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

在地球深处的碳循环中，碳在地球近地表的储存库(包括海洋、大气和地壳)和地幔之间进行交换。碳在不同储存库中的储存及其之间的通量对于在数百万到数十亿年的时间尺度上维持地球的气候和宜居性至关重要，并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、不同地球化学库的形成和钻石的起源。参与地球深层碳循环的大部分碳储存在地幔中，但储存模式却鲜为人知。可能的相包括碳酸盐、钻石、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，但这些变化也不受很好的约束。在大部分地幔中，碳可能以还原的形式存在，因此Fe-Ni-C系统中的相关系对稳定的相组合有关键的影响。在这个项目中，该团队将对Fe-Ni-C系统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系统的高压实验数据很少。为了更好地了解地幔中还原碳的宿主，计划对Fe-Ni-C系统进行实验和热力学研究。实验将在2到15 Gpa之间进行，重点放在6 Gpa，并将讨论(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和(D)C在FeNi合金中的溶解度随温度和压力的变化将通过电子探针进行分析，使用经过仔细校准的程序并详细注意分析空白。此外，这些合金将通过SIMS进行碳分析。为了更好地了解碳化物在地幔中的稳定性，建议对Fe3C和Fe7C3的热容和热熵进行4-1900K的量热研究。Fe3C和Fe7C3的热容已经75年没有测量过了，Fe7C3的热容也从来没有测量过。热量测量将与3M公司的Jean Tangeman和萨尔茨堡大学的Edga Daches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，以建立与橄榄岩平衡的还原碳相的稳定性的热力学模型，接近适用于地幔的P-T-fO2条件。该项目的更广泛影响包括UMN实验岩石学实验室与3M材料科学家之间的合作，UMN集团与萨尔茨堡大学埃德加·达赫之间的国际合作，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.