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

哈萨克斯坦和德国超高压长英质片麻岩变质钻石的生长机制和条件

• 批准号: 0107118
• 负责人: Larissa Dobrzhinetskaya
• 金额: $ 7万
• 依托单位: University of California-Riverside
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2001
• 资助国家: 美国
• 起止时间: 2001-08-01 至 2003-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0107118&HistoricalAwards=false
• 关键词: Mechanism; Conditions; Growth; Metamorphic; Diamonds

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.

Dobrizhinetskaya-0107118哈萨克斯坦Kokchetav山的微金刚石是在俯冲带石英长石沉积物的“超高压”（UHP）变质作用条件下生长的。10年前对这一现象的认识引发了对大陆碰撞事件理解的革命。 在德国的萨克森地区，也发现了类似的含金刚石岩石，属于瓦里斯坎时代的厄尔士山脉。Erzgebirge和Kokchetav钻石惊人地相似。然而，厄尔士山脉金刚石似乎在较低的压力下遭受了对石墨的更大的反反应和/或溶解。 对石榴石和锆石中金刚石的原位研究表明，在这两种石榴石中，金刚石包裹体经常伴随着含水相和指示自由流体的空腔。这一试点项目将进行两项与微型钻石成因有关的研究。第一个将是一个包裹体的研究，在最近发现的钻石从萨克森Erzgebirge，德国的TEM利用类似的技术，我们最近成功地开发了Kokchetav钻石。Erzgebirge钻石中包裹体的成分和结构的测定，加上我们以前对来自Kasakhtan的类似钻石形成的研究，将为钻石结晶的压力条件的评价和对地壳石英长石岩被构造过程所取深度的理解提供限制。这种技术以后可以用于地球和材料科学中的许多其他研究，涉及纳米级天然和合成矿物的TEM研究。第二项研究将对De Corte等人（1999年）和Dobrzhinetskaya等人（2001年）提出的金刚石生长假说进行实验测试，以限制可能导致漏斗/中空、骨架、立方体和立方体八面体金刚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内复制天然金刚石形成的实验尝试是基于在高度还原环境中的金属-碳系统中的合成。 直到最近，在高P和T条件下天然金刚石的实验建模才扩展到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。 现在认为，C-O-H流体在金刚石形成中可以是非常有效的。我们的重点将是在金刚石稳定性领域的微型金刚石的实验合成，以测试目前的增长模型。 我们将使用由天然矿物混合物组成的起始材料进行几次试点实验，该天然矿物混合物的组成与我们在Dobrzhenitskaya等人（2001）中报道的来自Kokchetav的天然含金刚石长英质片麻岩相似。 我们将探讨C-O-H流体相的存在和不存在的影响，尽管我们预计在流体不存在的条件下几乎没有结果。 氧逸度将通过用金属氧化物对缓冲来控制。 这项试点研究将使我们能够初步了解石墨-金刚石动力学的一些方面，在水的存在，以及这种实验的技术方面，使后续的追求。我们的实验计划将是第一次研究这种条件下的金刚石生长。具体来说，我们将在相对氧化的条件下（金刚石和碳酸盐稳定共存的条件下）使用C-O-H流体在起始材料中合成金刚石，其主体成分接近Kokchetav和Erzgebirge含金刚石的石英长石质岩石。