Isotopic and element exchange during serpentinization and metasomatism at the Atlantis Massif (MAR 30 degrees N): Insights from B and Sr isotope data

The Lost City hydrothermal system at the southern Atlantis Massif (Mid-Atlantic Ridge, 30 degrees N) provides a natural laboratory for studying serpentinization processes, the temporal evolution of ultramafic-hosted hydrothermal systems, and alteration conditions during formation and emplacement of an oceanic core complex. Here we present B, 0, and Sr isotope data to investigate fluid/rock interaction and mass transfer during detachment faulting and exhumation of lithospheric sequences within the Atlantis Massif. Our data indicate that extensive serpentinization was a seawater-dominated process that occurred predominately at temperatures of 150-250 degrees C and at high integrated W/R ratios that led to a marked boron enrichment (3491 ppm). Boron removal from seawater during serpentinization is positively correlated with changes in delta B-11 (11-16 parts per thousand) but shows no correlation with O-isotope composition. Modeling indicates that B concentrations and isotope values of the serpentinites are controlled by transient temperature-pH conditions. In contrast to prior studies, we conclude that low-temperature marine weathering processes are insignificant for boron geochemistry of the Atlantis Massif serpentinites. Talc- and amphibole-rich fault rocks formed within a zone of detachment faulting at temperatures of approximately 270-350 degrees C and at low W/R ratios. Talc formation in ultramafic domains in the massif was subsequent to an early stage of serpentinization and was controlled by the access of Si-rich fluids derived through seawater-gabbro interactions. Replacement of serpentine by talc resulted in boron loss and significant lowering of delta B-11 values (9-10 parts per thousand), which we model as the product of progressive extraction of boron. Our study provides new constraints on the boron geochemical cycle at oceanic spreading ridges and suggests that serpentinization associated with ultramafic-hosted hydrothermal systems may have important implications for the behavior of boron in subduction zone settings. (C) 2008 Elsevier Ltd. All rights reserved.