Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by Gaudin, A. Articles by Decarreau, A. Search for Related Content GeoRef GeoRef Citation

CHARACTERIZATION AND ORIGIN OF Fe³⁺-MONTMORILLONITE IN DEEP-WATER CALCAREOUS SEDIMENTS (PACIFIC OCEAN, COSTA RICA MARGIN)

¹ CNRS-UMR 6112, Laboratoire de Planétologie et Géodynamique, Faculté des Sciences et Techniques, Université de Nantes, BP 92208, 44322 Nantes Cedex 03, France
² Département de Géosciences, EA2642, UFR Sciences et Technique, Université de Franche Comté, 16 Route de Gray, 25065 Besançon, France
³ CNRS-UMR 6532, Laboratoire HydrASA, Faculté des Sciences, 86022 Poitiers Cedex, France
⁴ CRMC2, CNRS-UPR 7251, Campus de Luminy, Case 913, F-13288 Marseille Cedex 09, France

^* E-mail address of corresponding author: martine.buatier{at}univ-fcomte.fr

Millimetric to centimetric green grains widespread in pelagic calcareous sediments recovered at a water depth of 3000 m near the Costa Rica margin were studied by X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy. Samples were collected, during the Ticoflux II expedition, from the upper bioturbated part of four sedimentary cores (0.13–3.75 m below seafloor). The sediments are calcareous and siliceous nanofossil oozes (coccoliths, diatoms, radiolarians, etc.).

Green grains show generally a concentric zoning with a green rim in which smectite largely predominates over pyrite and a black core in which pyrite is prevalent. Observations by SEM indicate that this zoning results from a progressive inward alteration and replacement of the accumulations of pyrites by smectites. The high-resolution TEM observations of the smectite-pyrite interfaces suggest that the replacement of pyrites by smectite occurs through a dissolution-precipitation process with the formation of a gel. The pyrite matrix is composed of a huge number of very small (0.5–2 µm) pyrite octahedra, a typical texture resulting from the pyritization of organic material in early diagenetic environments.

The accurate mineralogical and crystal chemical characterization of the smectites indicate that they are Fe³⁺-montmorillonites (Fe³⁺-rich smectite with a dominant octahedral charge, rarely recorded in the literature). The formation of such Fe³⁺-montmorillonites forming green grains could be explained by two successive diagenetic redox stages: (1) reducing stage: early pyritization of the organic matter by microbial reduction within reducing micro-environments; (2) oxidizing stage: Fe³⁺-montmorillonite crystallized in space liberated after dissolution of pyrite connected with the rebalancing of the redox conditions of the micro-environments with the oxidizing surrounding sediments.

Key Words: Calcareous Sediments • Costa Rica Margin • Montmorillonite • Pyrite • Smectite

This article has been cited by other articles:

				M. A. S. Braga, C. L. Gomes, J. Duplay, and H. Paquet Clay minerals in the Namacotche Pegmatite Group from Zambezia Province, Mozambique: main constituents of late-stage secondary paragenesis Clay Minerals, December 1, 2008; 43(4): 597 - 613. [Abstract] [Full Text] [PDF]