Experimental Studies of the Mechanism of Diamond Crystallization From COH Supercritical Fluid in SiO2-Rich Media

DobrzhinetskayaEAR-0229666十年前，超高压变质岩中微钻石的识别引发了对大陆碰撞地体的认识革命。已知有两个概念来解释这种钻石的起源：(1)来自超临界COH流体的结晶；(2)来自含流体的碱-碳酸盐熔体的结晶。传统上，复制天然钻石形成的实验尝试一直基于在高度还原的环境中在金属-碳系统中进行合成。直到最近，在高P和T条件下对天然钻石的实验模拟才扩展到有水存在的碱-碳酸盐-碳体系。实验证明，在高温高压条件下，水是促进石墨晶化的活性催化剂，孕育时间与溶液中的金属离子有关。特别是，我们发现镁促进了形核，而硅不促进形核。这项工作将扩展我们在金刚石合成方面的工作，重点研究不同参数(如压力、温度、时间和氧逸度)下COH-SiO_2系统中的金刚石成核，以限制金刚石在超高压变质地体中的石英长片麻岩和石英岩中的生长机制。超临界COH流体合成金刚石的实验研究在金刚石稳定性领域是一个新的、快速发展的研究领域，对于理解大陆亲和性俯冲岩石中的金刚石结晶机制具有重要意义，并可能为工业合成带来有趣的应用。

DobrzhinetskayaEAR-0229666The recognition of microdiamonds within ultra-high pressure metamorphic rocks a decade ago initiated a revolution in the understanding of continental collision terranes. Two concepts are known to explain the origin of such diamonds: (1) crystallization from a supercritical COH fluid; (2) crystallization from fluid-bearing alkaline-carbonate melt. Traditionally, experimental attempts to replicate natural diamond formation have been based on synthesis in metal-carbon systems in a highly reduced environment. Only recently has experimental modeling of natural diamonds at high P and T conditions been extended to the alkaline-carbonate-carbon system in the presence of H2O. We have established experimentally that H2O is an active catalyst promoting diamond crystallization from graphite at high pressure and high temperature and that incubation time is dependent on metal ions in solution. In particular, we find that Mg facilitates nucleation and Si does not. This work will extend our work in diamond synthesis with a focus on diamond nucleation in the COH-SiO2 system at varying parameters such as pressure, temperature, time and oxygen fugacity to constrain the mechanism of diamond growth in quartzofeldspathic gneisses and quartzites from ultra-high pressure metamorphic terranes. Experimental investigations related to diamond synthesis from supercritical COH fluid in the diamond stability field is a new but fast-growing research field of great importance for understanding of the mechanism of diamond crystallization within subducted rocks of continental affinity and may bring interesting application for industrial diamond synthesis.