Water Incorporation in Garnet: Coesite versus Quartz Eclogite from Erzgebirge and Fichtelgebirge
Water Incorporation in Garnet: Coesite versus Quartz Eclogite from Erzgebirge and Fichtelgebirge
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DOI：
10.1093/petrology/egy022
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发表时间：
2018-02
影响因子：
3.9
通讯作者：
J. Gose;E. Schmädicke
中科院分区：
文献类型：
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作者：
J. Gose;E. Schmädicke
石榴石的含水量是由德国两个 Variscan 杂岩中的榴辉岩测定的：萨克森州的厄尔士山脉 (EG) 和巴伐利亚的菲希特尔山脉 (FG)。厄尔士山脉榴辉岩分布在三个单元中，每个单元都经历了特定的峰值条件（单元 1：840–920 C/30 kbar，单元 2：670–730 C/24–26 kbar，单元 3：600–650 C/20–22 kbar）。 FG 榴辉岩的峰值条件 (690–750 C/25–28 kbar) 与 EG 2 号机组榴辉岩的峰值条件接近。柯石英榴辉岩仅限于 EG 超高压 (UHP) 1 号机组。石榴石在约 1 号机组中显示出红外吸收带。 3650、3580-3630 和 3570 厘米，归因于结构水。许多石榴石还含有分子水（在亚微观流体包裹体中），其不规则地分布在颗粒尺度上并且是次生来源的。含有分子水的颗粒体积总是显示出 3580-3630 cm 的谱带，归因于水石榴石的取代。由于这种替代产生的结构水与分子水呈正相关，因此结构水的主要含量只能从不含分子水的颗粒体积中推断出来，如 Schmädicke & Gose (2017; American Mineraggier 102, 975–986) 所证明的那样。石英榴辉岩中的石榴石的主要含量通常较低 (<2–50 ppm)；大多数样品的平均值在 8–28 ppm 范围内。来自柯石英榴辉岩的石榴石含有更多的水（50-180 ppm），但来自不寻常的含金云母柯石英榴辉岩的石榴石仅含水 19-55 ppm。石榴石中的结构水与变质峰压力无关，但受榴辉岩相含水矿物（例如钙角闪石、黝帘石和/或金云母）的存在（或不存在）控制。如果含水矿物在变质作用高峰时保持稳定（如石英榴辉岩），石榴石中几乎不含水或不含水。如果含水矿物不是峰组合的一部分（如常见的柯石英榴辉岩），则石榴石含有明显更多的水。后者显然源自榴辉岩相含水矿物，在超高压变质作用过程中由于超出其稳定场而分解并释放出 H2O。而且，柯石英榴辉岩中的石榴石比石英榴辉岩中的石榴石富含钙。这归因于原产黝帘石的分解，释放出钙并促进更高的钙铝榴石含量，从而增强了石榴石的储水能力。本研究进一步表明：（1）峰后变质作用引入次生流体； (2) 在流体流入之前相对干燥的条件，因为只有缺水的石榴石才能吸收额外的结构水； (3) 确定的结构水主要含量可能不会因减压失水而改变，因为减压失水只有在峰值压力含水量为最大可储存量的75%时才会发生； (4) 由于两种榴辉岩类型的石榴石在变质峰时都是缺水的，因此不同的含水量不太可能与压力有关； (5) 在此背景下，矿物组合和含水矿物的脱水无疑更为重要； (6) 石榴石和两种榴辉岩类型的绿辉石只能吸收含水矿物释放的部分水，很大一部分肯定已释放到上盘岩石中； (7) 该研究指向 VC The Author(s) 2018。由牛津大学出版社出版。版权所有。如需许可，请发送电子邮件至：journals.permissions@oup.com 207 J O U R N A L O F P E T R O L O G Y Journal of Petrology，2018，Vol。 59，第 2 期，207–232 doi：10.1093/petrology/egy022 提前访问发布日期：2018 年 2 月 28 日
The water content of garnet was determined for eclogite from two Variscan complexes in Germany: the Erzgebirge (EG), Saxony, and the Fichtelgebirge (FG), Bavaria. Erzgebirge eclogites occur in three units, each of which experienced specific peak conditions (unit 1: 840–920 C/ 30 kbar, unit 2: 670–730 C/24–26 kbar, unit 3: 600–650 C/20–22 kbar). Peak conditions of the FG eclogite (690–750 C/25–28 kbar) are close to those of eclogite from EG unit 2. Coesite eclogite is restricted to the EG ultra-high pressure (UHP) unit 1. Garnet shows infrared absorption bands at ca. 3650, 3580–3630, and 3570 cm, ascribed to structural water. Many garnets also contain molecular water (in sub-microscopic fluid inclusions), which is irregularly distributed on the grain scale and of secondary origin. Grain volumes with molecular water invariably reveal a band at 3580–3630 cm attributed to a hydrogarnet substitution. Because structural water due to this substitution positively correlates with molecular water, the primary content of structural water can only be deduced from grain volumes that are free of molecular water as demonstrated by Schmädicke & Gose (2017; American Mineralogist 102, 975–986). This primary content is typically low in garnet from quartz eclogite (<2–50 ppm); averages for most samples fall in the range of 8–28 ppm. Garnet from coesite eclogite hosts more water (50–180 ppm) except for garnet from an unusual, phlogopite-bearing coesite eclogite that contains only 19–55 ppm. Structural water in garnet is unrelated to metamorphic peak pressure but governed by the presence (or absence) of eclogite-facies hydrous minerals such as calcic amphibole, zoisite, and, or, phlogopite. In the case that hydrous minerals were stable at peak metamorphism—as in quartz eclogite—garnet hosts little or no water. If hydrous minerals are not part of the peak assemblage— as in common coesite eclogite—garnet contains distinctly more water. The latter was apparently derived from eclogite-facies hydrous minerals, which decomposed and liberated their H2O due to overstepping their stability field during UHP metamorphism. Moreover, garnet in coesite eclogite is more Ca-rich than garnet in quartz eclogite. This is ascribed to the breakdown of prograde zoisite, liberating Ca and facilitating a higher grossular content, which, in turn, enhances the garnet’s capacity for water storage. This study further suggests: (1) post-peak metamorphic introduction of secondary fluid; (2) relatively dry conditions prior to fluid influx, because only water-deficient garnet is able to incorporate additional structural water; (3) The determined primary contents of structural water were probably not modified by decompressional water loss, because the latter should only occur if the water content at peak pressure is 75 % of the maximum storable amount; (4) Since garnet from both eclogite types was water-deficient at the metamorphic peak it is unlikely that the different water contents are related to pressure; (5) The mineral assemblage and the dehydration of hydrous minerals is definitely more important in this context; (6) Garnet and, by implication, omphacite from both eclogite types was able to incorporate only part of the water liberated by hydrous minerals, a great part must have been released to hanging-wall rocks; and (7) The study points to a VC The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com 207 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2018, Vol. 59, No. 2, 207–232 doi: 10.1093/petrology/egy022 Advance Access Publication Date: 28 February 2018