Trace element systematics of tourmaline in pegmatitic and hydrothermal systems from the Variscan Schwarzwald (Germany): The importance of major element composition, sector zoning, and fluid or melt composition
Trace element systematics of tourmaline in pegmatitic and hydrothermal systems from the Variscan Schwarzwald (Germany): The importance of major element composition, sector zoning, and fluid or melt composition
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DOI：
10.1016/j.chemgeo.2013.02.025
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发表时间：
2013-04
期刊：
影响因子：
3.9
通讯作者：
M. Marks;H. Marschall;P. Schühle;A. Guth;T. Wenzel;Dorrit E. Jacob-;M. Barth;G. Markl
中科院分区：
文献类型：
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作者：
M. Marks;H. Marschall;P. Schühle;A. Guth;T. Wenzel;Dorrit E. Jacob-;M. Barth;G. Markl
提供了德国黑森林花岗伟晶岩、混合片麻岩和各种热液脉中电气石成分变化的广泛数据集。研究的电气石是碱和X-空缺组的成员，主要代表镁铝榴石-黑云母固溶体，一些分析属于钙镁铁矿-镁钙铁矿系列。石英-电气石对的氧同位素数据表明，大多数石英-电气石脉的形成温度在550至350°C之间。大多数碧玺显示出强烈的扇形分带，将某些主要的（例如，Na，Mg），微量（例如，Ti、Ca）和微量元素（例如，Sr、Pb、REE）的差异。我们的结论是，比较的电气石组成的岩石成因解释必须根据在中性的一个部门的浓度，以消除晶内分馏的影响。大多数微量元素的含量在几个数量级上变化，中位浓度通常在0.1和10μg/g之间。发现Zn、V、Sr、Sc、Sn和Li的浓度最高（中位浓度为几十至几百微克/克），而U、Th、Bi、Tl和Cs的中位浓度非常低（<0.1微克/克）。几种微量元素的浓度（例如，Sr，REE，Co）与电气石的主量元素组成相关，说明它们的掺入可能受到晶体化学作用的影响。因此，一般假设电气石是一个被动监测的化学成分的熔体或流体，它从结晶必须提出质疑。然而，我们的研究表明，微量元素组成的电气石允许区分地球化学不同的流体来源，和微量元素组成的电气石，因此可以用来解开花岗岩相关的热液系统的化学复杂性。
An extensive data set on the compositional variation of tourmaline from granitic pegmatites, from migmatitic gneisses and from various types of hydrothermal veins from the Schwarzwald, Germany, is provided. The investigated tourmalines are members of the alkali and X-vacant groups representing mostly dravite–schorl solid solutions with some analyses belonging to the foitite–Mg–foitite series. Oxygen isotope data on quartz–tourmaline pairs indicate formation temperatures between 550 and 350°C for most of the quartz–tourmaline veins. Most of the tourmalines show strong sector zonation, fractionating certain major (e.g., Na, Mg), minor (e.g., Ti, Ca) and trace elements (e.g., Sr, Pb, REE) among the different crystallographic sectors of the crystals. We conclude that comparison of tourmaline compositions for petrogenetic interpretations must be based on the concentrations in the neutral a sector in order to eliminate the effects of intra-crystalline fractionation. Most trace element contents vary over several orders of magnitude with median concentrations generally between 0.1 and 10μg/g. The highest concentrations are found for Zn, V, Sr, Sc, Sn and Li (median concentrations of several tens to hundreds of μg/g) whereas very low median concentrations (<0.1μg/g) were found for U, Th, Bi, Tl and Cs. The concentration of several trace elements (e.g., Sr, REE, Co) correlates with the major element composition of the tourmaline, which implies potential crystal chemical effects on their incorporation. Consequently, the general assumption that tourmaline is a passive monitor for the chemical compositions of the melts or fluids from which it crystallizes has to be questioned. Nevertheless, our study demonstrates that the trace element composition of tourmaline allows to distinguish between geochemically distinct fluid sources, and the trace element composition of tourmaline may therefore be used to unravel the chemical complexity of granite-related hydrothermal systems.