The evolution of deeply subducted carbonates and the formation of microdiamonds

这一研究揭示了深部俯冲过程中大陆物质中的水流体循环形成超高压变质岩的证据，以及超高压条件下的交代作用。提出了台内超高压交代模型来解释天然超高压变质岩中发生的这些过程。俯冲硅酸盐岩释放的多组分含水流体渗入碳酸盐岩层，导致强烈交代作用。在这种交代过程中，碳酸盐岩，特别是白云岩大理岩中可以形成丰富的微金刚石。在超高压条件下，回收的水溶液中可以溶解多种组分，其中具有代表性的是二氧化钛。在Kokchetav超高压变质岩中，钛斜方辉石赋存于无金刚石的超高压白云岩大理岩中，斜方辉石中的二氧化钛可能是水流体中的溶解组分，表明具有较高的H2O活度。这表明，即使在Carbo…中，钻石在非常高的H2O活度下也可能不稳定更多的Nate岩石和碳可以溶解到流体中。根据显微FT-IR研究，该岩石中透辉石的羟基含量超过850ppm。这也意味着，在非常高的H2O活度条件下，可以形成无钻石的白云石大理石。与含金刚石白云岩大理岩相反，该大理岩中的透辉石不含K2O出溶相。在电气石-钾长石-石英岩中发现了含K2O的电气石，在电气石-钾长石-石英岩中发现了含K2O的电气石，这可能是一种新的矿物，因为在电气石-钾长石-石英岩中未见报道。电气石的出现也表明了这类岩石中挥发性组分的高活动性，以及水流体作为这些挥发性组分的载体的可能性。所有这些结果都有力地表明了水流体在超高压变质作用中的重要性，并导致了提出的“台内超高压交代作用”的模式。

This study revealed the evidence for the aqueous fluid recycling in the continental materials during deep subduction to form UHP metamorphic rocks and for the metasomatism under UHP conditions. Intraslab UHP metasomatism model was proposed to explain these processes occurred in natural UHP metamorphic rocks. Multicomponent aqueous fluid released from subducted silicate rocks had been infiltrated into carbonate layers and had led to strong metasomatism. During such a metasomatism process, abundant microdiamond could form in carbonate rocks, particularly dolomite marble. Several components could be solved in recycled aqueous fluids under UHP conditions ; TiO2 was one of the representative one. In the Kokchetav UHP metamorphic rocks, Ti-clinohumite occurs in diamond-free UHP dolomitic marble, and the origin of TiO2 in clinohumite could be a solved component in aqueous fluid indicating high H2O activity. This suggest that diamond could be unstable under very high H2O activity even in carbo … More nate rocks and carbon could dissolve into fluid phase. Diopside in this rock contains over 850ppm of H2O as hydroxyl on the basis of micro-FT-IR study. This also means diamond-free dolomitic marble could form under very high H2O activity conditions. Diopside in this marble does not contain K2O-bearing exsolved phases contrary to diamond-bearing dolomite marble. Not only the bulk K2O composition, the solubility of K2O in aqueous fluid under UHP conditions might control the presence of exsolved lamellae in diopside, and probably, K2O had been completely leached out into fluid phase in this rock.K2O-bearing tourmaline had been discovered in tourmaline-K-feldspar-quartz rock, and could be a new mineral because no such high-K2O compositions were reported in tourmaline. The occurrence of this tourmaline also indicates the high activity of volatile components in such rocks and the possibility of aqueous fluid as a carrier for these volatile components.All these results described above strongly suggest the importance of the role of aqueous fluid during UHP metamorphism and lead to the proposed model of“Intraslab UHP metasomatism". Less