HR: 09:45h
AN: OS11C-08    [Abstracts]
TI: Geochemical Conditions of Magnetic Iron Sulphide Formation in Southern Hydrate Ridge Sediments (ODP Leg 204): Preliminary Results
AU: * Pinero, E
EM: epinero@cmima.csic.es
AU: Larrasoana, J
EM: jclarra@ija.csic.es
AU: Gracia, E
EM: egracia@cmima.csic.es
AU: Martinez-Ruiz, F
EM: fmruiz@ugr.es
AB: Southern Hydrate Ridge sediments are composed by hemipelagic silty-clays interbedded with numerous turbidite and debris flow deposits. The magnetic mineralogy of these gas hydrate-rich sediments is dominated by magnetite and the magnetic iron sulphides greigite and pyrrhotite. Magnetite is more abundant in coarse-grained sediments and it's therefore interpreted to be detrital in origin. Greigite and pyrrhotite have been recognized as being diagenetically produced during microbial reduction of sulphate in the sulphate, the anaerobic oxidation of methane, and the methanic zones. In order to study the chemical conditions in which magnetic iron sulphides are produced and preserved, we have measured the total Fe (TFe), reactive-Fe, TOC and total S (TS) of a set of ca. 100 samples with distinctive magnetic assemblages (magnetite+greigite, greigite-, and pyrrhotite-dominated). TOC and TS were analyzed by an elemental analyzer and, TFe and reactive-Fe were measured by atomic absorption spectrometry. TOC contents range between 0.38 and 1.22%, having the greigite-dominated samples the highest relative TOC contents (>0.77%). TS contents vary between 0.2 and 0.9%. TFe values range between 4.3 and 7.3%, in which the reactive-Fe represents the 23-39%. TS/TOC ratio values are near the oxic and normal marine seawater conditions (TS/TOC = 0.36), except for two magnetite+greigite and greigite-dominated samples that have higher TS contents. All the samples show TS/reactive Fe ratios considerably lower that of saturated pyrite (1.15), which implies a low degree of pyritization in the environment. Only 2 samples dominated by greigite+magnetite show TS/reactive Fe contents higher than the saturated pyrrhotite ratio. The reactive Fe/TFe values are low, under reactive Fe/TFe =1/3, which implies that there is not enough reactive-Fe in the system. These preliminary results reinforce the view that formation and preservation of greigite and pyrrhotite in southern Hydrate Ridge sediments is favoured by low reactive-Fe contents, so that pyritization reactions at the different diagenetic zones are not driven to completion.
DE: 1012 Reactions and phase equilibria (3612, 8412)
DE: 1051 Sedimentary geochemistry
DE: 3004 Gas and hydrate systems
DE: 3022 Marine sediments: processes and transport
DE: 4825 Geochemistry
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting