Osmium behavior in a subduction system elucidated from chromian spinel in Bonin island beach sands
Osmium behavior in a subduction system elucidated from chromian spinel in Bonin island beach sands
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DOI：
10.1130/g32044.1
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发表时间：
2011-11
期刊：
影响因子：
5.8
通讯作者：
Katsuhiko Suzuki;R. Senda;K. Shimizu
中科院分区：
文献类型：
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作者：
Katsuhiko Suzuki;R. Senda;K. Shimizu
由于壳幔Os同位素比值的显著差异，Os同位素体系是地幔或火山岩中再循环地壳组分的潜在示踪剂。然而，潜在的地壳污染叠加这一可能的信号阻碍了Os同位素比值的效用。以三个不同岛屿的海滩砂为玻安岩和拉斑玄武岩岩浆的复合样品，探讨了用铬尖晶石（Cr-尖晶石）中的Os同位素作为伊豆-小笠原弧原始岩浆Os组成的指示剂。铬尖晶石是一种早期晶体，即使在大块岩石的后期地壳混染期间也能保持其同位素组成。我们发现，高度非放射成因的锇同位素组成的铬尖晶石玻安岩，这表明他们代表原始岩浆与轻微或没有Os的贡献，俯冲板片在婴儿弧阶段（48-46 Ma）的玻安岩的产生。相反，放射性成因的Os同位素比值的铬尖晶石拉斑玄武岩最有可能反映了贡献板派生的组件，因为更多的氧化条件下，在弧下地幔可能允许Os动员俯冲板在过渡弧阶段（45-41马）。虽然浅层次的同化地壳成分与放射成因的Os可能会叠印原来的Os签名拉斑玄武岩岩浆在其上升过程中，铬尖晶石使我们能够补偿这种可能性。
The Os isotopic system is a potential tracer of a recycled crustal component in mantle or volcanic rocks because of the significant contrast between Os isotope ratios of crust and mantle. However, the potential for crustal contamination overprinting this possible signal has hampered the utility of Os isotopic ratios. We explored the use of Os isotopes in chromian spinel (Cr-spinel) as a discriminator of primitive magma Os compositions in the Izu-Bonin arc, using beach sands as composite samples of the boninite and tholeiite magmas in three different islands. Cr-spinel is an early-stage crystal that preserves its isotopic composition even during later crustal contamination of the bulk rock. We found highly unradiogenic Os isotopic compositions in Cr-spinels from boninites, suggesting that they represent primitive magmas with slight or no Os contribution from the subducting slab during the generation of boninites in the infant arc stage (48–46 Ma). Conversely, the radiogenic Os isotopic ratios in Cr-spinels from tholeiites most likely reflect the contribution from a slab-derived component, because more oxidative conditions in the subarc mantle probably allowed Os to mobilize from the subducting slab during the transitional arc stage (45–41 Ma). Although shallow-level assimilation of crustal components with radiogenic Os may overprint the original Os signature of tholeiite magma during its ascent, Cr-spinel allows us to compensate such possibilities.