The process of serpentinization in dunite from New Caledonia
MetadataShow full item record
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 are lizardite, contain abundant brucite and never contain magnetite. Type 2 veins are 0.01 to 0.1 mm wide, extend across several olivine grains and cut across the type 1 veins. They are lizardite, contain magnetite, often in vein interiors, and contain less brucite than type 1 veins. Type 3 veins are dominantly chrysotile, cm-scale, have a magnetite-rich core, and extend for meters or more. Analyses of two representative samples indicate that the type 1 veins have relatively Fe-rich serpentine (XMg = 0.92) and brucite (XMg = 0.82). These minerals are less magnesian than those in the type 2 veins; serpentine has XMg = 0.93–0.94 and brucite has XMg = 0.84. In the magnetite-rich core to the type 3 vein both serpentine (XMg = 0.94–0.97) and one of the two brucite populations (XMg = 0.94) are Mg-rich. Opx in harzburgite layers in these samples is cut by serpentine veins that are on the order of 0.05 mm wide. The serpentine veins after Opx lack talc or magnetite and, as with veins cutting olivine, the older veins are more Fe rich (XMg = 0.84) than the younger veins (XMg = 0.90). We conclude that the formation of magnetite was accompanied by the extraction of iron from the early-formed serpentine and brucite.Thermodynamic calculations suggest that the type 1 veins formed in a rock-dominated system where the activities of FeO, MgO, and SiO2 were dictated by the compositions of olivine and orthopyroxene. In contrast the type 2 veins were formed in a more fluid-dominated system where the infiltrating fluid was relatively oxidizing and out of equilibrium with the original brucite–serpentine assemblage. Reduction of this fluid was accompanied by reaction of brucite and serpentine to magnetite and hydrogen. By producing magnetite, this reaction extracted iron from brucite and serpentine, making them both more magnesian. This would drive the brucite–serpentine–magnetite assemblage to higher oxygen fugacity, progressively decreasing the efficiency of the magnetite-forming reactions.
NOTICE: this is the author’s version of a work that was accepted for publication in Lithos. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Lithos, Vol. 178 (2013). DOI: 10.1016/j.lithos.2013.02.002
Showing items related by title, author, creator and subject.
Modal mineralogy of CM2 chondrites by X-ray diffraction (PSD-XRD). Part 1: Total phyllosilicate abundance and the degree of aqueous alterationHoward, K.; Benedix, G.; Bland, Phil; Cressey, G. (2009)CM carbonaceous chondrites are samples of incompletely serpentinized primitive asteroids. Using position sensitive detector X-ray diffraction (PSD-XRD) and a pattern stripping technique, we quantify the modal mineralogy ...
CO2 sequestration and extreme Mg depletion in serpentinized peridotite clasts from the Devonian Solund basin, SW-NorwayBeinlich, Andreas; Austrheim, H.; Glodny, J.; Erambert, M.; Andersen, T. (2010)The conglomerates of the Solund Devonian basin of SW-Norway contain numerous (locally up to 20vol.%) peridotitic clasts with concentric mm- to 10-cm thick zones of varying red to black color. The peridotite clasts show a ...
Formation of mg-rich olivine pseudomorphs in serpentinized dunite from the Mesoarchean Nuasahi Massif, Eastern India: Insights into the evolution of fluid composition at the mineral-fluid interfaceMajumdar, A.; Hövelmann, J.; Vollmer, C.; Berndt, J.; Mondal, S.; Putnis, Andrew (2016)We present a detailed investigation of the micrometer-to nanometer-scale textural-chemical features in partially serpentinized dunites from the lower ultramafic unit of the Mesoarchean Nuasahi Massif, eastern India; these ...