Apparent magnetic polarity reversals due to remagnetization resulting from late diagenetic growth of greigite from siderite

• Published in: Geophysical Journal International
• Main Authors: Sagnotti, L., Roberts, A. P., Weaver, R., Verosub, K. L., Florindo, F., Pike, C. R., Clayton, T., Wilson, G. S.
• Other Authors: Sagnotti, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia, Roberts, A. P.; School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK, Weaver, R.; School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK, Verosub, K. L.; Department of Geology, University of California, Davis, CA 95616, USA, Florindo, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia, Pike, C. R.; 3Department of Geology, University of California, Davis, CA 95616, USA, Clayton, T.; School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK, Wilson, G. S.; Department of Geology, University of Otago, Dunedin, New Zealand, Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia, School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK, Department of Geology, University of California, Davis, CA 95616, USA, 3Department of Geology, University of California, Davis, CA 95616, USA, Department of Geology, University of Otago, Dunedin, New Zealand
• Format: Article in Journal/Newspaper
• Language: English
• Published: Blackwell Publishing 2005
• Subjects: Antarctica; diagenesis; greigite; iron carbonate; iron sulphide; remagnetization; siderite; 04. Solid Earth::04.05. Geomagnetism::04.05.06. Paleomagnetism; 04. Solid Earth::04.05. Geomagnetism::04.05.07. Rock magnetism; Ross Sea; Antarc*
• Online Access: http://hdl.handle.net/2122/3992; https://doi.org/10.1111/j.1365-246X.2005.02485.x

A mixed-polarity zone, representing alternations between remagnetized and non-remagnetized strata, has been documented within the lower few metres of the CRP-1 core (Ross Sea, Antarctica). Detailed rock magnetic investigation of this interval indicates that the normal polarity remagnetization is carried by magnetostatically interacting single-domain particles of a ferrimagnetic iron sulphide mineral, while the reversed-polarity magnetization of non-remagnetized strata is carried by magnetite with a broad range of grain sizes and negligible magnetostatic interactions. Scanning electron microscope observations of polished sections indicate that the ferrimagnetic iron sulphide mineral is greigite (Fe3S4). Based on microtextural relationships, it is not possible to determine the relative timing of formation for much of the greigite. However, a significant proportion of the greigite has grown on the surface of authigenic siderite (FeCO3) grains that occur as microconcretions and as cement surrounding detrital matrix grains. In such cases, microtextural relationships indicate that siderite post-dates early diagenetic pyrite and that greigite post-dates the siderite. Siderite usually forms in environments with abundant dissolved iron and carbonate, but without dissolved pore water H2S. This set of geochemical conditions occurs in methanic settings below the sulphate reduction zone (in which early diagenetic pyrite forms).We interpret the observed remagnetization of the lower part of the CRP-1 core as due to a late diagenetic pore water migration event where abundant iron on the surface of siderite grains reacted with fluids containing limited dissolved sulphide, thereby causing precipitation of greigite. The distribution of siderite (and associated greigite) in the lower part of the CRP-1 core is patchy, which accounts for the apparent alternation of polarities. This study is part of a growing catalogue of remagnetizations involving greigite, which suggests that occurrences of greigite should be treated with caution in ...