Abstract Carbon isotope exchange between carbon-bearing high temperature phases records the carbon (re-) processing in the Earth's interior, where the vast majority of global carbon is stored. Redox reactions between carbonate phases and elemental carbon govern the mobility of carbon, which then can be traced by its isotopes. We determined the carbon isotope fractionation factor between graphite and a Na2CO3-CaCO3 melt at 900–1500 °C and 1 GPa; The failure to isotopically equilibrate preexisting graphite led us to synthesize graphite anew from organic material during the melting of the carbonate mixture. Graphite growth proceeds by (1) decomposition of organic material into globular amorphous carbon, (2) restructuring into nano-crystalline graphite, and (3) recrystallization into hexagonal graphite flakes. Each transition is accompanied by carbon isotope exchange with the carbonate melt. High-temperature (1200–1500 °C) equilibrium isotope fractionation with type (3) graphite can be described by Δ 13 C c a r b o n a t e - g r a p h i t e = 3.17 ( ± 0.07 ) · 10 6 T 2 (temperature T in K). As the experiments do not yield equilibrated bulk graphite at lower temperatures, we combined the ≥1200 °C experimental data with those derived from upper amphibolite and lower granulite facies carbonate-graphite pairs (Kitchen and Valley, 1995; Valley and O'Neil, 1981). This yields the general fractionation function Δ 13 C c a r b o n a t e - g r a p h i t e = 3.37 ± 0.04 · 10 6 T 2 usable as a geothermometer for solid or liquid carbonate at ≥600 °C. Similar to previous observations, lower-temperature experiments (≤1100 °C) deviate from equilibrium. By comparing our results to diffusion and growth rates in graphite, we show that at ≤1100 °C carbon diffusion is slower than graphite growth, hence equilibrium surface isotope effects govern isotope fractionation between graphite and carbonate melt and determine the isotopic composition of newly formed graphite. The competition between diffusive isotope exchange and growth rates requires a more careful interpretation of isotope zoning in graphite and diamond. Based on graphite crystallization rates and bulk isotope equilibration, a minimum diffusivity of Dgraphite = 2 × 10−17 m2s−1 for T > 1150 °C is required. This value is significantly higher than calculated from experimental carbon self-diffusion constants (∼1.6 × 10−29 m2 s−1) but in good agreement with the value calculated for mono-vacancy migration (∼2.8 × 10−16 m2 s−1).

中文翻译：

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实验碳酸盐/石墨碳同位素分馏和碳酸盐/石墨地球测温法

摘要 含碳高温相之间的碳同位素交换记录了地球内部的碳（再）加工过程，地球内部储存了绝大多数全球碳。碳酸盐相和元素碳之间的氧化还原反应控制着碳的流动性，然后可以通过其同位素进​​行追踪。我们确定了石墨和 Na2CO3-CaCO3 熔体在 900–1500 °C 和 1 GPa 之间的碳同位素分馏因子；由于未能对预先存在的石墨进行同位素平衡，我们在碳酸盐混合物的熔化过程中从有机材料重新合成石墨。石墨生长通过 (1) 有机材料分解为球状无定形碳，(2) 重组为纳米晶石墨，以及 (3) 重结晶为六方石墨薄片。每一次转变都伴随着碳同位素与碳酸盐熔体的交换。高温 (1200–1500 °C) 平衡同位素分馏与类型 (3) 石墨可以描述为 Δ 13 C 碳酸盐 - 石墨 = 3.17 (± 0.07 ) · 10 6 T 2 (温度 T in K)。由于实验在较低温度下不会产生平衡的块状石墨，我们将≥1200 °C 的实验数据与来自上部角闪岩和下部麻粒岩相碳酸盐-石墨对的数据相结合（Kitchen 和 Valley，1995 年；Valley 和 O'Neil，1981 年）。这产生了一般分馏函数 Δ 13 C 碳酸盐 - 石墨 = 3.37 ± 0.04 · 10 6 T 2 可用作 ≥600 °C 的固体或液体碳酸盐的地温计。与之前的观察类似，低温实验 (≤1100 °C) 偏离平衡。通过将我们的结果与石墨中的扩散和生长速率进行比较，我们表明在≤1100 °C 时，碳扩散比石墨生长慢，因此平衡表面同位素效应控制着石墨和碳酸盐熔体之间的同位素分馏，并确定了新形成石墨的同位素组成. 扩散同位素交换和增长率之间的竞争需要对石墨和金刚石中的同位素分带进行更仔细的解释。基于石墨结晶速率和体同位素平衡，对于 T > 1150 °C，需要 Dgraphite 的最小扩散率 = 2 × 10−17 m2s−1。该值显着高于根据实验碳自扩散常数（~1.6 × 10-29 m2 s-1）计算的值，但与单空位迁移计算的值（~2.8 × 10-16 m2 s-1 ）。