Large iron isotope fractionation during mantle wedge serpentinization: implications for iron isotopes of arc magmas
Large iron isotope fractionation during mantle wedge serpentinization: implications for iron isotopes of arc magmas
复制标题
地幔楔蛇纹石化过程中的大铁同位素分馏：对弧岩浆铁同位素的影响
DOI：
10.1016/j.epsl.2022.117423
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发表时间：
2022
影响因子：
5.3
通讯作者：
Sun Weidong
中科院分区：
文献类型：
--
作者：
Deng Jianghong;He Yongsheng;Zartman Robert E.;Yang Xiaoyong;Sun Weidong
弧岩浆通常比来自板内环境和扩张脊的熔岩更氧化。然而，弧岩浆的铁同位素普遍较轻，其机制仍有争议。地幔楔蛇纹岩为解决这一问题提供了新的思路。研究表明，马里亚纳弧前地幔楔中的蛇纹岩/蛇纹化橄榄岩具有重Fe同位素（δ 56 Fe=− 0.01-0.15‰）和较高的Fe 3+/Fe 3+ Fe比值（0.50-0.88），二者呈正相关。这表明地幔楔中的蛇纹岩化作用引起了铁氧化和铁同位素分馏的耦合作用。成熟蛇纹岩的高Fe 3+/Fe 2Fe值和高SO 3含量进一步表明弧前地幔楔的高度氧化。本文提出了地幔楔状蛇纹岩化过程中铁同位素分馏和铁氧化的新机制。由于铁的溶解度低，在所产生的高pH值的流体在蛇纹石化的早期阶段，在橄榄石蛇纹石化过程中释放的亚铁优先沉淀的蛇纹岩，富铁水镁石，磁铁矿。在晚期蛇纹岩化过程中橄榄石被完全消耗后，亚铁在低pH值的富氯/硫酸盐渗透流体中变得可溶并移动的，因此在进行性蛇纹岩化过程中释放到具有轻铁同位素的流体中，留下具有重铁同位素的残余蛇纹岩。这表明，地幔楔底部的成熟蛇纹岩可作为一种过滤器，将重铁同位素的三价铁封存起来，将轻铁同位素的二价铁释放到渗透的板片流体中，并向上迁移到地幔楔上部甚至地幔熔融带。高度氧化的弧前地幔楔蛇纹岩也可以被拉至弧下深度，通过脱水释放氧化流体。这些过程可能氧化弧地幔楔与轻铁同位素，其熔融的部分熔融，从而产生氧化弧岩浆与轻铁同位素。
Arc magmas are usually more oxidized than lavas from intraplate settings and spreading ridges. However, iron isotopes of arc magmas are generally lighter, and the mechanism is still controversial. The mantle wedge serpentinite provides new insights to resolve this issue. Here we show that mantle wedge serpentinites/serpentinized peridotites from the Mariana forearc mantle wedge have both heavy Fe isotopes (δ 56 Fe=− 0.01–0.15‰) and high Fe 3+/ΣFe values (0.50–0.88), which are positively correlated. This indicates coupled iron oxidation and iron isotope fractionation induced by serpentinization in the mantle wedge. The high Fe 3+/ΣFe values and high SO 3 contents of the mature serpentinites further suggest that the forearc mantle wedge is highly oxidized. Here we present a new mechanism for iron isotope fractionation and iron oxidation during mantle wedge serpentinization. Because iron solubility is low in the resultant high-pH fluids at the early stage of serpentinization, ferrous iron liberated during olivine serpentinization preferentially precipitates as serpentine, Fe-rich brucite, and magnetite in the serpentinite. After the full consumption of olivine during the late-stage serpentinization, ferrous iron becomes soluble and mobile in low-pH chlorine/sulfate-rich penetrating fluids, and is therefore released to the fluid with light Fe isotopes during progressive serpentinization, leaving the residual serpentinite with heavy iron isotopes. This suggests that mature serpentinites at the base of mantle wedge can act as a filter that sequesters ferric iron with heavy iron isotopes and liberates ferrous iron with light iron isotopes to the penetrating slab fluid, which migrate up to the above mantle wedge and even the mantle melting zone. The highly oxidized forearc mantle wedge serpentinites can also be dragged down to the subarc depth, releasing oxidizing fluids by dehydration. These processes may oxidize the arc mantle wedge coupled with light Fe isotopes, the fluxed partial melting of which consequently produces oxidized arc magmas with light Fe isotopes.