Fluid–rock interactions in the shallow Mariana forearc: carbon cycling and redox conditions
Fluid–rock interactions in the shallow Mariana forearc: carbon cycling and redox conditions
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DOI：
10.5194/se-10-907-2019
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发表时间：
2019-03
期刊：
影响因子：
3.4
通讯作者：
E. Albers;Wolfgang Bach;F. Klein;C. Menzies;Friedrich Lucassen;D. Teagle
中科院分区：
文献类型：
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作者：
E. Albers;Wolfgang Bach;F. Klein;C. Menzies;Friedrich Lucassen;D. Teagle
抽象的。很少有数据能够深入了解影响弧前浅层长期碳循环的过程。为了更好地了解俯冲板块沉积物和地壳中碳的活化情况，我们研究了起源于马里亚纳弧前(<；20公里)俯冲通道的碳酸盐物质，并在国际海洋发现计划366考察期间发现了这些碳酸盐物质。碳酸钙以脉状沉淀物的形式出现在变质火山岩和变质沉积碎屑中。这些碎屑代表了俯冲岩石圈的一部分，包括洋岛玄武岩，它们在较低的蓝片岩相条件下发生了变化，随后被蛇纹岩泥火山作用输送到了弧前海底。无面体文石和方解石以及矿脉内缺乏变形表明，在峰期变质作用影响其寄主后，碳酸盐形成在无应力环境中。重晶石的共生和碳酸盐中显著的Ce负异常证明了沉淀是在一般的氧化环境中进行的，即不受蛇纹岩作用控制的环境。碳酸盐中的锶和氧同位素组成(87Sr/86Sr=0.7052~0.7054，δ18OVSMOW=20~24‰)反映了板片流体在∼130~300∘C之间的沉积。这些温度估算与某些脉体中存在蓝片岩相的存在相一致。掺入的C是无机的(δ13CVPDB=−1‰~+4‰)，可能来源于钙质沉积物和/或洋壳的脱碳作用。这些发现为C在下行板20公里深处的迁移提供了证据。我们的研究首次详细地表明，这种碳的一部分在活动会聚边缘的俯冲通道中形成碳酸盐沉淀物。这一过程可能是理解深层碳循环的重要资产，因为它突显了一些碳在到达更深的深度之前从俯冲的岩石圈中损失。
Abstract. Few data exist that provide insight into processes affecting the long-term carbon cycle at shallow forearc depths. To better understand the mobilization of C in sediments and crust of the subducting slab, we investigated carbonate materials that originate from the subduction channel at the Mariana forearc (< 20 km) and were recovered during International Ocean Discovery Program Expedition 366. Calcium carbonates occur as vein precipitates within metavolcanic and metasedimentary clasts. The clasts represent portions of the subducting lithosphere, including ocean island basalt, that were altered at lower blueschist facies conditions and were subsequently transported to the forearc seafloor by serpentinite mud volcanism. Euhedral aragonite and calcite and the lack of deformation within the veins suggest carbonate formation in a stress-free environment after peak metamorphism affected their hosts. Intergrowth with barite and marked negative Ce anomalies in carbonate attest the precipitation within a generally oxic environment, that is an environment not controlled by serpentinization. Strontium and O isotopic compositions in carbonate (87Sr∕86Sr = 0.7052 to 0.7054, δ18OVSMOW = 20 to 24 ‰) imply precipitation from slab-derived fluids at temperatures between ∼130 and 300 ∘C. These temperature estimates are consistent with the presence of blueschist facies phases such as lawsonite coexisting with the carbonates in some veins. Incorporated C is inorganic (δ13CVPDB = −1 ‰ to +4 ‰) and likely derived from the decarbonation of calcareous sediment and/or oceanic crust. These findings provide evidence for the mobilization of C in the downgoing slab at depths of < 20 km. Our study shows for the first time in detail that a portion of this C forms carbonate precipitates in the subduction channel of an active convergent margin. This process may be an important asset in understanding the deep carbon cycle since it highlights that some C is lost from the subducting lithosphere before reaching greater depths.