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

bell太古宙次大陆地幔相对于上地幔的其他区域通常富含硅，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，但交代作用的性质和起源以及它们与地幔岩石圈反应的细节仍然知之甚少。本研究将尝试量化太古宙地幔橄榄岩中Si和C沉积的化学和矿物学反应，并限制交代流体的组成、起源和时间。这项工作的具体目标是确定脉状哈尔茨基岩化学成分的空间变化，并表征碳质哈尔茨基捕虏体的矿物学、结构和化学特征。这些结果将用于计算在亚钙石榴石辉石中产生富顽辉石脉的化学反应的化学计量学，评估难熔辉石中碳沉积的岩石学背景，并测试这两个过程在遗传上相关的假设，并负责在克拉通范围内的交代作用。这个问题将通过分析南非金伯利Bultfontein金伯利岩的地幔捕虏体样品来解决。其中包括因缺乏中生代叠印证据而被选中的难熔碳酸盐，以及被解释为流体反应的有说服力的纹理证据的不寻常的脉状样本。矿物主要和微量元素组成将通过EMP和LA-ICP-MS分析确定，并从大薄片的化学图计算出散装成分。添加成分的组成将通过脉状样品的化学图谱的数字空间分析来确定。碳酸盐的矿物学证据将使用EMP， SEM， XRD和TEM进行检查。用x射线层析成像技术评价亚钙石榴石辉石中金刚石的丰度和分布。将通过检查石榴石的微量元素非均质性来测试钻石形成交代作用的年轻年龄。通过Re-Os同位素分析和矿物微量元素、Sr和Nd同位素组成对原岩年龄和交代流体来源进行约束。定量的交代作用可以让我们更清楚地了解太古宙地幔的历史，并加深对岩石成因的理解，这是地球早期大陆稳定的关键。研究结果对太古宙克拉通的形成与演化、地幔中挥发性和微量元素的储存与运输机制以及全球碳循环的演化具有重要意义。

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.