Dissolution susceptibility of glass-like carbon versus crystalline graphite in high-pressure aqueous fluids and implications for the behavior of organic matter in subduction zones
Dissolution susceptibility of glass-like carbon versus crystalline graphite in high-pressure aqueous fluids and implications for the behavior of organic matter in subduction zones
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DOI：
10.1016/j.gca.2020.01.030
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发表时间：
2020-03-15
影响因子：
5
通讯作者：
Poli, Stefano
中科院分区：
文献类型：
--
作者：
Tumiati, Simone;Tiraboschi, Carla;Poli, Stefano
有机物质的结晶度和石墨化程度各不相同，是俯冲沉积物中碳的一个重要来源，深海和超深海钻石的同位素特征以及弧环境中的火山喷发物都证明了这一点。在这项实验研究中，我们研究了sp（2）杂化碳在1和3GPa，800 ℃的水溶液中的溶解，以（i）结晶合成石墨和（ii）X射线无定形玻璃状碳作为端员。我们选择了玻璃状碳作为天然的“无序”石墨碳的类似物从有机物中衍生出来，因为不像其他形式的无序碳，它不会在研究的实验条件下进行任何结构修饰，允许接近热力学平衡。纹理的观察，拉曼光谱，同步辐射X射线衍射和溶解敏感性的碳热分解葡萄糖（代表非转化的有机物）在相同的实验条件下支持这一假设。使用双胶囊和铁橄榄石-磁铁矿-石英（FMQ）或镍-氧化镍（NNO）缓冲液将实验的氧化还原状态缓冲在约-0.5的Delta FMQ。在研究的P-T-fO（2）条件下，主要的水溶解产物是二氧化碳，由固体碳氧化形成。在1GPa和800摄氏度下，玻璃状碳的氧化溶解产生的二氧化碳比结晶石墨多16-19摩尔%。相比之下，在3GPa的较高压力下与玻璃状碳相互作用的流体仅显示出有限的CO2增加（fH（2）NNO），或者相对于与结晶石墨相互作用的流体，甚至更低的CO2含量（fH 2FMQ）。测量的流体成分允许恢复玻璃状碳和石墨之间的吉布斯自由能（DG）的差异，其在1GPa-800摄氏度下为+1.7（1）kJ/mol，在3GPa-800摄氏度下为+0.51（1）kJ/mol（fH（2）NNO）。热力学模型表明，在高压下溶解敏感性的下降与玻璃状碳相对于结晶石墨的更高的压缩性有关，导致G-P曲线在800度（C）下在约3.4GPa处交叉，接近石墨-金刚石转变。新的实验数据表明，在存在冲洗俯冲沉积物的水性流体的情况下，去除结晶不良的“无序”石墨碳比结晶石墨更有效。这尤其发生在俯冲带的浅层，在那里自由能的差异更高，并且组织不良的亚稳碳质物质和由下行板的脱挥发分产生的含水流体的可用性最大化。在大于110 km的深度处，Δ G的微小差异意味着将“无序”石墨碳转化为有序石墨的能量驱动最小;“无序”石墨碳甚至可以在窄P间隔中在能量上略微有利。(C)2020爱思唯尔有限公司保留所有权利。
Organic matter, showing variable degrees of crystallinity and thus of graphitization, is an important source of carbon in subducted sediments, as demonstrated by the isotopic signatures of deep and ultra-deep diamonds and volcanic emissions in arc settings. In this experimental study, we investigated the dissolution of sp(2) hybridized carbon in aqueous fluids at 1 and 3GPa, and 800 degrees C, taking as end-members (i) crystalline synthetic graphite and (ii) X-ray amorphous glass-like carbon. We chose glass-like carbon as an analogue of natural ``disordered" graphitic carbon derived from organic matter, because unlike other forms of poorly ordered carbon, it does not undergo any structural modification at the investigated experimental conditions, allowing approach to thermodynamic equilibrium. Textural observations, Raman spectroscopy, synchrotron Xray diffraction and dissolution susceptibility of char produced by thermal decomposition of glucose (representative of nontransformed organic matter) at the same experimental conditions support this assumption. The redox state of the experiments was buffered at Delta FMQ approximate to -0.5 using double capsules and either fayalite-magnetite-quartz (FMQ) or nickel-nickel oxide (NNO) buffers. At the investigated P-T-fO(2) conditions, the dominant aqueous dissolution product is carbon dioxide, formed by oxidation of solid carbon. At 1GPa and 800 degrees C, oxidative dissolution of glass-like carbon produces 16-19 mol% more carbon dioxide than crystalline graphite. In contrast, fluids interacting with glass-like carbon at the higher pressure of 3GPa show only a limited increase in CO2 (fH(2)NNO) or even a lower CO2 content (fH2FMQ) with respect to fluids interacting with crystalline graphite. The measured fluid compositions allowed retrieval of the difference in Gibbs free energy (DG) between glass-like carbon and graphite, which is +1.7(1) kJ/mol at 1GPa-800 degrees C and +0.51(1) kJ/mol (fH(2)NNO) at 3GPa-800 degrees C. Thermodynamic modeling suggests that the decline in dissolution susceptibility at high pressure is related to the higher compressibility of glass-like carbon with respect to crystalline graphite, resulting in G-P curves crossing at about 3.4GPa at 800 degrees(C), close to the graphite-diamond transition. The new experimental data suggest that, in the presence of aqueous fluids that flush subducted sediments, the removal of poorly crystalline "disordered" graphitic carbon is more efficient than that of crystalline graphite. This occurs especially at shallow levels of subduction zones, where the difference in free energy is higher and the availability of poorly organized metastable carbonaceous matter and of aqueous fluids produced by devolatilization of the downgoing slab is maximized. At depths greater than 110 km, the small differences in Delta G imply that there is minimal energetic drive for transforming "disordered" graphitic carbon to ordered graphite; "disordered" graphitic carbon could even be energetically slightly favored in a narrow P interval. (C) 2020 Elsevier Ltd. All rights reserved.