Al-Zoning of Serpentine Aggregates in Mesh Texture Induced by Metasomatic Replacement Reactions

Serpentinization of oceanic lithosphere commonly proceeds with the development of mesh texture. Examination of a serpentinized harzburgite and a plagioclase-bearing wehrlite revealed conspicuous zoning of Al in a serpentine mesh texture, with Al-rich cores and Al-poor rims, as well as Al-rich veins, indicating local transport of Al from plagioclase and pyroxene during serpentinization. To reveal the influences of Al on the reaction mechanisms and textural development during serpentinization, we conducted hydrothermal experiments in the olivine (Ol)–plagioclase (Pl)–H2O system and analyzed the variations in mineralogy and microtexture of olivine replacements as a function of distance from the Ol–Pl boundary. The Al–Si metasomatic zone, where Al-serpentine and a minor amount of Ca-saponite were formed, was developed in the Ol-hosted region close to the Ol–Pl boundary. Far from the Ol–Pl boundary, Al-free serpentine, brucite, and magnetite were formed, indicating the progress of an ‘isochemical’ reaction (separate from water). The aggregates of Al-serpentine after olivine in the metasomatic zone showed a characteristic zoning of Al. Microtextural evidence indicates that the zoning was produced in response to the migration of an Al metasomatic front that involved an early-stage of serpentinization with an Al-free solution, and the subsequent pseudomorphic replacement of olivine and simultaneous development of overgrowths as the amount of Al increased in the solution. The Al-bearing aqueous solution caused the formation of olivine pseudomorphs and this contrasts with the lack of preservation of original olivine outlines in the isochemical zones. Comparisons of zoning in natural and experimentally produced mesh textures suggest that Al-poor rims in the mesh texture form at the start of the serpentinization process, followed by the coupled formation of Al-rich mesh cores and Al-rich veins. Our experimental results indicate that Al-zoning in the mesh texture represent the transition from a closed to an open system during serpentinization.