This study explores the fractionation of iron isotopes (57 Fe/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 δ57Fe 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 dolostoes range between -0.1 and 0.5‰, although in very organic-rich shale samples below 360 m core depth higher δ57Fe values are noted. Pyrite nodules and pyritized ammonites from the type exposure yield δ57Fe values of -0.3 to -0.45‰. A fractionation model consistent with the δ57Fe variations relates the lower δ57Fe 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 δ57Fe, 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) δ57Fe 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.
Bibliographical noteFunding Information:
This iron-isotope research was supported by the Israel Science Foundation (Grant 455/00). The Anatomy of a Source Rock project was funded by the NERC (Research grant numbers GST/02/1346 and GST/06/1346: Rapid Global Geological Events) and by a consortium of oil companies (Arco British, Conoco Norway, Enterprise, FINA, Phillips, Saga Petroleum, Shell, Statoil and Texaco). The Ministry of Defence and the Smedmore and Encombe estates kindly gave access to the cliffs near Kimmeridge. The British Geological Survey are thanked for providing access to the RGGE Kimmeridge Clay Cores. Nurith Vistowsky, and Reuben Belmaker are thanked for their help with the clean laboratory work and Dr. Irene Segal of the Geological Survey of Israel for her generous assistance with the mass spectrometry. Dr. C. M. Johnson and S. L. Brantley are thanked for kindly making available unpublished experimental data during the writing of the paper. The critical comments of Drs. C. M. Johnson, S. L. Brantley and R. Raiswell led to substantial revision of the manuscript and we would like to express our gratitude to these reviewers and to the Associate Editor, Dr. D. E. Canfield.