Chlorite Geothermometry?—Contamination and Apparent Octahedral Vacancies
Chlorite Geothermometry?—Contamination and Apparent Octahedral Vacancies
复制标题
绿泥石地温测量？——污染和表观八面体空位
DOI：
10.1346/ccmn.1994.0420512
复制
发表时间：
1994
影响因子：
2.2
通讯作者：
P. Buseck
中科院分区：
文献类型：
--
作者：
W. Jiang;D. Peacor;P. Buseck
已发表的热液体系和碎屑层序绿泥石分子式中八面体空位含量高，主要是由其他矿物包裹物引起的。通过分析电子显微镜（AEM）对魁北克加斯皮尔半岛和德克萨斯州墨西哥湾沿岸的泥质岩石中的绿泥石进行了验证。gaspaspal绿泥石以离散晶体形式出现，局部与绿泥石共存，墨西哥湾沿岸绿泥石除了局部蛇纹石样7-Å层外，没有混合层。与大多数电子探针分析（EMPA）不同，与其他AEM分析一样，所报道的绿泥石配方没有明显的八面体空位，相对于经典变质绿泥石来说，不富含si和（Fe + Mg）。所研究的绿泥石以及变质岩和碎屑岩中的绿泥石可以肯定地识别为不含或只含少量混合层状或亚显微共生体，它们含有很少或不含钙或碱。相比之下，其他碎屑序列的绿泥石的EMPA显示不同数量的Na + K + 2Ca，与八面体空位的比例表现出模糊的正相关关系。来自Salton海和Los azures地热田的绿泥石的EMPA被认为含有与温度相关的四面体Al（因此被用作“绿泥石地温计”），其组成特征与在变质岩中报道的几个皂石-绿泥石过渡系列相似。随着变质等级的增加，八面体占比和四面体Al的持续增加归因于混合层或细粒共生矿物的丰度减少，这些矿物通常是由于晶体尺寸和均匀性的增加而导致的。因此，根据八面体空位或四面体铝的比例使用“绿泥石地热测量法”是没有根据的，并且会导致不准确的温度估计。
The large contents of octahedral vacancies in published formulae of chlorite from hydrothermal systems and clastic sequences are shown to be largely caused by inclusion of other minerals. Verification is provided by analytical electron microscope (AEM) analyses of chlorite in pelitic rocks from the Gaspé Peninsula, Quebec and the Gulf Coast, Texas. The Gaspé chlorite occurs as discrete crystals locally coexisting with corrensite, and the Gulf Coast chlorite is free of mixed layers other than local serpentine-like 7-Å layers. Unlike most electron microprobe analyses (EMPA), but like other AEM analyses, the reported chlorite formulae do not have significant octahedral vacancies, are not Si-rich and (Fe + Mg)-poor relative to classic metamorphic chlorite, and have nearly equal amounts of tetrahedral and octahedral Al. The studied chlorites and those in metabasites and clastic rocks that could be positively identified as containing no or minimal mixed layering or submicroscopic intergrowths have little or no Ca or alkalis. In contrast, EMPA of chlorite reported for other clastic sequences show variable amounts of Na + K + 2Ca that exhibit a poorly defined positive correlation with the proportion of octahedral vacancies. The EMPA of chlorite from the Salton Sea and Los Azufres geothermal fields that were suggested to contain temperature-dependent amounts of tetrahedral Al (and thus used as “chlorite geothermometers”) show compositional characteristics similar to those reported for several saponite-chlorite transition series in metabasites.Continuous increases in octahedral occupancy and tetrahedral Al with increasing metamorphic grade are attributed to decreases in abundance of mixed layers or fine-grained intergrown minerals that commonly occur as a result of increasing crystal size and homogeneity in prograde sequences. Use of “chlorite geothermometry” based on the proportion of apparent octahedral vacancies or tetrahedral Al is therefore unwarranted and leads to inaccurate temperature estimates.