No significant boron in the hydrated mantle of most subducting slabs.
No significant boron in the hydrated mantle of most subducting slabs.
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DOI：
10.1038/s41467-018-07064-6
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发表时间：
2018-11-02
影响因子：
16.6
通讯作者：
Agostini S
中科院分区：
文献类型：
--
作者：
McCaig AM;Titarenko SS;Savov IP;Cliff RA;Banks D;Boyce A;Agostini S
硼已成为海水衍生流体向弧火山释放的主要代用物，与硼含量和同位素比值的弧交叉变化有关。由于迄今为止分析的所有海底蛇纹岩都富含硼，因此一般认为，如果最上层的板块地幔水合，它也将富含硼。在这里，我们提出了在太平洋快速扩散的海洋辉长岩中硼和硼同位素的第一次测量，表明在蚀变过程中强烈吸收了来自海水的硼。研究表明，在上地幔的一次水化作用中，如弯曲断裂蛇纹岩化所提出的，硼不会到达水化的板块地幔。只有延长的热液循环，例如在长寿命的转换断层中，才能向板块地幔中添加大量硼。我们认为，俯冲板块中的水合地幔对弧火山岩中的硼富集或深部地幔再循环的贡献很小。硼是俯冲带火山岩中海水衍生流体的主要代用物之一，对确定海洋岩石圈中硼的位置和浓度具有重要意义。在这里，作者表明，由于硼在水化过程中与水强烈解耦，下行板块地幔中的硼浓度被高估了。
Boron has become the principle proxy for the release of seawater-derived fluids into arc volcanics, linked to cross-arc variations in boron content and isotopic ratio. Because all ocean floor serpentinites so far analysed are strongly enriched in boron, it is generally assumed that if the uppermost slab mantle is hydrated, it will also be enriched in boron. Here we present the first measurements of boron and boron isotopes in fast-spread oceanic gabbros in the Pacific, showing strong take-up of seawater-derived boron during alteration. We show that in one-pass hydration of the upper mantle, as proposed for bend fault serpentinisation, boron will not reach the hydrated slab mantle. Only prolonged hydrothermal circulation, for example in a long-lived transform fault, can add significant boron to the slab mantle. We conclude that hydrated mantle in subducting slabs will only rarely contribute to boron enrichment in arc volcanics, or to deep mantle recycling. Boron is one of the main proxies for seawater-derived fluids in subduction zone volcanics and it is vital to characterise the location and concentration of boron in the oceanic lithosphere. Here the authors show that boron concentration in the mantle of downgoing slabs has been overestimated, because boron is strongly decoupled from water in the hydration process.
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