Rhenium-Osmium Isotope Investigations of the Taitao Ophiolite

准确限制上地幔的平均 187Os/188Os 比率（其中 187Re 衰变为 187Os，半衰期约为 420 亿年）应该有助于确定俯冲洋壳与中洋脊玄武岩的上地幔源区保持隔离的时间。当地幔熔化形成洋壳时，铼-锇同位素系统的母体元素铼优先从地幔中去除并富集在洋壳中。子元素 Os 大部分留在地幔残渣中。由于残余地幔中母子比的减少，长期将洋壳从上地幔去除和隔离（可能通过去除到下地幔）将导致上地幔的 187Os/188Os 比值比没有从上地幔去除的情况下增长迟缓。因此，确定上地幔的平均 187Os/188Os 比率可以揭示有多少洋壳已从上地幔中移出，并且尚未混合回上地幔中。 之前的一些研究以蛇绿岩（上海洋地幔和地壳碎片进入长寿大陆地壳）为目标，以揭示上地幔中骨同位素随时间的逐渐演化。然而，由于蛇绿岩不同岩石中存在的锇起源的不确定性，对蛇绿岩岩石中锇同位素数据的解释已被证明是有争议的/有问题的。在这里，我们建议研究智利西南部泰涛蛇绿岩中不同岩石和矿物的 187Os/188Os 比值以及 Re 和 Os 浓度。 Taitao 蛇绿岩可能是最年轻的蛇绿岩（16 Ma），与智利三重交界处有关。对非常年轻的蛇绿岩的研究对于了解蛇绿岩中锇同位素的起源非常重要，因为蛇绿岩的年轻性减少了年龄修正引起的初始同位素组成的不确定性，而年龄修正是此类岩石中铼-锇系统不确定性的一大来源。这种简化将允许关注可能使蛇绿岩组合的 Re-Os 系统复杂化的其他过程，例如熔岩和水-岩石相互作用。这项工作的结果将允许对大块橄榄岩、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。有关 Taitao 蛇绿岩中 Re-Os 同位素系统行为的知识最终将应用于解释不断增长的古老蛇绿岩全球数据库。这些信息反过来又可以用来对上地幔 187Os/188Os 比率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.