Liu Yongsheng, He Detao, Gao Changgui, Foley Stephen, Gao Shan, Hu Zhaochu, Zong Keqing, Chen Haihong
State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan, 430074, China.
ARC Centre of Excellence for Core to Crust Fluid Systems, Dept. of Earth and Planetary Sciences, Macquarie University, North Ryde, New South Wales 2109, Australia.
Sci Rep. 2015 Jun 23;5:11547. doi: 10.1038/srep11547.
Carbon in rocks and its rate of exchange with the exosphere is the least understood part of the carbon cycle. The amount of carbonate subducted as sediments and ocean crust is poorly known, but essential to mass balance the cycle. We describe carbonatite melt pockets in mantle peridotite xenoliths from Dalihu (northern China), which provide firsthand evidence for the recycling of carbonate sediments within the subduction system. These pockets retain the low trace element contents and δ(18)OSMOW = 21.1 ± 0.3 of argillaceous carbonate sediments, representing wholesale melting of carbonates instead of filtered recycling of carbon by redox freezing and melting. They also contain microscopic diamonds, partly transformed to graphite, indicating that depths >120 km were reached, as well as a bizarre mixture of carbides and metal alloys indicative of extremely reducing conditions. Subducted carbonates form diapirs that move rapidly upwards through the mantle wedge, reacting with peridotite, assimilating silicate minerals and releasing CO2, thus promoting their rapid emplacement. The assimilation process produces very local disequilibrium and divergent redox conditions that result in carbides and metal alloys, which help to interpret other occurrences of rock exhumed from ultra-deep conditions.
岩石中的碳及其与外逸层的交换速率是碳循环中最不为人所了解的部分。作为沉积物和洋壳俯冲的碳酸盐数量鲜为人知，但对于碳循环的质量平衡至关重要。我们描述了来自中国北方大湖的地幔橄榄岩捕虏体中的碳酸岩熔体囊，它们为俯冲系统内碳酸盐沉积物的再循环提供了第一手证据。这些囊保留了泥质碳酸盐沉积物的低微量元素含量和δ(18)OSMOW = 21.1 ± 0.3，代表了碳酸盐的整体熔融，而不是通过氧化还原冻结和熔融对碳进行过滤再循环。它们还含有部分转变为石墨的显微金刚石，表明达到了>120公里的深度，以及指示极端还原条件的碳化物和金属合金的奇异混合物。俯冲的碳酸盐形成底辟，迅速向上穿过地幔楔，与橄榄岩反应，同化硅酸盐矿物并释放二氧化碳，从而促进它们的快速就位。同化过程产生非常局部的不平衡和不同的氧化还原条件，导致碳化物和金属合金的形成，这有助于解释从超深条件下挖掘出的其他岩石的出现情况。
