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.

在地球深部碳循环中，碳在地球近地表的储层（包括海洋、大气和地壳）与地幔之间交换。 碳在不同储层中的储存以及它们之间的通量对于维持数百万到数十亿年的地球气候和可居住性至关重要，并且可能对地球内部的动力学产生主要影响，包括板块构造的运作，熔融的轨迹，独特的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储存在地幔中，但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，但这些变化也没有很好的约束。 在地幔的大部分，碳可能是在还原形式，在系统Fe-Ni-C的相关系有关键的影响，稳定的相组合。 在这个项目中，该团队将进行Fe-Ni-C系统的相关系的实验测定，并对可能的地幔碳化物Fe 3C和Fe 7 C3的性质进行热力学测定。除了在适用于商业金刚石合成的窄压力区间（5.4-5.7 GPa）进行研究外，Fe-Ni-C系统的高压实验数据很少。 为了提高对地幔中还原碳的宿主的认识，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）7 C3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）FeNi合金中C的溶解度作为温度和压力的函数。使用仔细校准的程序和对分析空白的详细关注。 此外，将通过西姆斯分析合金的C。为了更好地理解地幔中碳化物的稳定性，建议对Fe 3C和Fe 7 C3在4 - 1900 K的热容和熵进行量热研究。 Fe 3C的热容已经有75年没有被测量过了，而Fe 7 C3的热容从来没有被测量过。 量热法将与3 M的Jean Tangeman和萨尔茨堡大学的埃德加达克斯合作进行。 实验和量热的结果将结合现有的限制碳化物和硅酸盐相平衡的热力学模型的还原碳相的稳定性与橄榄岩接近的P-T-fO 2条件适用于地幔。 该项目的更广泛影响包括UMN实验岩石学实验室与3 M材料科学家之间的合作，UMN集团与萨尔茨堡大学埃德加·达克斯之间的国际合作以及本科生和研究生的培训。