Carbon in Reduced Mantle: Stability of Carbides and C-bearing Alloys in the System Fe-Ni-C

在地球深部碳循环中，碳在地球近地表储层（包括海洋、大气和地壳）和地幔之间交换。在数百万年至数十亿年的时间尺度上，不同储层中碳的储存和它们之间的通量对维持地球气候和宜居性至关重要，还可能对地球内部的动力学产生主要影响，包括板块构造的作用、熔化的轨迹、不同地球化学储层的形成以及钻石的起源。参与地球深部碳循环的大部分碳都储存在地幔中，但人们对其储存方式知之甚少。可能的相包括碳酸盐、金刚石、FeNi合金、碳化物或碳化物熔体。稳定相可能随深度和横向变化，但这些变化也没有很好的约束。在大部分地幔中，碳可能以还原形式存在，因此Fe-Ni-C体系中的相关系对稳定相组合具有关键影响。在本项目中，团队将对Fe-Ni-C体系中的相关系进行实验测定，并对可能的地幔碳化物Fe3C和Fe7C3的性质进行热力学测定。除了适用于商业金刚石合成的较窄压力区间（5.4-5.7 GPa）的研究外，Fe-Ni-C体系的高压实验数据较少。为进一步了解地幔中还原碳的寄主，计划开展Fe-Ni-C体系的实验和热力学研究。实验将在2到15 GPa之间进行，重点是&amp；#8805；(a) Fe-Ni- c体系相稳定性的拓扑结构(b) (Fe,Ni)3C和（Fe,Ni）7C3碳化物的相对稳定性(c) Fe-Ni- c碳化物熔体的稳定性轨迹和(d) c在FeNi合金中的溶解度随温度和压力的函数分析FeNi合金中的c将通过电子探针进行，使用仔细校准的程序和对分析空白的详细关注。此外，合金将通过SIMS进行C分析。为了更好地了解地幔中碳化物的稳定性，建议对Fe3C和Fe7C3在4 ~ 1900 K范围内的热容和熵进行量热研究。Fe3C的热容已经有75年没有测量过了，Fe7C3的热容也没有测量过。量热法将与3M的Jean Tangeman和Universtät Salzburg的Edgar Dachs合作进行。实验和量热结果将结合现有的碳化物和硅酸盐相平衡约束，建立与橄榄岩相平衡的还原碳相稳定性热力学模型，接近于适用于地幔的P-T-fO2条件。该项目的广泛影响包括UMN实验岩石学实验室与3M材料科学家之间的合作，UMN集团与Universtät Salzburg的Edgar Dachs之间的国际合作以及本科生和研究生的培训。

In the deep Earth carbon cycle, carbon is exchanged between Earth's near surface reservoirs (including the oceans, atmosphere, and crust) and the mantle. The storage of carbon in different reservoirs and the fluxes between them are of key importance to maintenance of Earth's climate and habitability on times scales of millions to billions of years and also may have principal influence on the dynamics of Earth's interior, including the operation of plate tectonics, the locus of melting, the formation of distinct geochemical reservoirs, and the origin of diamonds. The majority of the carbon participating in the deep Earth carbon cycle is stored in the mantle, but the mode of storage is poorly understood. Possible phases include carbonates, diamonds, FeNi alloys, carbides, or carbide melts. The stable phases are likely to vary with depth and laterally, but these variations are also not well-constrained. In much of the mantle, carbon is likely in reduced form, for which phase relations in the system Fe-Ni-C have key influence on the stable phase assemblages. In this project, the team will conduct experimental determinations of phase relations in the system Fe-Ni-C and make thermodynamic determinations of the properties of possible mantle carbides Fe3C and Fe7C3.Apart from studies in a narrow pressure interval applicable to commercial diamond synthesis (5.4-5.7 GPa), high pressure experimental data for the system Fe-Ni-C are sparse. To improve understanding of the hosts of reduced carbon in the mantle, it is planned to conduct experimental and thermodynamic studies of the system Fe-Ni-C. Experiments will be conducted between 2 and 15 GPa with a focus at ≥6 GPa and will address (a) the topology of phase stability in the system Fe-Ni-C (b) the relative stabilities of (Fe,Ni)3C and (Fe,Ni)7C3 carbides (c) the locus of stability of Fe-Ni-C carbide melts and (d) the solubility of C in FeNi alloy as a function of temperature and pressure Analyses of C in FeNi alloy will be performed by electron microprobe, using carefully calibrated procedures and detailed attention to analytical blanks. Additionally, the alloys will be analyzed for C by SIMS. To better understand the stability of carbides in the mantle, it is proposed to perform a calorimetric study of the heat capacities and entropies of Fe3C and Fe7C3 from 4 to 1900 K. The heat capacities of Fe3C have not been measured in 75 years and those of Fe7C3 have never been measured. Calorimetry will be performed in collaboration with Jean Tangeman of 3M and Edgar Dachs of Universtät Salzburg. The experimental and calorimetric results will be combined with existing constraints on carbide and silicate phase equilibria to construct thermodynamic models of the stability of reduced carbon phases in equilibrium with peridotite close to the P-T-fO2 conditions applicable to the mantle. Broader impacts of the project include collaboration between the UMN experimental petrology laboratory and materials scientists at 3M, international collaboration between the UMN group and Edgar Dachs at the Universtät Salzburg and the training of undergraduate and graduate students.