Molybdenum and boron isotopic evidence for carbon-recycling via carbonate dissolution in subduction zones
Molybdenum and boron isotopic evidence for carbon-recycling via carbonate dissolution in subduction zones
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DOI：
10.1016/j.gca.2019.12.013
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发表时间：
2020-06
影响因子：
5
通讯作者：
Yunying Zhang;C. Yuan;M. Sun;Jie Li;X. Long;Yingde Jiang;Zongying Huang
中科院分区：
文献类型：
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作者：
Yunying Zhang;C. Yuan;M. Sun;Jie Li;X. Long;Yingde Jiang;Zongying Huang
俯冲带是地球表面和内部之间碳循环的关键地点。然而，俯冲的碳是如何释放和转移到地表的还不是很清楚，特别是关于板条衍生的流体在深层碳循环中的作用。本文报道了中国北天山志留系正弧安山岩和埃达克安山岩的Mo和B同位素数据，它们分别代表了流体变质的地幔楔体和脱水洋壳的部分熔体。正常弧安山岩的δ-98Mo值(0.33-1.08‰)明显高于亏损地幔的值(约-0.20‰)。由于其有限的SiO_2范围(53.8-55.3wt.%)排除了δ98Mo值变化的分异原因，仅靠流体活化的Mo同位素分馏也是有限的(≤0.3‰)，δ98Mo值的升高可归因于地幔源区中含有重Mo同位素的地壳物质的作用。由于海相碳酸盐岩具有重Mo和B同位素特征，而我们的正常弧安山岩也具有重δ11B(−1.63~+4.00‰)值，我们认为海相碳酸盐可能参与了俯冲板块的成分，改变了地幔源区的Mo-B同位素组成。δ98Mo与δ11B和δ98Mo与Ba/Rb的正相关表明，俯冲碳酸盐通过板条流体向上覆地幔楔体输送，从而为碳酸盐溶解向上覆板块输送俯冲碳提供了强有力的证据。相比之下，较年轻的埃达克质安山岩具有轻的δ98Mo(−0.48~−0.27‰)和δ11B(−9.43~−2.05‰)值，这意味着钼和B的同位素光源。考虑到重的98Mo和11B在板片脱水过程中优先向流体相迁移，它们非常轻的δ98Mo和δ11B值支持脱水洋壳作为其岩浆来源。这两种安山岩的Mo-B同位素对比突出表明，在大洋俯冲过程中，大部分碳酸盐可以从俯冲板块移至俯冲板块。
Subduction zones are critical sites for carbon cycling between Earth’s surface and interior. However, how subducted carbon is released and transferred to the surface is not well understood, especially regarding the role of slab-derived fluids in the deep carbon cycle. Here we report Mo and B isotopic data for the Silurian normal arc andesites and adakitic andesites from the Chinese North Tianshan, which represent partial melts of fluid-modified mantle wedge and dehydrated oceanic crust, respectively. The normal arc andesites yielded δ98Mo values (0.33–1.08‰) significantly higher than that (about –0.20‰) of the depleted mantle. Because their limited range of SiO2(53.8–55.3 wt.%) precludes differentiation as a cause for their variable δ98Mo values and Mo isotopic fractionation solely by fluid mobilization is limited (≤0.3‰), the elevated δ98Mo values could be ascribed to the incorporation of crustal material with heavy Mo isotopes in the mantle source. Since marine carbonate is featured by both heavy Mo and B isotopes and our normal arc andesites also give heavy δ11B (−1.63 to +4.00‰) values, we consider that marine carbonate was possibly involved as a component of the subducted slab, which modified Mo–B isotopic compositions of the mantle source. The positive correlations between δ98Mo and δ11B and between δ98Mo and Ba/Rb suggest transport of subducted carbonates to the overlying mantle wedge via slab fluids, thus providing robust evidence for transfer of subducted carbon to the overriding plate by carbonate dissolution. In contrast, the younger adakitic andesites have light δ98Mo (−0.48 to −0.27‰) and δ11B (−9.43 to −2.05‰) values, implying an isotopically Mo- and B-light source. Given the preferential transport of heavy98Mo and11B to the fluid phase during slab dehydration, their remarkably light δ98Mo and δ11B values support a dehydrated oceanic crust as their magma source. The contrasting Mo–B isotopes for such two kinds of andesites highlight that most carbonates can be removed from the subducted slab to the overriding plate during oceanic subduction.