Olivine-hosted spinel–diopside symplectic intergrowths in Eoarchean dunites: Records of dehydrogenation-oxidation of a hydrous precursor

The role of water in the deep Earth has been recognized as critical for influencing thermal conditions, mineral stability, rheology, electrical properties, and element transport. Its influence is particularly interesting in studying Archean samples, which could provide insights into early Earth mantle processes. In this paper, we examine olivine from a dunite lens (Lens A) in the northwestern limb of the Eoarchean Isua Supracrustal Belt (ISB), southern West Greenland. Previous studies in the area demonstrate that this lens preserves portions of its primary features, which is notable given its age. Olivine in Lens A dunite samples hosts fine-scale spinel (Cr-magnetite) and pyroxene (diopside) symplectic intergrowths that are not associated with fractures and grain boundaries. Electron backscatter diffraction (EBSD) mapping shows a strong topotaxial relationship between the spinel component of the symplectic inclusion and host olivine, as defined by (100) Ol //{111} Spl . The host olivine is characterized by Fo 89-92 , NiO = 0.43–0.57 wt%, MnO = 0.10–0.18 wt% (cores). Compared with neighboring Lens B, which contains highly forsteritic olivine (Fo 96-98 ), Ti-rich humite-group inclusions, and stronger multiple hydration-dehydration imprints, Lens A retains features comparable to primary igneous attributes. Similar textures and mineralogical characteristics have been documented in symplectite inclusions that are strongly linked to formation through dehydrogenation, oxidation, and Fe-Cr cation diffusion along crystal defects under high-temperature conditions. The process results in the crystallographically controlled, topotactic replacement of a precursor phase. Hydrous olivine is known to segregate as planar or lamellar features that develop parallel to the (100) plane of the host olivine, whereas humite-group minerals typically develop along (001). The symplectic intergrowths in this study exhibit a consistent crystallographic orientation [(100) Ol //{111} Spl ], indicating structural control by the (100) planes of the host olivine. The trace amounts of Ti in the symplectic intergrowths and the absence of an ilmenite phase argue against replacement of Ti-clinohumite. Instead, the observed crystallographic relationship and composition are consistent with hydrous olivine 3(Mg,Fe) 2 SiO 4 ·MgH 2 SiO 4 as the precursor phase. The Cr-magnetite-diopside symplectic intergrowths record a closed-system, high-temperature, solid-state replacement process driven by dehydrogenation of an OH-bearing precursor phase within the host olivine. Assuming that one mole of hydrogen is released per mole of Cr oxidized, we provide a first-order stoichiometric estimate of approximately 10 ppm H in the precursor phase. These mineralogical features provide fine-scale evidence for the presence of structurally-bound hydrogen and hydrous components and, more broadly, the preservation of hydrous signatures in one of Earth’s oldest ultramafic rocks.