Dehydrogenation of deep-seated hydrous olivine in “black-colored” dunites of arc origin
Dehydrogenation of deep-seated hydrous olivine in “black-colored” dunites of arc origin
复制标题
弧成因“黑色”沙丘中深层含水橄榄石的脱氢
DOI：
10.1016/j.lithos.2021.105967
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发表时间：
2021
期刊：
影响因子：
3.5
通讯作者：
Tamura Akihiro
中科院分区：
文献类型：
--
作者：
Arai Shoji;Hoshikawa Chihiro;Miura Makoto;Ando Jun-ichi;Ishimaru Satoko;Mochizuki Nobutatsu;Tamura Akihiro
橄榄石和地幔中其他名义上无水矿物中氢或水的行为和含量一直存在争议，因为它们对各种地幔过程具有重大影响。氢在其中扩散得非常快，因此很难在地质样品中检查其行为。在这里，我们报告了它在地幔橄榄石中存在的痕迹，尽管橄榄石完全新鲜，但颜色为黑色。黑色橄榄石形成纯橄榄岩或橄榄石，至少部分是粗粒的，在日本的 Horoman 杂岩以及日本的 Iwanai-dake 杂岩和阿曼蛇绿岩中都有发现。黑色是由于橄榄石中磁铁矿和透辉石的微小棒状复合溶出物分布所致。它们的分布是均匀的，至少在某些区域是均匀的，但与橄榄石的任何裂缝和晶界无关，这排除了外部氧气侵入的可能性。这种独特的橄榄石可能是通过从最初含水的含有羟基的橄榄石中释放氢而产生的。随着脱氢和结构重结晶的进展，橄榄石变得越来越清澈。含水橄榄石从最初含水的岩浆中沉淀出来，形成杜长岩，并且随后在冷却和/或减压时脱氢。释放的氢与表面氧结合时可能形成水。水最终融入俯冲板片并被消耗，再次形成含水岩浆。这个过程有助于地球系统中氢或水的循环利用。我们还认为，由与弧相关的含水岩浆形成的一些橄榄岩（尤其是纯橄榄岩）中的橄榄石已通过短暂的黑色橄榄石从最初的含羟基橄榄石转变为橄榄岩。
Behavior and amount of hydrogen or water in olivine and other nominally anhydrous minerals in the mantle have been a matter of debate because of their significant influence on various mantle processes. Hydrogen very quickly diffuses in them so that its behavior is difficult to examine in geologic samples. Here we report traces of its presence in mantle-derived olivine, which is black in color despite its complete freshness. The black-colored olivine forms dunite or wehrlite, coarse-grained at least in part, found in the Horoman complex, Japan as well as in the Iwanai-dake complex, Japan and the Oman ophiolite. The black color is due to distribution of minute rod-like composite exsolutions of magnetite and diopside in the olivine. Their distribution is homogeneous, at least in certain areas, but is not related with any fractures and grain boundaries of the olivine, which precludes the possibility of invasion of external oxygen. This unique olivine has been possibly produced by hydrogen release from an initially hydrous olivine containing hydroxyl. The olivine has been getting clearer with a progress of dehydrogenation coupled with textural recrystallization. The hydrous olivine was precipitated from an initially hydrous magma, forming dunitic rocks, and has been subsequently dehydrogenated upon cooling and/or decompression. The released hydrogen possibly form water when combined with surface oxygen. The water is ultimately incorporated in the subducted slab and consumed to form hydrous magmas again. This process contributes to recycling of hydrogen or water in the Earth system. We also suggest that olivines in some of the peridotites, especially dunites, formed from arc-related hydrous magmas have been converted from initial hydroxyl-bearing ones via the transient black-colored olivines.