Relationship between structure, morphology, and carbon isotopic composition of graphite in marbles: Implications for calcite-graphite carbon isotope thermometry
Relationship between structure, morphology, and carbon isotopic composition of graphite in marbles: Implications for calcite-graphite carbon isotope thermometry
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DOI：
10.2138/am.2011.3576
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发表时间：
2011-04
影响因子：
3.1
通讯作者：
M. Satish-Kumar;J. Jaszczak;T. Hamamatsu;Hideki Wada
中科院分区：
文献类型：
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作者：
M. Satish-Kumar;J. Jaszczak;T. Hamamatsu;Hideki Wada
摘要方解石与石墨之间的碳同位素交换是一种实用、可靠的中高档大理石地质温度计。然而，在极少数情况下，例如在希腊的纳克索斯，以前就有过方解石和石墨之间明显的不平衡碳同位素分馏的报道。本研究在石墨的形态特征、X射线衍射研究、拉曼光谱研究和碳同位素研究等方面取得了新的结果。确定了三种不同形态的石墨类型。第一种类型是细粒的，具有明显的多边形晶体聚集体，具有光滑的片状表面。第二种类型由大的梨状集合体组成，具有典型的晶体过度生长特征；第三种类型是与金云母共生的“正常”大片状晶体。FE-SEM的石墨形态特征表明，葡萄状聚集体由复杂的相互生长和堆积的石墨层组成，由锥形螺旋结构组成。不同类型的石墨具有不同的LC(002)和DG值，在-1580 cm-1处有一个尖锐的一阶拉曼峰，并有无序的谱带。细晶石墨中拉曼无序带的存在归因于边缘效应。石墨的碳同位素分析表明，含有细粒结晶良好的石墨的大理岩与方解石具有一致的同位素分馏，这与基于矿物等值线的温度估计完全匹配。相反，粗的梨状石墨提供了不均匀的碳同位素值(大约每磨几个)，并在方解石和石墨之间显示了异常的碳同位素分馏。与金云母共生的石墨碳同位素值也有变化。结合形态、结晶学和碳同位素数据，我们认为碳同位素组成的变化是由于先前存在的颗粒上石墨的过度生长造成的，而石墨晶体的形态观察和结构特征是预测变质过程中同位素平衡的关键。
Abstract Carbon isotope exchange between calcite and graphite is a useful and reliable geothermometer for medium- to high-grade marbles. However, in rare instances, such as at Naxos, Greece, apparent disequilibrium carbon isotope fractionation between calcite and graphite has been previously reported. In this study, new results are presented on the morphological features, X-ray diffraction studies, Raman spectroscopic studies, and carbon isotope studies of graphite. Three morphologically distinct graphite types are identified. The first type is fine grained with distinct polygonal crystal aggregates having smooth pinacoid faces. The second type consists of large botryoidal aggregates with typical crystal overgrowth features, and the third type is “normal” large platy crystals associated with phlogopite. Graphite morphological features using an FE-SEM suggests that the botryoidal aggregates are composed of complexly intergrown and stacked graphite layers, comprised of cone-helix structures. Different types of graphite display distinct Lc(002) and DG values as well as a sharp first-order Raman peak at -1580 cm-1 with disordered bands. The presence of Raman disordered bands in fine-grained well-crystallized graphite is attributed to edge effects. Carbon isotope analysis of graphite reveals that marble with fine-grained well-crystallized graphite show a consistent isotope fractionation with the calcite, which perfectly match temperature estimates based on the mineral isograds. In contrast, coarse botryoidal graphite gives heterogeneous carbon isotope values (about a few per mill) and show anomalous carbon isotope fractionation between the calcite and graphite. The graphite associated with phlogopite also shows variations in the carbon isotope values. Combining morphologic, crystallographic, and carbon isotopic data we conclude that the variations in carbon isotopic composition were caused by the overgrowth of graphite on the preexisting grain and that morphological observations and structural characterizations of graphite crystals is a key for predicting isotopic equilibration during metamorphism.