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.

地幔中橄榄石和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都有重大影响。氢在其中迅速扩散，因此很难在地质样品中检查其行为。在这里，我们报告了其存在于地幔衍生的橄榄石中的痕迹，尽管其完全新鲜，但它还是黑色的。黑色的橄榄石形式的辉光岩或辉绿岩至少部分呈粗粒状，存在于日本的荷鲁曼岩群以及日本的岩内-代克岩群和阿曼蛇绿岩中。黑色是由于橄榄石中的磁铁矿和透辉石的微小棒状复合溶液的分布所致。它们的分布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，从而排除了外部氧气入侵的可能性。这种独特的橄榄石可能是由氢从最初含有羟基的含水橄榄石中释放出来而产生的。随着脱氢和组织重结晶的进展，橄榄石变得越来越清晰。含水橄榄石从最初的含水岩浆中沉淀出来，形成辉长岩，随后经冷却和/或减压脱氢。当与表面氧结合时，释放的氢可能形成水。最终将水掺入俯冲板中，并再次消耗以形成含水岩浆。该过程有助于地球系统中氢或水的循环利用。我们还建议某些橄榄石中的橄榄石，