A Geochemical and 57Fe Mössbauer Investigation of East Pacific Rise and Red Sea Metalliferous Sediments and Other Selected Marine Sedimentary Deposits

Abstract

Marine sediments and sedimentary deposits from several distinct environments have been investigated as part of a reconnaissance study to characterize better the iron-containing phases in areas of metalliferous, pelagic and shelf sedimentation and to assess the contribution that Mössbauer spectroscopy can make in such studies. Iron in metalliferous sediments from four locations on the East Pacific Rise (9-41 °S) is predominantly in the +3 oxidation state. In surface sediments, iron is mainly in ferrihydrite and to a lesser extent in smectite. The crystallinity of the iron oxide / oxide hydroxide component tends to increase with increasing latitude between 9-31 °S and at 31 °S disordered, crystalline goethite is found. On the basis of Mössbauer data, ageing of ferrihydrite to goethite over an interval of 1 m is proposed at 9 °S. The iron oxide / oxide hydroxide component of basin sediments adjacent to the East Pacific Rise is more crystalline and in Bauer Basin sediments the increase in crystallinity to the south parallels that observed in East Pacific Rise sediments. Goethite, rather than ferrihydrite, is the dominant iron oxide hydroxide in Bauer, Yupanqui and Roggeveen Basin sediments and smectite is a relatively more significant iron-containing phase in these sediments. Sediments examined from the Atlantis II Deep, Red Sea, contain a more diverse suite of iron-containing minerals and studies were restricted to the iron oxides and oxide hydroxides (ferrihydrite, goethite, haematite and magnetite) and the genetic relationships between them. The most significant finding is that haematite, in the presently discharging Southwest Basin, formed from ferrihydrite via a distinct haematite-precursor mineral and not by the dehydration of goethite as proposed by Bischoff (1969a). X-ray diffraction and Mössbauer data are presented together with a proposed model of formation in support of this hypothesis. Sediments northwest of Fiji in the Yasawa Trough and Braemar Ridge are not metalliferous. The ferrous:ferric iron ratio is higher for the Yasawa Trough sediments principally because of the contribution of detrital chlorite and hornblende from the Fiji Group and decreases with distance from the Fiji Group reflecting a greater contribution of halmyrolytic / authigenic iron-containing minerals. Irrespective of location, the ratio decreases also with decreasing particle size of the sediment fraction. The iron in sediments from the western continental shelf of India is also largely contained in detrital sediments, especially clay minerals. Poorly ordered goethite is intimately associated with carbonate-fluorapatite in ferruginous, phosphatic rock dredged from Barrier Bank, New Zealand and in iron-rich phosphatic nodules and slabs from the east Australian continental shelf. Goethite in the former is superparamagnetic whereas both superparamagnetic and antiferromagnetic goethite with slightly greater mean crystallite dimensions occur in the latter deposit. The disordered nature of the goethite in these deposits is attributed to the influence of transition metal and phosphate ions rather than to Al content. Room temperature Mössbauer spectra of the sediments comprising this study tend to be complex. The spectra are difficult to computer-fit satisfactorily because the iron-containing portion of the sediment can be made up of several minerals, each of which may have ferric and/or ferrous iron distributed over a number of sites. The difficulty is compounded in cases where the minerals are disordered or have partially relaxed magnetic character. Low temperature studies are considered essential in such cases but, despite their absence, useful qualitative data were still obtained from the non-fitted sediment spectra.