Work

Hydrogen concentrations in Horoman peridotites (Hokkaido, Japan) and implications for the ductile deformation and exhumation of the oceanic lithosphere

Year: 2026

Type: article

Abstract: We characterized and quantified hydrogen concentrations in nominally anhydrous minerals (NAMs) of the Horoman peridotite complex in the Hidaka metamorphic belt (Hokkaido, Japan). Twelve representative spinel peridotites (lherzolites and harzburgites) with microstructures ranging from coarse-porphyroclastic to mylonitic were investigated. We conducted unpolarized Fourier-transform infrared spectroscopy (FTIR) measurements on thick, doubly polished sections. Despite serpentine contamination (35% in average), typical OH bands were identified in all NAMs. Only one analyzed olivine yielded a reliable hydrogen concentration (8 wt ppm H 2 O). Porphyroclastic orthopyroxenes contain 69–198 wt ppm H 2 O, and clinopyroxenes have a large range of concentrations (283–630 wt ppm H 2 O). Using mineral fractions previously obtained by electron backscatter diffraction and FTIR results from this study on NAMs, the average bulk water content is estimated to be approximately 47–56 wt ppm H 2 O for the Horoman peridotites. A minute amount of pargasite is present, and if taken into account, the bulk water content is 136–145 wt ppm H 2 O . These average bulk water contents are in agreement with previous estimates for the oceanic mantle lithosphere based on peridotite xenoliths and peridotites from massifs. However, the impact of tectonic exhumation and late, pervasive serpentinization on the observed hydrogen concentrations remains unclear. Furthermore, within the Horoman peridotite complex, hydrogen concentrations show no clear relationship with olivine CPO symmetry. These observations suggest that variations in hydrogen content alone cannot account for the massif-scale transition in olivine CPOs from E to AG types in the structurally lower units to the upper units. Thus, using olivine CPO to approximate olivine water content may be inappropriate. Our results support a model in which olivine CPO evolution reflects the combined effects of deformation history, including the level of finite strain, the inheritance of pre-existing fabrics, or variations in strain geometry, and not the effect of hydrogen. The observed diversity and spatial organization of olivine CPOs are best explained by the interplay of kinematic and microstructural factors under broadly comparable hydration conditions.

Source: Lithos

DOI: 10.1016/j.lithos.2026.108774

Authors: Kazuki Matsuyama, Sylvie Demouchy