Osmium isotopes and mantle convection
Osmium isotopes and mantle convection
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DOI：
10.1098/rsta.2002.1073
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发表时间：
2002-11
期刊：
影响因子：
--
通讯作者：
E. Hauri
中科院分区：
文献类型：
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作者：
E. Hauri
187Re 衰变到 187Os（半衰期为 420 亿年）为追踪对流地幔中地壳物质和地幔残留物的演化提供了独特的同位素指纹。由于大程度的熔体萃取，古代次大陆地幔岩石圈具有独特的低 Re/Os 和 187Os/188Os 比值，记录了长达 32 亿年的古代熔耗事件。部分熔体的 Re/Os 比率比其来源高几个数量级，并且海洋或大陆地壳的俯冲引入地幔物质，迅速积累放射性 187Os。来自次大陆岩石圈的榴辉岩具有极高的 187Os/188Os 比率，记录的年龄与最古老的橄榄岩一样古老。数据显示橄榄岩（低）和榴辉岩（高）之间 Re/Os 和 187Os/188Os 比率近乎完美的分配。对流地幔保留了一定程度的与岩石圈地幔相似的Os-同位素异质性，尽管其幅度受到对流混合的调节。来自洋脊的深渊橄榄岩具有较低的 Os 同位素比率，表明在最近的熔融事件产生洋中脊玄武岩之前，上地幔已经经历过熔融消耗阶段。据估计，上地幔中铼的消耗量是大陆地壳铼预算的 10 倍，因此需要一个单独的储层来关闭质量平衡。一个由 5-10% 的地幔组成的储层，其铼浓度与洋中脊玄武岩相似，可以平衡上地幔的铼消耗。该储层很可能由在地球历史上循环进入地幔的镁铁质海洋地壳组成，并提供在海洋热点熔化产生海洋岛玄武岩（OIB）的物质。 OIB 中普遍存在的高锇同位素比率，加上其他地球化学示踪剂，表明热点的地幔源含有大量（大于 10%）的岩石学上不同的镁铁质材料，这些材料代表了古代海洋岩石圈在对流地幔中以 8 亿年或更长的时间尺度循环。
The decay of 187Re to 187Os (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 187Os/188Os 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 187Os. Eclogites from the subcontinental lithosphere have extremely high 187Os/188Os ratios, and record ages as old as the oldest peridotites. The data show a near–perfect partitioning of Re/Os and 187Os/188Os 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.