IRON ISOTOPE FRACTIONATION DURING OCEANIC CRUST ALTERATION

Abstract

The purpose of this work is to study the mobility and budget of Fe isotopes in the oceanic crust and in particular during low-temperature interaction of seawater with oceanic basalt. We carried out this investigation using samples from Ocean Drilling Program (ODP) Site 801C drilled during Leg 129 and Leg 185 in Jurassic Pacific oceanic crust seaward of the Mariana Trench. The site comprises approximately 450 m of sediment overlying a section of 500 m of basalt, which includes intercalated pelagic and chemical sediments in the upper basaltic units and two low-temperature (10–30 °C) ocherous Si–Fe hydrothermal deposits. Fe was chemically separated from 70 selected samples, and 57Fe/54Fe ratios were measured by MC-ICP-MS Isoprobe. The isotopic ratios were measured relative to an internal standard solution and are reported relative to the international Fe-standard IRMM-14. Based on duplicate measurements of natural samples, an external precision of 0.2‰ (2σ) has been obtained. The results indicate that the deep-sea sediment section has a restricted range of δ57Fe, which is close to the igneous rock value. In contrast, large variations are observed in the basaltic section with positive δ57Fe values (up to 2.05‰) for highly altered basalts and negative values (down to −2.49‰) for the associated alteration products and hydrothermal deposits. Secondary Fe-minerals, such as Fe-oxyhydroxides or Fe-bearing clays (celadonite and saponite), have highly variable δ57Fe values that have been interpreted as resulting from the partial oxidation of Fe2+ leached during basalt alteration and precipitated as Fe3+-rich minerals. In contrast, altered basalts at Site 801C, which are depleted in Fe (up to 80%), display an increase in δ57Fe values relative to fresh values, which suggest a preferential leaching of light iron during alteration. The apparent fractionation factor between dissolved Fe2+ and Fe remaining in the mineral is from 0.5‰ to 1.3‰ and may be consistent with a kinetic isotope fractionation where light Fe is stripped from the minerals. Alternatively, the formation of secondary clays minerals, such as celadonite during basalt alteration may incorporate preferentially the heavy Fe isotopes, resulting in the loss of light Fe isotopes in the fluids.Because microbial processes within the oceanic crust are of potential importance in controlling rates of chemical reactions, Fe redox state and Fe-isotope fractionation, we evaluated the possible effect of this deep biosphere on Fe-isotope signatures. The Fe-isotope systematics presented in this study suggest that, even though iron behavior during seafloor weathering may be mediated by microbes, such as iron-oxidizers, δ57Fe variations of more than 4‰ may also be explained by abiotic processes. Further laboratory experiments are now required to distinguish between various processes of Fe-isotope fractionation during seafloor weathering.