Melt-Rock and Fluid-Rock Reaction in the Petrogenesis of Archean Subcontinental Mantle

相对于上地幔的其他区域，太古宙次大陆地幔通常富硅，也是天然钻石的几乎唯一来源。有人提出，这两种现象都是交代作用的结果，但交代剂的性质和来源以及它们与地幔岩石圈反应的细节尚不清楚。这项研究将试图量化伴随着硅和碳沉积到太古代地幔橄榄岩中的化学反应和矿物学反应，并限制交代流体的组成、来源和时间。这项工作的具体目标是确定脉状方辉橄榄岩化学成分的空间变化，并表征碳质方辉橄榄岩包体的矿物学、结构和化学特征。这些结果将被用来计算在亚钙石榴石方辉橄榄岩中产生富顽火辉石矿脉的化学反应的化学计量，评估难熔方辉橄榄岩中碳沉积的岩石学背景，并检验这两个过程在成因上相关并导致克拉通范围内交代的假设。这个问题将通过分析南非金伯利Bultfontein金伯利岩中的地幔包裹体样本来解决。其中包括因缺乏中生代叠印证据而被选中的难熔方辉橄榄岩，以及被解释为流体反应的有说服力的结构证据的不寻常的脉状样本。矿物主量元素和微量元素组成将通过EMP和LA-ICPMS分析以及根据大片薄片化学图计算的总体成分来确定。添加组分的组成将通过对脉状样品的化学地图进行数字空间分析来确定。利用电磁脉冲、扫描电子显微镜、X射线衍射仪和透射电子显微镜对碳酸盐的矿物学证据进行研究。利用X射线层析成像技术评价亚钙石榴石方辉橄榄岩中金刚石的丰度和分布。将通过检查石榴石的微量元素不均一性来测试形成钻石交代作用的年轻年龄。原岩年龄将通过Re-Os同位素分析和矿物微量元素、锶、钕同位素组成确定交代流体的来源。通过对交代作用的量化，可以更清楚地了解太古宙地幔的历史，并增加对岩石成因的理解，这是早期地球大陆稳定的关键。预计这一研究结果将对太古宙克拉通的形成和演化、挥发分和微量元素在地幔中的储存和运输机制以及全球碳循环的演化具有重要的意义。

EAR-0310330BellArchean subcontinental mantle is commonly enriched in Si with respect to other regions of the upper mantle and is also the virtually exclusive source of natural diamond. It has been proposed that both these phenomena are the result of metasomatism, yet the nature and origins of the metasomatic agents and the details of their reaction with the mantle lithosphere remain poorly understood. The proposed research will attempt to quantify the chemical and mineralogical reactions accompanying the deposition of Si and C into Archean mantle peridotite and constrain the compositions, origin and timing of the metasomatic fluids.The specific objectives of this work are to determine the spatial variations in chemical composition of veined harzburgites and to characterize the mineralogy, texture and chemical features of carbonaceous harzburgite xenoliths. These results will be used to calculate the stoichiometry of chemical reactions producing enstatite-rich veins in subcalcic garnet harzburgite, to assess the petrologic context of carbon deposition in refractory harzburgite, and to test the hypotheses that the two processes are genetically related and responsible for metasomatism on a craton-wide scale. The problem will be approached by the analysis of mantle xenolith samples from the Bultfontein kimberlite, Kimberley, South Africa. These include refractory harzburgites selected for their lack of evidence of Mesozoic overprinting and unusual veined examples interpreted as persuasive textural evidence for fluid reaction. Mineral major and trace element compositions will be determined by EMP and LA-ICP-MS analysis and bulk compositions calculated from chemical maps of large thin sections. Compositions of added components will be determined by digital spatial analysis of chemical maps of veined samples. Mineralogical evidence for carbonate will be examined using EMP, SEM, XRD and TEM. Abundance and distribution of diamond in subcalcic garnet harzburgite will be evaluated by X-ray tomography. A young age for diamond-forming metasomatism will be tested by examining garnet for trace element heterogeneity. Protolith ages will be constrained using Re-Os isotope analysis and source of metasomatizing fluids by mineral trace element, and Sr and Nd isotope compositions. Quantifying metasomatic effects allows a clearer window into the history of Archean mantle and an increased understanding of petrogenesis that is the key to continent stabilization in the early Earth. It is anticipated that the results of this study will have important implications for the Archean craton formation and evolution, mechanisms of volatile and trace element storage and transport in the mantle, and the evolution of the global carbon cycle.