Magmatic ^18O in Zircons From Gabbros and Serpentinized Peridotite at the Mid-Atlantic Ridge (ODP Leg 153)

Zircons from gabbros and serpentinized ultramafic rocks from the Mid-Atlantic Ridge near the Kane Transform (MARK area) drilled during Leg 153 of the Ocean Drilling Program were analyzed for δ18O in situ in rock chips by ion microprobe. The gabbros contain clinopyroxene, plagioclase, apatite, Fe-oxides, with zircon occurring primarily along grain boundaries and as inclusions in other minerals (e.g. plagioclase). The mineralogy of the serpentinites is more complex, as they are comprised predominantly of a serpentine matrix that is cross-cut by multiple generations of intrusions, including zircon-bearing magmas emplaced as cm- to mm-scale gabbroic dikes and also lower temperature hydrothermal veins. The gabbroic dikes were pervasively altered at greenschist facies conditions, leaving zircon ± apatite as the only preserved magmatic phases. In some serpentinites it is difficult to distinguish altered magmatic veins from lower temperature hydrothermal veins. Zircons in serpentinite and gabbroic samples yield average δ18O values of 4.94±0.80‰ VSMOW (2 SD, N=33 analyses on 12 grains). This value would be in high temperature, magmatic equilibrium with MORB if δ18O (WR) ~ 5.3‰, or mantle peridotite if δ18O (Ol) = ~4.8‰. Equilibrium fractionation factors for δ18O between zircon-water (Zrc-H2O) were calculated by combining fractionation factors for Zrc-quartz (Valley et al. 2003) and quartz-H2O (Clayton et al., 1972; Matsuhisa et al., 1979). Over the temperature range of the calibrated fractionation factors (i.e. 500-800°C), calculated δ18O (zircon) values would be < 1 ‰ for zircon in equilibrium with previously measured MARK hydrothermal fluids (e.g. δ18O = 2.3‰). Zircon in equilibrium with seawater with δ18O = 0.0‰ would have 2.3‰ lower values. We note that extrapolation of the Zrc-H2O fractionation factors to temperatures below the calibrations of Zrc-Qtz and Qtz-H2O (e.g. <500°C) does not yield a typical mineral-water `crossover' at lower temperatures; thus the calculated fractionation of δ18O for Zrc-H2O is less certain below 500°C. A magmatic origin is thus inferred for the zircons based on δ18O , and is consistent with the observation of oscillatory zoning in many grains. All of the MARK zircons appear to be associated with gabbroic melts; zircons in serpentinites are not found as a matrix phase. These results demonstrate a wider compositional range of ocean crust that contains magmatic zircon than previously recognized. If the MARK samples are representative of `typical' ocean crust, the results of this study suggest that magmatic zircon may be an ubiquitous accessory phase in oceanic gabbros of any age, and are highly amenable to geochemical/geochronological study, particularly if the protoliths have experienced significant alteration and/or metamorphism (e.g. eclogites). The MARK data supports findings of prior studies that zircons in uncontaminated mantle melts will have δ18O values of 5.3±0.6‰ and that magmatic δ18O (zircon) values above ~6‰ require a high δ18O component (i.e. recycled crust) in the melt. Cavosie et al., 2005 EPSL; Valley et al. 2005 CMP