Abstract The chemical and physical processes operating during subduction-zone metamorphism can profoundly influence the cycling of elements on Earth. Deep-Earth carbon (C) cycling and mobility in subduction zones has been of particular recent interest to the scientific community. Here, we present textural and geochemical data (C O, Sr isotopes and bulk and in-situ trace element concentrations) for a suite of ophicarbonate rocks (carbonate-bearing serpentinites) metamorphosed over a range of peak pressure-temperature (P-T) conditions together representing a prograde subduction zone P-T path. These rocks, in order of increasing peak P-T conditions, are the Internal Liguride ophicarbonates (from the Bracco unit, N. Apennines), pumpellyite- and blueschist-facies ophicarbonates from the Sestri-Voltaggio zone (W. Ligurian Alps) and the Queyras (W. Alps), respectively, and eclogite-facies ophicarbonates from the Voltri Massif. The Bracco oceanic ophicarbonates retain breccia-like textures associated with their seafloor hydrothermal and sedimentary origins. Their trace element concentrations and δ18OVSMOW (+15.6 to +18.2‰), δ13CVPDB (+1.1 to +2.5‰) and their 87Sr/86Sr (0.7058 to 0.7068), appear to reflect equilibration during Jurassic seawater-rock interactions. Intense shear deformation characterizes the more deeply subducted ophicarbonates, in which prominent calcite recrystallization and carbonation of serpentinite clasts occurred. The isotopic compositions of the pumpellyite-facies ophicarbonates overlap those of their oceanic equivalents whereas the most deformed blueschist-facies sample shows enrichments in radiogenic Sr (87Sr/86Sr = 0.7075) and depletion in 13C (with δ13C as low as −2.0‰). These differing textural and geochemical features for the two suites reflect interaction with fluids in closed and open systems, respectively. The higher-P-metamorphosed ophicarbonates show strong shear textures, with coexisting antigorite and dolomite, carbonate veins crosscutting prograde antigorite foliation and, in some cases, relics of magnesite-nodules enclosed in the foliation. These rocks are characterized by lower δ18O (+10.3 to 13.0‰), enrichment in radiogenic Sr (87Sr/86Sr up to 0.7096) and enrichment in incompatible and fluid-mobile element (FME; e.g., As, Sb, Pb). These data seemingly reflect interaction with externally-derived metamorphic fluids and the infiltrating fluids likely were derived from dehydrating serpentinites with hybrid serpentinite-sediment compositions. The interaction between these two lithologies could have occurred prior to or after dehydration of the serpentinites elsewhere. We suggest that decarbonation and dissolution/precipitation processes operating in ancient subduction zones, and resulting in the mobilization of C, are best traced by a combination of detailed field and petrographic observations, C, O and Sr isotope systematics (i.e., 3D isotopes), and FME inventories. Demonstration of such processes is key to advancing our understanding of the influence of subduction zone metamorphism on the mobilization of C in subducting reservoirs and the efficiency of delivery of this C to depths beneath volcanic arcs and into the deeper mantle.

中文翻译：

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从海底到俯冲的 Ophicarbonate 演化及其对深地球 C 循环的影响

摘要 俯冲带变质作用过程中的化学和物理过程可以深刻地影响地球上元素的循环。俯冲带中的深地碳 (C) 循环和流动性最近引起了科学界的特别关注。在这里，我们展示了一组在一系列峰值压力 - 温度 (PT) 条件下变质的钙碳酸盐岩石（含碳酸盐蛇纹岩）的结构和地球化学数据（CO、Sr 同位素以及整体和原位微量元素浓度），共同代表前进俯冲带 PT 路径。这些岩石，按照 PT 峰值条件的增加顺序，是 Internal Liguride ophicarbonates（来自 Bracco 单元，N. Apennines）、来自 Sestri-Voltaggio 区（W. Ligurian Alps）和 Queyras（ W. 阿尔卑斯山），分别是来自 Voltri 地块的榴辉岩相异碳酸盐。Bracco 海洋异碳酸盐保留了与其海底热液和沉积成因相关的角砾状结构。它们的微量元素浓度和 δ18OVSMOW（+15.6 至 +18.2‰）、δ13CVPDB（+1.1 至 +2.5‰）和它们的 87Sr/86Sr（0.7058 至 0.7068）似乎反映了侏罗纪海水-岩石相互作用期间的平衡。强烈的剪切变形表征了更深俯冲的蛇纹石碎屑，其中发生了显着的方解石再结晶和碳化作用。水泵岩相磷碳酸盐的同位素组成与其海洋等价物的同位素组成重叠，而变形最严重的蓝片岩相样品显示放射性 Sr 富集（87Sr/86Sr = 0.7075）和 ​​13C 耗尽（δ13C 低至 -2.0‰）。这两套不同的结构和地球化学特征分别反映了封闭系统和开放系统中流体的相互作用。高 P 变质的蛇纹石显示出强烈的剪切结构，与叶蛇纹石和白云石共存，碳酸盐矿脉横切前行叶蛇纹石叶理，在某些情况下，包含在叶理中的菱镁矿-结核遗迹。这些岩石的特点是 δ18O 较低（+10.3 至 13.0‰），富含放射成因 Sr（87Sr/86Sr 高达 0.7096）和富含不相容和流体流动元素（FME；例如 As、Sb、Pb）。这些数据似乎反映了与外部衍生的变质流体的相互作用，并且渗透流体可能来自具有混合蛇纹岩-沉积物成分的脱水蛇纹岩。这两种岩性之间的相互作用可能发生在其他地方蛇纹岩脱水之前或之后。我们建议在古代俯冲带中运行的脱碳和溶解/沉淀过程，并导致 C 的移动，最好通过详细的现场和岩相观察、C、O 和 Sr 同位素系统学（即 3D 同位素）的组合来追踪，和 FME 库存。这些过程的证明是促进我们理解俯冲带变质作用对俯冲储层中 C 迁移的影响以及 C 将 C 输送到火山弧下方深处和更深地幔的效率的关键。我们建议在古代俯冲带中运行的脱碳和溶解/沉淀过程，并导致 C 的移动，最好通过详细的现场和岩相观察、C、O 和 Sr 同位素系统学（即 3D 同位素）的组合来追踪，和 FME 库存。这些过程的证明是促进我们理解俯冲带变质作用对俯冲储层中 C 迁移的影响以及 C 将 C 输送到火山弧下方深处和更深地幔的效率的关键。我们建议在古代俯冲带中运行的脱碳和溶解/沉淀过程，并导致 C 的移动，最好通过详细的现场和岩相观察、C、O 和 Sr 同位素系统学（即 3D 同位素）的组合来追踪，和 FME 库存。这些过程的证明是促进我们理解俯冲带变质作用对俯冲储层中 C 迁移的影响以及 C 将 C 输送到火山弧下方深处和更深地幔的效率的关键。