Controls on iron-isotope fractionation in organic-rich sediments (Kimmeridge Clay, Upper Jurassic, Southern England)

Abstract

This study explores the fractionation of iron isotopes ( 57Fe/ 54Fe) in an organic-rich mudstone succession, focusing on core and outcrop material sampled from the Upper Jurassic Kimmeridge Clay Formation type locality in south Dorset, UK. The organic-rich environments recorded by the succession provide an excellent setting for an investigation of the mechanisms by which iron isotopes are partitioned among mineral phases during biogeochemical sedimentary processes. Two main types of iron-bearing assemblage are defined in the core material: mudstones with calcite ± pyrite ± siderite mineralogy, and ferroan dolomite (dolostone) bands. A cyclic data distribution is apparent, which reflects variations in isotopic composition from a lower range of δ 57Fe values associated with the pyrite/siderite mudstone samples to the generally higher values of the adjacent dolostone samples. Most pyrite/siderite mudstones vary between −0.4 and 0.1‰ while dolostones range between −0.1 and 0.5‰, although in very organic-rich shale samples below 360 m core depth higher δ 57Fe values are noted. Pyrite nodules and pyritized ammonites from the type exposure yield δ 57Fe values of −0.3 to −0.45‰. A fractionation model consistent with the δ 57Fe variations relates the lower δ 57Fe pyrite and siderite ± pyrite mudstones values to the production of isotopically depleted Fe(II) during biogenic reduction of the isotopically heavier lithogenic Fe(III) oxides. A consequence of this reductive dissolution is that a 57Fe-enriched iron species must be produced that potentially becomes available for the formation of the higher δ 57Fe dolostones. An isotopic profile across a dolostone band reveals distinct zonal variations in δ 57Fe, characterized by two peaks, respectively located above and below the central part of the band, and decoupling of the isotopic composition from the iron content. This form of isotopic zoning is shown to be consistent with a one-dimensional model of diffusional-chromatographic Fe-isotope exchange between dolomite and isotopically enriched pore water. An alternative mechanism envisages the infiltration of dissolved ferrous iron from variable (high and low) δ 57Fe sources during coprecipitation of Fe(II) ion with dolomite. The study provides clear evidence that iron isotopes are cycled during the formation and diagenesis of organic carbon-rich sediments.