Mechanism(s)and Conditions(s) of Growth of Metamorphic Diamonds from Ultra-High Pressure Felsic Gneisses in Kazakhstan and Germany

哈萨克斯坦Kokchetav地块的微金刚石是在俯冲带石英岩质沉积物的“超高压”变质作用条件下生长的。十年前对这一现象的认识引发了对大陆碰撞地体认识的一场革命。如今，在德国萨克森地区的瓦里斯坎时代的埃尔兹盖比热，也发现了类似的含金刚石岩石。Erzgebirge和Kokchetav钻石惊人地相似。然而，在较低的压力下，Erzgebirge钻石似乎遭受了更大的石墨反反应和/或溶解。对石榴石和锆石中原位钻石的研究表明，在这两种地体中，钻石包裹体经常伴有含水相和指示自由流体的空腔。这个试点项目将进行两项有关微钻石成因的研究。第一个将是利用TEM技术研究最近在德国Saxonian Erzgebirge发现的钻石中的包裹体，该技术与我们最近成功开发的Kokchetav钻石相似。额尔齐格钻石中包裹体的组成和结构的确定，加上我们之前对哈萨克斯坦类似钻石地层的研究，将为评估钻石结晶的压力条件和了解地壳石英田的深度提供约束。这些技术后来可用于地球科学和材料科学的许多其他研究，这些研究涉及纳米级天然矿物和合成矿物的透射电镜研究。第二项研究将是对De Corte等人（1999年）和Dobrzhinetskaya等人（2001年）提出的钻石生长假设进行实验测试，以限制可能导致漏斗/空心、骨架、立方体和立方体八面体钻石在富含流体的俯冲带环境中优先生长的过程。传统上，在金刚石稳定性领域复制天然金刚石形成的实验尝试都是基于高度还原环境下金属-碳体系的合成。直到最近，高P和高T条件下天然钻石的实验模型才扩展到H2O存在下的碳酸盐-碳体系。后一种观点已得到天然钻石中流体包裹体研究结果的证实。现在人们认为碳氢化合物在金刚石地层中非常有效。我们的重点将放在金刚石稳定性领域的微金刚石的实验性合成上，以测试目前的生长模型。我们将使用一种由天然矿物混合物组成的起始材料进行若干试点实验，该矿物混合物的成分类似于我们在Dobrzhenitskaya等人（2001年）中报道的来自Kokchetav的天然含金刚石长英质片麻岩。我们将探讨存在和不存在C-O-H流体相的影响，尽管我们期望在没有流体的条件下得到很少的结果。氧逸度将通过金属氧化物对的缓冲来控制。这项试点研究将使我们对H2O存在下石墨-金刚石动力学的某些方面以及此类实验的技术方面有初步的了解，从而使后续的研究成为可能。我们的实验项目将是第一个在这种条件下研究钻石生长的项目。具体来说，我们将在相对氧化的条件下（金刚石与碳酸盐稳定共存的条件下）合成金刚石，原料的体成分接近Kokchetav和Erzgebirge含金刚石的准长石，使用C-O-H流体。

DobrizhinetskayaEAR-0107118Microdiamonds of the Kokchetav massif in Kazakhstan grew under conditions of "ultra-high-pressure"(UHP) metamorphism of quartzofeldspathic sediments in a subduction zone. The recognition of this phenomenon a decade ago initiated a revolution in the understanding of continental collision terranes. Similar diamond-bearing rocks have now been discovered in the Saxony region of Germany, in the Erzgebirge of Variscan age. Erzgebirge and Kokchetav diamonds are strikingly similar. However, Erzgebirge diamonds appear to have suffered greater back-reaction to graphite and/or dissolution at lower pressures. The study of diamonds in situ in garnet and zircon shows that in both terranes, diamond inclusions are very frequently accompanied by a hydrous phase and cavities indicative of a free fluid. This pilot project will pursue two studies relating to the genesis of microdiamonds. The first will be a study of inclusions in recently discovered diamonds from Saxonian Erzgebirge, Germany by TEM utilizing techniques similar to those we recently successfully developed for Kokchetav diamonds. Determinations of the composition and structure of inclusions in Erzgebirge diamonds coupled with our previous research on similar diamond formation from Kasakhstan will provide constraints for the evaluation of pressure conditions of diamond crystallization and an understanding of the depth to which crustal quartzofeldspathic rocks have been taken by tectonic processes. Such techniques could later be used in many other studies in both Earth and material sciences dealing with TEM research on the nanometric sized natural and synthetic minerals. The second study will be an experimental test of the diamond growth hypotheses suggested by De Corte et al. (1999) and Dobrzhinetskaya et al. (2001) to put constraints on the process(es) which may be responsible for preferential growth of hopper/hollow, skeletal, cuboidal and cube-octahedral diamonds in fluid-rich subduction-zone environments. Traditionally, experimental attempts to replicate natural diamond formation within the diamond stability field have been based on syntheses 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 carbonate-carbon system in presence of H2O. The latter ideas have been corroborated by results of fluid-inclusion studies in natural diamonds. It is now believed that C-O-H fluids can be very effective in diamond formation. Our focus will be on experimental syntheses of microdiamonds in the diamond stability field to test current models of growth. We will conduct several pilot experiments using a starting material composed of natural mineral mix of composition similar to the natural diamond-bearing felsic gneiss from Kokchetav upon which we reported in Dobrzhenitskaya et al. (2001). We will explore the effects of presence and absence of a C-O-H fluid phase, although we expect little results under fluid-absent conditions. Oxygen fugacity will be controlled by buffering with metal-oxide pairs. This pilot study will allow us to develop initial understanding of some aspects of graphite-diamond kinetics in presence of H2O as well as technical aspects of such experimentation to enable subsequent pursuit. Our experimental program will be the first study of diamond growth under such conditions. Specifically, we will synthesize diamonds under relatively oxidizing conditions (conditions under which diamonds and carbonates coexist stably) using C-O-H fluid in starting material whose bulk composition is close to Kokchetav and Erzgebirge diamond-bearing quatzofeldspathic rocks.