Dehydrogenation of deep-seated hydrous olivine in “black-colored” dunites of arc origin

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.