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Dunite from New Caledonia displays three types of veins. The earliest, type 1 veins are narrow (50-100 m wide) and rarely extend across more than a single olivine grain. They 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 cored by magnetite and contain less brucite than type 1 veins. Type 3 veins are cm-scale, have a magnetite-rich core, and extend for meters or more. The type 1 veins have relatively Fe-rich serpentine (X Mg = 0.92) and brucite (X Mg = 0.82). These minerals are more magnesian than those in the type 2 veins; serpentine has X Mg = 0.93-0.94 and brucite has X Mg = 0.84. In the magnetite-rich core to the type 3 vein both serpentine (X Mg = 0.94-0.97) and brucite (X Mg = 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 veins lack talc or magnetite and, as with veins cutting olivine, the older veins are more Fe rich (X Mg = 0.84) than the younger veins (X Mg = 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 rockbuffered system where the activities of FeO, MgO, and SiO 2 were dictated by the compositions of olivine and orthopyroxene. In contrast the type 2 veins were formed in a more fluid-buffered 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. In addition to producing magnetite this reaction also extracted iron from brucite and serpentine, making them both more magnesian. This would drive the brucite-serpentinemagnetite buffer to higher oxygen fugacity, progressively decreasing the efficiency of the magnetite-forming reactions.

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We present the iron isotope composition of primary, diagenetic and metamorphic minerals in five samples from the contact metamorphosed Biwabik Iron Formation. These samples attained peak metamorphic temperatures of <200, <340,~500, <550, and <740°C respectively. d 56 Fe of bulk layers ranges from-0.8 to +0.8&; in some samples the layers may differ by >1& on the millimeter scale. Minerals in the lowest grade samples consistently show a sequence in which d 56 Fe of magnetite > silicate ‡ carbonate. The intermineral Fe isotope differences vary in a fashion that cannot be reconciled with theoretical temperaturedependent fractionation factors. Textural evidence reveals that most, if not all, magnetite in the Biwabik Formation is diagenetic, not primary, and that there was tremendous element mobility during diagenesis. The short duration of contact metamorphism allowed diagenetic magnetite compositions to be preserved throughout prograde metamorphism until at least the appearance of olivine. Magnetite compositions therefore act as an isotope record of the environment in which these sediments formed. Larger-scale fluid flow and longer timescales may allow equilibration of Fe isotopes in regionally metamorphosed rocks to lower temperatures than in contact metamorphic environments, but weakly regionally metamorphosed rocks may preserve small-scale Fe isotopic heterogeneities like those observed in the Biwabik Iron Formation. Importantly, Fe isotope compositions that are characteristic of chemical sedimentation or hydrothermal processes are preserved at low grade in the form of large inter-mineral variations, and at high grade in the form of unique bulk rock compositions. This observation confirms earlier work that has suggested that Fe isotopes can be used to identify sedimentary processes in the Precambrian rock record. Communicated by F. Poitrasson.

[1] The depth extent, strength, and composition of oceanic detachment faults remain poorly understood because the grade of deformation-related fabrics varies widely among sampled oceanic core complexes (OCCs). We address this issue by analyzing fault rocks collected from the Kane oceanic core complex at 23 30 0 N on the Mid-Atlantic Ridge. A portion of the sample suite was collected from a younger fault scarp that cuts the detachment surface and exposes the interior of the most prominent dome. The style of deformation was assessed as a function of proximity to the detachment surface, revealing a $450 m thick zone of high-temperature mylonitization overprinted by a $200 m thick zone of brittle deformation. Geothermometry of deformed gabbros demonstrates that crystal-plastic deformation occurred at temperatures >700 C. Analysis of the morphology of the complex in conjunction with recent thermochronology suggests that deformation initiated at depths of $7 km. Thus we suggest the detachment system extended into or below the brittle-plastic transition (BPT). Microstructural evidence suggests that gabbros and peridotites with high-temperature fabrics were dominantly deforming by dislocationaccommodated processes and diffusion creep. Recrystallized grain size piezometry yields differential stresses consistent with those predicted by dry-plagioclase flow laws. The temperature and stress at the BPT determined from laboratory-derived constitutive models agree well with the lowest temperatures and highest stresses estimated from gabbro mylonites. We suggest that the variation in abundance of mylonites among oceanic core complexes can be explained by variation in the depth of the BPT, which depends to a first order on the thermal structure and water content of newly forming oceanic lithosphere.