马里亚纳弧前地幔楔蛇纹石化橄榄岩Fe同位素研究

Serpentinite is quite common in the mantle wedge, and it is very important to the oxygen fugacity of subduction zones. Iron, which is sensitive to oxygen fugacity, is one of the most important elements that is responsible to oxygen fugacity during serpentinization. There is great significance to study the Fe mobility and oxygen fugacity of mantle wedge serpentinite. Mariana forearc mantle serpentinites/ serpentinized peridotites obtained from active serpentinite mud seamounts are the ideal samples to study these issues. The Exp. 366 of International Ocean Discovery Project (IODP), have carried out drilling project on three of the Mariana serpentinite mud seamounts (affected by subduction fluids from different depths) from Dec. 2016 - Feb. 2017. We have obtained many core samples after attending this ocean drilling expedition. In this application, we are planning to choose the Mariana forearc mantle wedge serpentinized peridotites obtained in the IODP Exp.366 as our objects, by the determination of fluid mobile elements and volatiles (F, Cl and B) of the serpentinized peridotites in the three different seamounts, which are metasomatized by subduction fluids from various depths, to study the nature of dehydration fluids from different subduction depths in the forearc region. We will also analyze the Fe3+/ΣFe ratios. Based on these, we will study Fe isotopes of these serpentinized peridotite, to reveal the Fe mobility during subduction dehydration and mantle serpentinization in the forearc region. The oxygen fugacity of the forearc mantle will also be investigated, to provide a clue to the oxygen fugacity of arc mantle and the mechanism behind it.

蛇纹岩在地幔楔中广泛存在，对于俯冲带氧逸度起到极其重要的作用。其中的核心元素—铁，对氧逸度十分敏感。研究地幔楔蛇纹岩中Fe的迁移性及氧化还原状态，具有重要意义。马里亚纳岛弧系统弧前蛇纹岩泥海山中的蛇纹岩/蛇纹石化橄榄岩碎块是研究相关问题的理想样品。国际大洋发现计划（IODP）366航次于2016年12月-2017年2月对马里亚纳弧前3个蛇纹岩泥海山（受不同深度俯冲流体影响形成）进行了钻探研究，申请人曾参加该航次并获得了相关岩石样品。本申请拟选取该航次在3个海山钻取的蛇纹石化橄榄岩为研究对象，通过测定其流体活动元素和挥发分（F、Cl、B等）含量，来反映弧前区域不同深度俯冲流体的性质；结合蛇纹石化橄榄岩Fe3+/ΣFe比值，测定其Fe同位素组成，探索弧前区域俯冲洋壳脱水—地幔蛇纹石化过程中Fe的活动性及其氧化还原过程，揭示蛇纹石化过程对地幔氧逸度的影响，为岛弧地幔的氧逸度状态和成因提供依据。

岛弧岩浆岩通常比产于板内环境和扩张洋脊的岩浆（如OIB和MORB）更加氧化。然而，岛弧岩浆岩的铁同位素（δ56Fe=~0.05‰）通常比OIB（δ56Fe=~0.1‰）和MORB（δ56Fe=~0.1‰）要更轻，产生这种差异的机制目前仍有很大争议。地幔楔蛇纹岩为解决这一问题提供了新的见解。本项目以IODP366航次钻探获得的马里亚纳俯冲带弧前地幔楔蛇纹岩/蛇纹石化橄榄岩为研究对象，结合全岩主量元素、S-Cl含量以及Fe3+/ΣFe比值测试，开展了系统的铁同位素研究工作。研究结果表明，马里亚纳弧前地幔楔蛇纹岩/蛇纹石化橄榄岩具有重的Fe同位素组成(δ56Fe=-0.01-0.15‰)以及高的Fe3+/ΣFe比值(0.50-0.88)，而且二者呈正相关。这表明地幔楔中存在着由蛇纹石化作用引起的铁氧化和铁同位素分馏耦合作用。成熟蛇纹岩的高Fe3+/ΣFe值和高SO3含量进一步表明弧前地幔楔处于高度氧化状态。本研究最终提出了地幔楔蛇纹石化过程中铁氧化和铁同位素分馏的新机制。在蛇纹石化早期，流体具有高pH值，由于铁在高pH值流体中的溶解度较低，在蛇纹石化过程中橄榄石分解释放的亚铁优先以蛇纹石、富铁水镁石和磁铁矿的形式固定在蛇纹岩中。在晚期蛇纹石化过程中，当橄榄石被完全消耗后，流体pH值降低，导致亚铁在富含氯/硫酸盐的渗滤流体中变得可溶、活动。伴随这个过程不断进行，具有轻铁同位素组成的亚铁离子被不断释放到渗滤流体中，导致残留的蛇纹岩逐渐富集重铁同位素。我们的研究表明，在俯冲带地幔楔底部的成熟地幔楔蛇纹岩可以作为一个过滤器固定富重铁同位素的三价铁，并释放轻铁同位素的亚铁离子进入渗滤的板片流体中并随流体迁移至上部地幔楔甚至地幔熔融区。高度氧化的弧前地幔楔蛇纹岩也可被俯冲拖拽至弧下深度并脱水分解释放出氧化性流体。以上这些过程可能会导致岛弧地幔楔逐渐氧化并具有轻的铁同位素组成，这样的地幔楔部分熔融就会产生具有轻铁同位素组成的氧化性岛弧岩浆。