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  • Carbon is one of the most

    2018-11-05

    Carbon is one of the most important elements that control life and the environment of Earth. The colorimetric assay and biosphere, however, are very small C reservoirs. By contrast, the mantle has 27,000 examoles C, whereas the total Earth surface (mostly sediments including fossil fuels) has ∼7350 examoles C, with only 0.07 in the atmosphere and 0.13 in the biosphere and 3.3 in the hydrosphere (). Therefore, carbon exchanges between the mantle and the surface, which are dominated by magmatism (outgassing) and plate subduction (down going), are the keys that strongly influence long term climate changes and the inhabitability of Earth. The C fluxes and the detailed processes that release C from deep in the mantle into the atmosphere remain obscure. There are four major forms of carbon in the mantle: diamond, graphite, carbide and carbonates, with minor CO and CH etc. Carbonate and also CO are very active in the mantle. Experiments showed that the solidus line of silicate is dramatically lowered once carbonated (), whereas CO can facilitate low-degree mantle-derived melt, reduce the SiO and increase the alkalis in the melt, i.e. promote the formation of alkali basalts (). In this case, alkali magmatism is an important source of CO emission from the mantle that must be considered sensibly. A recent paper in Nature Geoscience reported for the first time, direct observation on the transition of carbonated magmas to alkali basalts in the South China Sea (). The authors argued that carbonated silicate melts reacted with the lithospheric mantle and were converted to alkali basaltic melts on their way up. They further proposed that the extremely thin lithosphere of less than 20 km in the South China Sea facilitates extrusion of the carbonated silicate melts. Alkali basalts are widely distributed worldwide, e.g., significant portion of ocean island basalts are alkali basalts. The questions to be tackled in the future include: Is the transition from carbonated magmas to alkali basalt a normal phenomenon that controls the formation of all alkali basalts? What is the source of carbonates in the melt, i.e., newly recycled or previously stored in the transition zone ()? Large scale Mg isotope anomalies of alkali basalts in eastern China indicate the incorporation of subducted carbonates, most likely during the westward subduction of the Pacific Plate (). In the case of the South China Sea basalt, the carbon may come from subducted Neo-Tethys oceanic crust () or paleo-Pacific plate. Nevertheless, the close association of CO and carbonated magmas with alkali basalts opens a new window that enable us to have a better perspective on the deep carbon recycling. Plate subduction is the main process that transports carbon down into the mantle. Carbonate is the predominant carbon spices in the subducted oceanic slab. As shown in , carbonated MORB get melted before it goes across the Upper–Lower Mantle boundary (). So, carbonate cannot be preserved in the mantle as residues of the subducted oceanic slab. In contrast, carbonated mantle peridotite is stable between 300 and 800 km, i.e. in the transition zone and the upper most Lower Mantle. Carbonate melts may be preserved there as carbonated peridotite after reacted with mantle peridotite. Therefore, the transition zone may have played a major role in carbon recycling.
    Introduction Sediment recycling has important impacts on the differentiation of the continental crust (Plank and Langmuir, 1993; Hawkesworth et al., 1997; Chauvel et al., 2008; Behn et al., 2011; Hacker et al., 2011; Liu and Rudnick, 2011; Marschall and Schumacher, 2012). Volcanic arc zones are commonly considered as the most important sites for the recycling of subducted marine sediments into the continental crust via arc magmatism (Kay, 1978; Plank and Langmuir, 1993; Hawkesworth et al., 1997; Shimoda et al., 1998; Chauvel et al., 2008; Behn et al., 2011). In collisional orogenic belts, however, except for element recycling (Cannaò et al., 2015) and fluid–rock interaction (Halama et al., 2014) in the Alps, sediment recycling processes have been comparatively less well understood.