Institut für Geochemie und Petrologie, ETH Zürich, Sonneggstrasse 5, 8092 Zürich, Switzerland.
Nature. 2011 Apr 14;472(7342):209-12. doi: 10.1038/nature09899. Epub 2011 Mar 23.
Very low seismic velocity anomalies in the Earth's mantle may reflect small amounts of melt present in the peridotite matrix, and the onset of melting in the Earth's upper mantle is likely to be triggered by the presence of small amounts of carbonate. Such carbonates stem from subducted oceanic lithosphere in part buried to depths below the 660-kilometre discontinuity and remixed into the mantle. Here we demonstrate that carbonate-induced melting may occur in deeply subducted lithosphere at near-adiabatic temperatures in the Earth's transition zone and lower mantle. We show experimentally that these carbonatite melts are unstable when infiltrating ambient mantle and are reduced to immobile diamond when recycled at depths greater than ∼250 kilometres, where mantle redox conditions are determined by the presence of an (Fe,Ni) metal phase. This 'redox freezing' process leads to diamond-enriched mantle domains in which the Fe(0), resulting from Fe(2+) disproportionation in perovskites and garnet, is consumed but the Fe(3+) preserved. When such carbon-enriched mantle heterogeneities become part of the upwelling mantle, diamond will inevitably react with the Fe(3+) leading to true carbonatite redox melting at ∼660 and ∼250 kilometres depth to form deep-seated melts in the Earth's mantle.
地幔中极低的地震速度异常可能反映了橄榄岩基质中存在少量熔体，而地幔上部的熔融可能是由少量碳酸盐的存在引发的。这些碳酸盐部分源于俯冲的大洋岩石圈，部分被埋藏在 660 公里不连续面以下，并重新混合到地幔中。在这里，我们证明了碳酸盐诱发的熔融可能发生在地球过渡带和下地幔中接近绝热温度的深部俯冲岩石圈中。我们通过实验表明，当这些碳酸盐熔体渗透到周围地幔中时是不稳定的，当它们在大于约 250 公里的深度循环时会被还原为不活动的金刚石，在这种深度下，地幔的氧化还原条件由（Fe，Ni）金属相的存在决定。这种“氧化还原冻结”过程导致富含金刚石的地幔域，其中来自钙钛矿和石榴石中 Fe(2+)歧化的 Fe(0)被消耗，但 Fe(3+)被保留。当这种富含碳的地幔不均匀体成为上涌地幔的一部分时，金刚石不可避免地会与 Fe(3+)反应，导致在约 660 和 250 公里的深度真正的碳酸盐氧化还原熔融，在地幔中形成深部熔体。
