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

还原地幔中的碳：Fe-Ni-C 体系中碳化物和含碳合金的稳定性

• 批准号: 1119295
• 负责人: Marc Hirschmann
• 金额: $ 24.44万
• 依托单位: University of Minnesota-Twin Cities
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-07-01 至 2014-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1119295&HistoricalAwards=false
• 关键词: Carbon; Reduced; Mantle; Stability; Carbides

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.

在地球深层碳循环中，碳在地球近地表储层（包括海洋、大气和地壳）和地幔之间进行交换。 不同储层中的碳储存以及它们之间的通量对于数百万至数十亿年时间尺度上地球气候和宜居性的维持至关重要，并且还可能对地球内部的动力学产生主要影响，包括板块构造的运行、熔化地点、独特地球化学储层的形成以及钻石的起源。 参与地球深层碳循环的大部分碳储存在地幔中，但人们对储存方式知之甚少。 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，但这些变化也没有得到很好的约束。 在地幔的大部分区域，碳可能以还原形式存在，因此 Fe-Ni-C 系统中的相关系对稳定的相组合具有关键影响。 在该项目中，该团队将对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很少。 为了加深对地幔中还原碳主体的了解，计划对 Fe-Ni-C 系统进行实验和热力学研究。实验将在 2 至 15 GPa 之间进行，重点是 ≥6 GPa，并将解决 (a) Fe-Ni-C 系统中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以及 (d) C 在 FeNi 合金中的溶解度与 温度和压力 FeNi 合金中的 C 分析将通过电子微探针进行，使用仔细校准的程序并详细关注分析空白。 此外，还将通过 SIMS 分析合金的 C。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的热容和熵进行量热研究。Fe3C的热容已经有75年没有被测量过，而Fe7C3的热容也从未被测量过。 量热法将与 3M 的 Jean Tangeman 和萨尔茨堡大学的 Edgar Dachs 合作进行。 实验和量热结果将与碳化物和硅酸盐相平衡的现有约束相结合，构建还原碳相与橄榄岩平衡的稳定性热力学模型，接近适用于地幔的 P-T-fO2 条件。 该项目更广泛的影响包括UMN实验岩石学实验室与3M材料科学家之间的合作、UMN集团与萨尔茨堡大学Edgar Dachs之间的国际合作以及本科生和研究生的培训。