Hauri Erik H
Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd NW, DC 20015, USA.
Philos Trans A Math Phys Eng Sci. 2002 Nov 15;360(1800):2371-82. doi: 10.1098/rsta.2002.1073.
The decay of (187)Re to (187)Os (with a half-life of 42 billion years) provides a unique isotopic fingerprint for tracing the evolution of crustal materials and mantle residues in the convecting mantle. Ancient subcontinental mantle lithosphere has uniquely low Re/Os and (187)Os/(188)Os ratios due to large-degree melt extraction, recording ancient melt-depletion events as old as 3.2 billion years. Partial melts have Re/Os ratios that are orders of magnitude higher than their sources, and the subduction of oceanic or continental crust introduces into the mantle materials that rapidly accumulate radiogenic (187)Os. Eclogites from the subcontinental lithosphere have extremely high (187)Os/(188)Os ratios, and record ages as old as the oldest peridotites. The data show a near-perfect partitioning of Re/Os and (187)Os/(188)Os ratios between peridotites (low) and eclogites (high). The convecting mantle retains a degree of Os-isotopic heterogeneity similar to the lithospheric mantle, although its amplitude is modulated by convective mixing. Abyssal peridotites from the ocean ridges have low Os isotope ratios, indicating that the upper mantle had undergone episodes of melt depletion prior to the most recent melting events to produce mid-ocean-ridge basalt. The amount of rhenium estimated to be depleted from the upper mantle is 10 times greater than the rhenium budget of the continental crust, requiring a separate reservoir to close the mass balance. A reservoir consisting of 5-10% of the mantle with a rhenium concentration similar to mid-ocean-ridge basalt would balance the rhenium depletion of the upper mantle. This reservoir most likely consists of mafic oceanic crust recycled into the mantle over Earth's history and provides the material that melts at oceanic hotspots to produce ocean-island basalts (OIBs). The ubiquity of high Os isotope ratios in OIB, coupled with other geochemical tracers, indicates that the mantle sources of hotspots contain significant quantities (greater than 10%) of lithologically distinct mafic material which represents ancient oceanic lithosphere cycled through the convecting mantle on a time-scale of 800 million years or more.
铼 - 187衰变为锇 - 187（半衰期为420亿年）为追踪地壳物质和对流地幔中地幔残余物的演化提供了独特的同位素指纹。由于经历了高度的熔体萃取，古老的次大陆地幔岩石圈具有独特的低铼/锇和锇 - 187/锇 - 188比值，记录了距今32亿年之久的古老熔体亏损事件。部分熔体的铼/锇比值比其源区高出几个数量级，而大洋地壳或大陆地壳的俯冲将能快速积累放射性锇 - 187的物质带入地幔。来自次大陆岩石圈的榴辉岩具有极高的锇 - 187/锇 - 188比值，并记录了与最古老橄榄岩一样古老的年龄。数据显示，橄榄岩（低）和榴辉岩（高）之间铼/锇和锇 - 187/锇 - 188比值近乎完美地分异。对流地幔保留了一定程度的锇同位素不均一性，类似于岩石圈地幔，尽管其幅度受对流混合作用的调节。来自大洋中脊的深海橄榄岩具有低锇同位素比值，表明上地幔在最近一次熔融事件产生中洋脊玄武岩之前经历过熔体亏损阶段。据估计，从上地幔中亏损的铼的量比大陆地壳的铼储量大10倍，这需要一个单独的储库来实现质量平衡。一个由5% - 10%的地幔组成、铼浓度与中洋脊玄武岩相似的储库将平衡上地幔的铼亏损。这个储库很可能由地球历史上再循环回地幔的镁铁质大洋地壳组成，并提供在大洋热点处熔融形成洋岛玄武岩（OIB）的物质。洋岛玄武岩中普遍存在的高锇同位素比值，再加上其他地球化学示踪剂，表明热点的地幔源区含有大量（超过10%）岩性不同的镁铁质物质，这些物质代表了在8亿年或更长时间尺度上循环通过对流地幔的古老大洋岩石圈。
