻的蛇绿岩（16 Ma），与智利三联点有关。研究一个非常年轻的蛇绿岩是非常重要的了解蛇绿岩中的Os同位素的起源，因为年轻的蛇绿岩减少了初始同位素组成的不确定性所造成的年龄校正，一个很大的来源的不确定性Re-Os系统在这样的岩石。这种简化将允许集中在其他过程中，可能会复杂的蛇绿岩组合的铼-锇系统学，如熔体岩石和水-岩石的相互作用。这项工作的结果将允许散装橄榄岩，铬铁矿，橄榄石，辉石和可能的硫化物之间的同位素比较。还可以比较各种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应用于解释日益增长的全球古老蛇绿岩数据库。这些信息反过来又可能被用来对上地幔187 Os/188 Os比值的演化进行更可靠的限制。

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.