Low water contents in garnet of orogenic peridotite: clues for an abyssal or mantle-wedge origin?
Low water contents in garnet of orogenic peridotite: clues for an abyssal or mantle-wedge origin?
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DOI：
10.1127/ejm/2019/0031-2880
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发表时间：
2019-07
影响因子：
2.1
通讯作者：
E. Schmädicke;J. Gose
中科院分区：
文献类型：
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作者：
E. Schmädicke;J. Gose
有关造山带含石榴石超镁铁岩(GBU)名义上的无水矿物(NAM)中的水的数据极其罕见。用红外光谱分析了德国ErzgeBirge(EG)橄榄岩和辉石岩中的石榴石，以及瑞士Alpe Arami(AA)的两个橄榄岩样品。来自EG橄榄岩和辉石岩的石榴石产生的红外吸收带位于3650x±1110 cm−1(类型I)和3570-3630 cm−1(类型II)的波数范围内，归属于结构羟基(俗称“水”)。在约一半的样品中，位于<；3460 cm−1处的额外宽带与分子水(MW)有关。在所有的EG样品中，由带型I、C、+、II确定的结构H2O的含量都很低(3-68ppm)。结构水与石榴石中的镁、钛呈负相关，与Y、HREE呈正相关。包括分子水在内，H2O与Li之间存在显著的正相关关系。由于第II类带的强度在有分子水的区域被增强，所以EG石榴石中原生、峰期变质H2O的含量可能低至0-11万亿ppm。AA石榴石中的结构水含量同样很低(10-1300ppm)，其中分子水可以忽略不计。这种浓度明显低于相应压力下石榴石的储水量。减压失水不能作为低含量的解释，相反，峰后变质水的流入导致石榴石吸收二次结构水。因此，这一结果被解释为变质峰期严重缺水的迹象。值得注意的是，所获得的数据与Cima di Gagone橄榄岩中报告的6ppm的石榴石中的H2O含量一致，该橄榄岩起源于深海橄榄岩。目前尚不清楚这些低含量的岩石是否是深海低压原岩的典型代表，但如果这些岩石是地幔楔形最低、最水化的部分的一部分，它们预计会含有更多的水。考虑到ErzgeBirge玄武质柯石英榴辉岩中的石榴石与同一单位的GBU样品一样缺水，因此至少有可能这两种岩石在大洋背景下都是低压成因。
Data on water in nominally anhydrous minerals (NAMs) of orogenic garnet-bearing ultramafic rocks (GBU) are extremely rare. In this study, garnet of peridotite and pyroxenite from Erzgebirge (EG), Germany, and two peridotite samples from Alpe Arami (AA), Switzerland, were analyzed by infrared (IR) spectroscopy. Garnet from EG peridotite and pyroxenite yielded IR absorption bands at 3650 ± 10 cm−1(type I) and in the wavenumber range of 3570–3630 cm−1(type II) that are ascribed to structural hydroxyl (colloquially “water”). Additional broad band’s centered at <3460 cm−1, present in about half of the samples, are related to molecular water (MW). The content of structural H2O defined by band types I + II is low (3–68 ppm) in all EG samples. Structural water is negatively correlated to Mg and Ti and positively to Y and HREE in EG garnet. Including molecular water, a pronounced positive correlation between H2O and Li is observed. Because the intensity of the type II band is enhanced in domains with molecular water, the primary, peak metamorphic H2O content in EG garnet was probably as low as 0–11 ppm. Equally low contents of structural water are present in AA garnet (10–13 ppm) in which molecular water is negligible. Such concentrations are distinctly lower than the water storage capacity of garnet at the relevant pressure. Water loss upon decompression cannot serve as an explanation for the low contents because, on the contrary, post-peak-metamorphic influx of H2O led garnet to take up secondary structural water. Hence, the results are interpreted as an indication of severe water deficiency at peak metamorphism. Notably, the obtained data agree with the H2O content of 6 ppm reported in garnet from Cima di Gagnone peridotite, which originated as abyssal peridotite. It remains unknown if these low contents are typical for an abyssal, low-pressure protolith but, if the rocks were part of the lowermost, most hydrated portion of the mantle wedge, they are expected to contain much more water. Given that garnet in basaltic coesite eclogite from the Erzgebirge is equally water-deficient as the GBU samples from the same unit, it is at least a possibility that both rock types share a low-pressure origin in an oceanic setting.