Water in garnet of garnetite (metarodingite) and eclogite from the Erzgebirge and the Lepontine Alps
Water in garnet of garnetite (metarodingite) and eclogite from the Erzgebirge and the Lepontine Alps
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来自厄尔士山脉和莱庞廷阿尔卑斯山的石榴石（变辉石）和榴辉岩的石榴石中的水
DOI：
10.1111/jmg.12554
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发表时间：
2020
影响因子：
3.4
通讯作者：
Schmädicke
中科院分区：
文献类型：
--
作者：
Schmädicke
对造山带石榴石橄榄岩和变质基性岩中名义上的无水矿物中的水知之甚少。本研究主要针对德国厄尔士山脉（EG）和瑞士勒蓬廷阿尔卑斯山（LA）的橄榄岩型榴辉岩和石榴子石（变柱晶岩）。在厄尔士山脉新发现的橄榄岩型榴辉岩与片麻岩型柯石英榴辉岩发生在同一超高压（UHP）单元中，在岩石学上无法区分。石榴石存在于所有镁铁质和超镁铁质高压（HP）岩石中，为比较不同岩石类型的H2O含量提供了理想的替代。EG和LA样品中的石榴石成分非常相似，取决于岩石类型。石榴子石比榴辉岩含有更多的CaO（石榴子石：10.5- 16.5wt%;榴辉岩：5- 11wt%），并具有稀土元素异常分布的特征。相比之下，从两种岩石类型的石榴石的红外（IR）光谱揭示了相同的OH吸收带，也是相同的那些以前研究的橄榄岩石榴石从相同的位置。两组红外带SW I（3，650 ± 10 cm−1）和SW II（3，570 - 3，630 cm−1）属于结构羟基（俗称“水”）。第三，宽带是存在于约一半的分析石榴石域和相关的分子水（MW）在亚显微流体包裹体。结构水的主要内容，保存在石榴石域没有流体包裹体（和MW带），系统地变化，这取决于位置和岩石类型。与LA样品相比，来自EG岩石的石榴石含有更多的水，并且来自石榴子石的石榴石（EG：121-241重量ppm H2O; LA：23-46重量ppm）比榴辉岩石榴石（EG：84重量ppm; LA：4-11重量ppm）含有更多的水。较高含量的结构水（SW）中观察到的分子水，其中SW II带（不限于HP条件）的域同时增强。这意味着在减压过程中的流体流入不仅导致流体包裹体，但也有利于二次SW的摄取。结果表明，石榴石从所有EG和LA样品最初是H2O-不饱和。结合来自榴辉岩、石榴子石和先前研究的橄榄岩的数据，H2O和CaO呈正相关，表明所有岩石类型在峰变质作用时的H2O欠饱和度相同。这种普遍存在的缺水现象与这些岩石中任何一种来自与俯冲板块接触的地幔楔最低部分的说法都不一致。这与先前推断的来自LA（Cima di Gagnone）的部分岩石的深海起源一致。Erzgebirge UHP机组也有类似的起源。我们认为，所有的镁铁质和超镁铁质岩石的这个单位不仅共享相同的变质演化，但也有一个共同的原岩起源，最有可能在海底。这一推断得到橄榄岩托管石榴石的存在的支持，代表变质的rodingite。
Little is known about water in nominally anhydrous minerals of orogenic garnet peridotite and enclosed metabasic rocks. This study is focused on peridotite‐hosted eclogite and garnetite (metarodingite) from the Erzgebirge (EG), Germany, and the Lepontine Alps (LA), Switzerland. Newly discovered, peridotite‐hosted eclogite in the Erzgebirge occurs in the same ultra‐high pressure (UHP) unit as gneiss‐hosted coesite eclogite, from which it is petrologically indistinguishable. Garnet is present in all mafic and ultramafic high pressure (HP) rocks providing for an ideal proxy to compare the H2O content of the different rock types. Garnet composition is very similar in EG and LA samples and depends on the rock type. Garnet from garnetite, compared to eclogite, contains more CaO (garnetite: 10.5–16.5 wt%; eclogite: 5–11 wt%) and is also characterized by an anomalous REE distribution. In contrast, the infrared (IR) spectra of garnet from both rock types reveal the same OH absorption bands that are also identical to those of previously studied peridotitic garnet from the same locations. Two groups of IR bands, SW I (3,650 ± 10 cm−1) and SW II (3,570–3,630 cm−1) are ascribed to structural hydroxyl (colloquially ‘water’). A third, broad band is present in about half of the analysed garnet domains and related to molecular water (MW) in submicroscopic fluid inclusions. The primary content of structural H2O, preserved in garnet domains without fluid inclusions (and MW bands), varies systematically—depending on both the location and the rock type. Garnet from EG rocks contains more water compared to LA samples, and garnet from garnetite (EG: 121–241 wt.ppm H2O; LA: 23–46 wt.ppm) hosts more water than eclogitic garnet (EG: 84 wt.ppm; LA: 4–11 wt.ppm). Higher contents of structural water (SW) are observed in domains with molecular water, in which the SW II band (being not restricted to HP conditions) is simultaneously enhanced. This implies that fluid influx during decompression not only led to fluid inclusions but also favoured the uptake of secondary SW. The results signify that garnet from all EG and LA samples was originally H2O‐undersaturated. Combining the data from eclogite, garnetite and previously studied peridotite, H2O and CaO are positively correlated, pointing to the same degree of H2O‐undersaturation at peak metamorphism in all rock types. This ubiquitous water‐deficiency cannot be reconciled with the derivation of any of these rocks from the lowermost part of the mantle wedge that was in contact with the subducting plate. This agrees with the previously inferred abyssal origin for part of the rocks from the LA (Cima di Gagnone). A similar origin has to be invoked for the Erzgebirge UHP unit. We suggest that all mafic and ultramafic rocks of this unit not only shared the same metamorphic evolution but also a common protolith origin, most probably on the ocean floor. This inference is supported by the presence of peridotite‐hosted garnetite, representing metamorphosed rodingite.
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影响因子：
3.4
作者：
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通讯作者：
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