Mixed incorporation of carbon and hydrogen in silicate melts under varying pressure and redox conditions
Mixed incorporation of carbon and hydrogen in silicate melts under varying pressure and redox conditions
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DOI：
10.1016/j.epsl.2020.116520
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发表时间：
2020-11-01
影响因子：
5.3
通讯作者：
Banjara, Dipendra
中科院分区：
文献类型：
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作者：
Karki, Bijaya B.;Ghosh, Dipta B.;Banjara, Dipendra
包括碳和氢在内的挥发物通常被认为在硅酸盐熔体中比在地幔岩石中更容易溶解。然而，在今天和早期演化过程中，这些融化物是如何促成地球内部关键挥发物的储存和分布的，在很大程度上仍是未知的。提高我们对整个地幔压力状态下含挥发物硅酸盐岩浆的认识至关重要。本文利用第一性原理分子动力学模拟研究了含碳和氢的熔融Mg1-xFexSiO3 （x = 0,0.25）。结果表明，在不同的压力和氧化还原条件下，二元挥发物在熔体中的溶解机理有很大的不同。当作为CO2和H2O组分（对应于氧化条件）加入时，几乎所有的碳和氢都与氧形成键。它们在低压下的形态主要由分离的CO2分子、碳酸盐和羟基组成。更多的含氧物质，包括四面体配位碳，氢（O-H-O）桥，各种氧连接的配合物随着熔体进一步压缩而出现。当两种挥发物结合为碳氢化合物CH4和C2H6（对应于还原条件）时，羟基普遍存在，分子氢显著存在。碳氧键几乎完全被抑制。相反，碳与自身、氢和硅直接相关。这两种挥发物对铁也表现出很强的亲和力。因此，还原挥发形态涉及由碳、氢、硅和铁组成的聚合配合物，主要可以用两种形式表示：c1 - 4h1 - 5si0 - 50 -2（无铁）和c5 - 8h1 - 8si0 - 6fe5 - 80 -2。计算得到的氧化和还原组分中二元挥发物的分摩尔体积最初随压力迅速减小，然后在更高的压力下逐渐减小，从而系统地降低硅酸盐熔体密度。我们对计算出的挥发性成分和铁对熔体密度的相反影响的评估表明，熔体-晶体密度交叉可能发生在今天的地幔中，也可能发生在早期的岩浆海洋环境中。上地幔和过渡带条件下的熔体可能以各种氧化和非氧化形式溶解碳和氢。深部部分熔体和下地幔条件下的岩浆海残余物可能在核-幔分异过程中将碳作为复杂的还原物析出到岩心，而保留了大部分氢作为羟基伴生物。(C) 2020 Elsevier B.V.版权所有
Volatiles including carbon and hydrogen are generally considered to be more soluble in silicate melts than in mantle rocks. How these melts contribute to the storage and distribution of key volatiles in Earth's interior today and during its early evolution, however, remains largely unknown. It is essential to improve our knowledge about volatiles-bearing silicate magmas over the entire mantle pressure regime. Here we investigate molten Mg1-xFexSiO3 (x = 0, 0.25) containing both carbon and hydrogen using first-principles molecular dynamics simulations. Our results show that the dissolution mechanism of the binary volatiles in melts varies considerably under different conditions of pressure and redox. When incorporated as CO2 and H2O components (corresponding to oxidizing conditions) almost all carbon and hydrogen form bonds with oxygen. Their speciation at low pressure consists of predominantly isolated molecular CO2, carbonates, and hydroxyls. More oxygenated species, including tetrahedrally coordinated carbons, hydrogen (O-H-O) bridges, various oxygen-joined complexes appear as melt is further compressed. When two volatiles are incorporated as hydrocarbons CH4 and C2H6 (corresponding to reducing conditions), hydroxyls are prevalent with notable presence of molecular hydrogen. Carbonoxygen bonding is almost completely suppressed. Instead carbon is directly correlated with itself, hydrogen, and silicon. Both volatiles also show strong affinity to iron. Reduced volatile speciation thus involves polymerized complexes comprising of carbon, hydrogen, silicon, and iron, which can be mostly represented by two forms: C1-4H1-5Si0-5O0-2 (iron-free) and C5-8H1-8Si0-6Fe5-8O0-2. The calculated partial molar volumes of binary volatiles in their oxidized and reduced incorporation decrease rapidly initially with pressure and then gradually at higher pressures, thereby systematically lowering silicate melt density. Our assessment of the calculated opposite effects of the volatile components and iron on melt density indicates that melt-crystal density crossovers are possible in the present-day mantle and also could have occurred in early magma ocean environments. Melts at upper mantle and transition zone conditions likely dissolve carbon and hydrogen in a wide variety of oxidized and non-oxygenated forms. Deep-seated partial melts and magma ocean remnants at lower mantle conditions may exsolve carbon as complex reduced species possibly to the core during core-mantle differentiation while retaining a majority of hydrogen as hydroxyls-associated species. (C) 2020 Elsevier B.V. All rights reserved.