The transport of carbon into Earth's mantle is a critical pathway in Earth's carbon cycle, affecting both the climate and the redox conditions of the surface and mantle. The largest unconstrained variables in this cycle are the depths to which carbon in sediments and altered oceanic crust can be subducted and the relative contributions of these reservoirs to the sequestration of carbon in the deep mantle1. Mineral inclusions in sublithospheric, or 'superdeep', diamonds (derived from depths greater than 250 kilometres) can be used to constrain these variables. Here we present oxygen isotope measurements of mineral inclusions within diamonds from Kankan, Guinea that are derived from depths extending from the lithosphere to the lower mantle (greater than 660 kilometres). These data, combined with the carbon and nitrogen isotope contents of the diamonds, indicate that carbonated igneous oceanic crust, not sediment, is the primary carbon-bearing reservoir in slabs subducted to deep-lithospheric and transition-zone depths (less than 660 kilometres). Within this depth regime, sublithospheric inclusions are distinctly enriched in 18O relative to eclogitic lithospheric inclusions derived from crustal protoliths. The increased 18O content of these sublithospheric inclusions results from their crystallization from melts of carbonate-rich subducted oceanic crust. In contrast, lower-mantle mineral inclusions and their host diamonds (deeper than 660 kilometres) have a narrow range of isotopic values that are typical of mantle that has experienced little or no crustal interaction. Because carbon is hosted in metals, rather than in diamond, in the reduced, volatile-poor lower mantle2, carbon must be mobilized and concentrated to form lower-mantle diamonds. Our data support a model in which the hydration of the uppermost lower mantle by subducted oceanic lithosphere destabilizes carbon-bearing metals to form diamond, without disturbing the ambient-mantle stable-isotope signatures. This transition from carbonate slab melting in the transition zone to slab dehydration in the lower mantle supports a lower-mantle barrier for carbon subduction.

中文翻译：

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超深金刚石中记录的岩石圈到下地幔碳循环

碳向地幔的运输是地球碳循环的关键途径，影响气候以及地表和地幔的氧化还原条件。这个循环中最大的不受约束的变量是沉积物和改变的洋壳中的碳可以俯冲到的深度，以及这些储层对深部地幔中碳的固存的相对贡献。亚岩石圈或“超深”钻石（源自大于 250 公里的深度）中的矿物包裹体可用于限制这些变量。在这里，我们展示了来自几内亚康康钻石中矿物包裹体的氧同位素测量值，这些包裹体来自从岩石圈延伸到下地幔（大于 660 公里）的深度。这些数据，结合钻石的碳和氮同位素含量，表明在俯冲到深岩石圈和过渡带深度（小于 660 公里）的板块中，碳化火成岩洋壳而不是沉积物是主要的含碳储层。在这个深度范围内，相对于来自地壳原岩的榴辉岩岩石圈包裹体，亚岩石圈包裹体明显富含 18O。这些亚岩石圈包裹体的 18O 含量增加是由于它们从富含碳酸盐的俯冲洋壳熔体结晶而成。相比之下，下地幔矿物包裹体及其主金刚石（深度超过 660 公里）的同位素值范围很窄，这是几乎没有或根本没有地壳相互作用的地幔的典型特征。因为碳存在于金属中，而不是在钻石中，在减少的、易挥发的下地幔 2 中，碳必须被动员和浓缩以形成下地幔钻石。我们的数据支持一个模型，其中俯冲海洋岩石圈对最上层下地幔的水合作用使含碳金属不稳定，形成金刚石，而不会干扰环境地幔稳定同位素特征。这种从过渡带中的碳酸盐板片熔化到下地幔中的板片脱水的转变支持了碳俯冲的下地幔屏障。在不干扰环境地幔稳定同位素特征的情况下。这种从过渡带中的碳酸盐板片熔化到下地幔中的板片脱水的转变支持了碳俯冲的下地幔屏障。在不干扰环境地幔稳定同位素特征的情况下。这种从过渡带中的碳酸盐板片熔化到下地幔中的板片脱水的转变支持了碳俯冲的下地幔屏障。