Carbonate Transfer during the Onset of Slab Devolatilization: New Insights from Fe and Zn Stable Isotopes
Carbonate Transfer during the Onset of Slab Devolatilization: New Insights from Fe and Zn Stable Isotopes
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DOI：
10.1093/petrology/egy057
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发表时间：
2018-06-01
影响因子：
3.9
通讯作者：
Williams, H.
中科院分区：
文献类型：
--
作者：
Debret, B.;Bouilhol, P.;Williams, H.
长期碳循环是最近争议的一个主题，因为新的质量平衡计算表明，大多数碳在俯冲过程中通过流体从板块转移到地幔楔，限制了碳循环到地幔深处的效率。在这里，我们使用对 H-C-O-S-Cl 流体敏感的新同位素示踪剂（即铁和锌稳定同位素）检查俯冲过程中碳的大规模流动性，这些示踪剂被解释为代表残余板块（奎拉斯，西阿尔卑斯山）和弧下地幔（科希斯坦，喜马拉雅山）的样本。我们表明，在俯冲过程中，板片中变沉积岩和超镁铁岩之间的交代界面处存在碳酸盐沉淀和溶解的几个阶段。在俯冲的早期阶段，在板块达到 300-400 摄氏度等温线之前，沉积物衍生的流体渗透到超镁铁岩岩性中，增强了含蛇纹石的蛇纹岩中碳酸盐的沉淀。因此，蛇纹岩中的碳酸盐储存充当俯冲带中碳的临时储存库。这一事件伴随着蛇纹石铁同位素组成（δ Fe-56）的减少，这是由于与低δ Fe-56沉积物衍生流体的相互作用，以及流体流动元素（例如B、Li、As）浓度的增加。在较高温度（> 400 摄氏度）下，碳酸盐会被流体从蛇纹岩中浸出。这伴随着蛇纹岩锌同位素组成（δ（66）zn）的减少，我们将其解释为具有重同位素δ（66）zn特征的含碳酸盐流体的释放。热力学模型表明，流体碳迁移率的突然变化是由于板片顺变质作用期间释放的流体aCO(2) 减少，从沉积物主导脱水转变为蛇纹岩主导脱水。这表明，在板片达到榴辉岩相 P-T 条件之前，含有氧化碳（例如 CO2）、与同位素轻 Fe、重 Zn 和流体流动元素相关的板片流体可以被释放。这些观测结果为俯冲早期流体中碳的流动性提供了有力的证据。此外，释放的流体将充当弧前地幔（或板片/地幔界面）的潜在交代剂。在喜马拉雅弧下地幔中观察到的含碳酸盐变质脉具有互补的轻三角洲 Fe-56 和重三角洲 Zn-66 特征，为俯冲早期阶段含硫酸盐和碳酸盐流体的大规模转移提供了进一步的支持。这表明弧前可能在向弧岩浆源头输送水、硫和碳方面发挥着重要作用。
Long-term carbon cycling is a subject of recent controversy as new mass balance calculations suggest that most carbon is transferred from the slab to the mantle wedge by fluids during subduction, limiting the efficiency of carbon recycling to the deep mantle. Here, we examine the large scale mobility of carbon during subduction using new isotopic tracers sensitive to H-C-O-S-Cl fluids, namely iron and zinc stable isotopes, in samples interpreted to represent residual slab (Queyras, Western Alps) and sub-arc mantle (Kohistan, Himalaya). We show that during subduction there are several stages of carbonate precipitation and dissolution at metasomatic interfaces between metasedimentary and ultramafic rocks in the slab. During the early stages of subduction, before the slab reaches the 300-400 degrees C isotherms, the infiltration of sediment-derived fluids into ultramafic lithologies enhances carbonate precipitation in antigorite-bearing serpentinites. Carbonate storage in serpentinites, therefore, acts as a temporary reservoir of carbon in subduction zones. This episode is accompanied by a decrease in serpentinite iron isotope composition (delta Fe-56), due to interaction with low-delta Fe-56 sediment-derived fluids, and an increase in the concentrations of fluid-mobile elements (e.g. B, Li, As). At higher temperatures (> 400 degrees C), carbonate is leached from the serpentinites by fluids. This is accompanied by a decrease in serpentinite zinc isotope composition (delta(66)zn) which we interpret as the release of a carbonate-bearing fluid with an isotopically heavy delta(66)zn signature. Thermodynamic modelling shows that the sudden change in fluid carbon mobility is due to a decrease in the aCO(2) of the fluids released during slab prograde metamorphism, which shifts from sediment- to serpentinite-dominated dehydration. This demonstrates that slab fluids bearing oxidized carbon (e.g. CO2), associated with isotopically light Fe, heavy Zn and fluid-mobile elements, can be released before the slab reaches eclogite facies P-Tconditions. These observations provide strong evidence for the mobility of carbon in fluids during the early stages of subduction. Moreover, the fluids released will act as a potential metasomatic agent for the fore-arc mantle (or slab/mantle interface). The observation of carbonate-bearing metamorphic veins in the Himalayan sub-arc mantle with complementary light delta Fe-56 and heavy delta Zn-66 signatures provides further support for the large scale transfer of both sulphate- and carbonate-bearing fluids during the early stages of subduction. This suggests that the fore-arc may have an important role in delivering water, sulfur and carbon to the source of arc-magmas.