Thermodynamic constraints on carbonate stability and carbon volatility during subduction
Thermodynamic constraints on carbonate stability and carbon volatility during subduction
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DOI：
10.1016/j.epsl.2019.04.047
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发表时间：
2019-08-01
影响因子：
5.3
通讯作者：
Bodnar, R. J.
中科院分区：
文献类型：
--
作者：
Gorce, J. S.;Caddick, M. J.;Bodnar, R. J.
碳酸盐矿物在高压下的分解经常被认为是导致俯冲岩石中碳释放的重要机制。然而，预计俯冲板片中的碳酸盐矿物在大于约150公里的弧生岩浆深度的深度内保持稳定，这意味着脱水MORB中碳酸盐相的分解可能不是弧火山碳预算的主要贡献者。为了解释这种差异，以前的研究表明，除了富含H2O的流体促进分解的碳酸盐丰富的岩性，从而产生挥发性的C物种，可以纳入弧岩浆。在这里，我们探讨了H2O介导的脱碳与一个简单的热力学模型的可行性。我们计算的平衡矿物组合和伴随的流体H2O/CO2比典型的俯冲岩性，假设一系列的俯冲带地热，并探讨了影响的额外的外部流体所产生的超镁铁岩在不同阶段的沙漠化。结果表明，C沿着火山弧的解放是由碳酸盐矿物的分解，由于在较热的俯冲系统的有利条件，或分解碳酸盐矿物在较高的流体生产力与沙漠化在适当的深度沿着冷俯冲地热期间。在火山弧测得的碳通量的比较表明，较冷的俯冲带产生较高的碳通量，这意味着深度在其中的沙漠化反应发生强烈控制的可用性板脱碳的含水流体，和流体的可用性代表的主要控制在俯冲过程中的碳挥发性。(C)2019爱思唯尔B. V.保留所有权利。
The breakdown of carbonate minerals at high pressure is frequently cited as an important mechanism that leads to carbon release from subducted rocks. However, carbonate minerals in the subducting slab are predicted to be stable to depths that are greater than arc-generating magma depths of approximately 150 km, implying that breakdown of carbonate phases in dehydrated MORB may not be a major contributor to arc volcano carbon budgets. To account for this discrepancy, previous studies have suggested that addition of H2O-rich fluids promotes the breakdown of carbonate-rich lithologies, thus generating volatile C species that could be incorporated into arc magmas. Here, we explore the feasibility of H2O-mediated decarbonation with a simple thermodynamic model. We calculate equilibrium mineral assemblages and accompanying fluid H2O/CO2 ratios for typical subducted lithologies, assuming a range of subduction zone geotherms, and explore the implications of addition of external fluids that are generated from deserpentinization of ultramafic lithologies at various stages. Results suggest that the liberation of C along volcanic arcs is facilitated by either the breakdown of carbonate minerals due to thermodynamically favorable conditions in hotter subduction systems, or by the breakdown of carbonate minerals during periods of higher fluid productivity associated with deserpentinization at appropriate depths along colder subduction geotherms. A comparison of C fluxes measured at volcanic arcs shows that colder subduction zones generate higher C fluxes, implying that the depth at which deserpentinization reactions occur strongly controls the availability of aqueous fluids for slab decarbonation, and that fluid availability represents the dominant control on carbon volatility during subduction. (C) 2019 Elsevier B.V. All rights reserved.