ORIGIN OF WATERS RESPONSIBLE FOR SERPENTINIZATION OF THE IZU-OGASAWARA-MARIANA FORE-ARC SEAMOUNTS IN VIEW OF HYDROGEN AND OXYGEN ISOTOPE RATIOS

Many serpentinite seamounts occur over a region 20-120 km west of the trench axis in the Izu-Ogasawara-Mariana forearc regions. The hydrogen and oxygen isotopic compositions of serpentine from these regions indicate that there are at least two kinds of waters responsible for serpentinization: seawater and water derived from dehydration of the descending slab. Serpentine from two Mariana and two Torishima samples with microscopically ductile and sheared texture (sheared-type) have lower δD (-63 to -52‰) and slightly higher δ18O values (+6.1 to +8.2‰) than that of other nine Ogasawara samples with mesh texture (mesh-type) (δD = -43 to -49‰ and δ18O = +5.8 to +6.7‰). This suggests that the sheared-type serpentine with lower δD and slightly higher δ18O values was formed within the wedge mantle by interaction with water derived from a descending slab. The sheared texture is likely to have been produced during diapiric uplift. The unaltered portion of the ultramafic bodies later interacted with seawater after emplacement at or near the seafloor, resulting in formation of the mesh-type serpentine with higher δD values. © 1990.