The process of serpentinization in dunite from New Caledonia

摘要: Abstract Dunite from New Caledonia displays three types of serpentine-dominated veins. The earliest, type 1 veins are narrow (50–100 μm wide) and rarely extend across more than a single olivine grain. They lizardite, contain abundant brucite never magnetite. Type 2 0.01 to 0.1 mm wide, several grains cut the magnetite, often in vein interiors, less 3 dominantly chrysotile, cm-scale, have magnetite-rich core, for meters or more. Analyses two representative samples indicate that relatively Fe-rich serpentine (XMg = 0.92) (XMg = 0.82). These minerals magnesian those veins; has XMg = 0.93–0.94 XMg = 0.84. In core both (XMg = 0.94–0.97) one populations (XMg = 0.94) Mg-rich. Opx harzburgite layers these is by on order 0.05 mm wide. after lack talc magnetite and, as with cutting olivine, older Fe rich (XMg = 0.84) younger (XMg = 0.90). We conclude formation was accompanied extraction iron early-formed brucite. Thermodynamic calculations suggest formed rock-dominated system where activities FeO, MgO, SiO2 were dictated compositions orthopyroxene. contrast fluid-dominated infiltrating fluid oxidizing out equilibrium original brucite–serpentine assemblage. Reduction this reaction hydrogen. By producing extracted serpentine, making them magnesian. This would drive brucite–serpentine–magnetite assemblage higher oxygen fugacity, progressively decreasing efficiency magnetite-forming reactions.