A Deep-Water Glauconitization Process on the Ivory Coast-Ghana Marginal Ridge (ODP Site 959): Determination of Fe3+-Rich Montmorillonite in Green Grains

A. Wiewióra1, P. Giresse2, S. Petit3 and A. Wilamowski1
1 Institute of Geological Sciences, Polish Academy of Sciences, ul. Twarda 51/55, 00-818 Warszawa, Poland
2 Laboratoire de Sédimentologie Marine, Université de Perpignan, Avenue de Villeneuve, 66860, Perpignan Cedex, France
3 UMR 6532 CNRS HydrASA, Université de Poitiers, 40, Avenue du Recteur Pineau, 86022 Poitiers Cedex, France
E-mail of corresponding author: wiewiora@twarda.pan.pl

Abstract: The mineral and chemical composition of green glauconitic grains from ODP Site 959 (2100 m water depth) located on the northern flank of the Ivory Coast-Ghana Marginal Ridge was studied. Recurrent winnowing of a 20 m thick Pleistocene succession resulted in a low accumulation rate and stratigraphic hiatuses. The green clay material typically occurs as fillings in the chambers of pelagic foraminifers. The amount of green clay present in sediments older than 1 Ma is small, and greater in younger material. Mud composed of smectite, kaolinite, traces of mica, calcite and quartz was the precursor material that filled the chambers of the foraminifers. Processes at the water-sediment interface slowly modified this composition. Kaolinite was dissolved; smectite lost Al but gained Fe, K and layer charge. In that matrix, the nanocrystals of neoformed smectite are observed. The infrared (IR) spectra showed OH-stretching and bending vibrations due to groups incorporating Fe3+. The spectra are in agreement with the crystallochemical formulae of Fe3+-rich montmorillonite as determined by point-by-point analyses on the neoformed crystallites and on the surrounding matrix. The layer charge in this Fe3+-rich montmorillonite is almost wholly octahedral as shown in crystallochemical formulae and documented independently by a new IR method. The tetrahedral charge appeared when the Fe content increased by >1.2 Fe per formula unit. With the maturation process, the increased role of the closed layers is observed, with the color of grains becoming greener. We have documented for the first time glauconitization proceeding at a depth of 2100 m at a temperature near 3°C. The most important factors of the process are: accumulation of terrigenous clayey material in the foraminiferal chambers, Fe supply from a nearby continent, and a lengthy residence at the water-sediment interface in the zone of the winnowing and low sediment accumulation rate.

Key Words: Eastern Atlantic • Fe3+-rich Montmorillonite • Foraminifer Fillings • Glauconitization • Green Clay • Pleistocene

Clays and Clay Minerals; December 2001 v. 49; no. 6; p. 540-558; DOI: 10.1346/CCMN.2001.0490606
© 2001, The Clay Minerals Society
Clay Minerals Society (www.clays.org)