HR: 08:55h
AN: GP31A-04    [PDF]
TI: Magnetite dissolution in siliceous sediments
AU: * Florindo, F
EM: florindo@ingv.it
AF: Istituto Nazionale di Geofisica Vulcanologia, Via di Vigna Murata 605, Rome, 00143 Italy
AU: * Florindo, F
EM: florindo@ingv.it
AF: Southampton Oceanography Centre, University of Southampton, European Way, Southampton, S014 3ZH United Kingdom
AU: Roberts, A P
EM: arob@soc.soton.ac.uk
AF: Southampton Oceanography Centre, University of Southampton, European Way, Southampton, S014 3ZH United Kingdom
AU: Palmer, M R
EM: martin.r.palmer@soc.soton.ac.uk
AF: Southampton Oceanography Centre, University of Southampton, European Way, Southampton, S014 3ZH United Kingdom
AB: Magnetite dissolution, and consequent loss of magnetization, is widely observed in reducing sedimentary environments, where the decrease in Eh-pH values with depth is driven by bacterially mediated degradation of organic carbon. We have observed low magnetizations in sediments with elevated porewater silica concentrations that arise from diagenesis of biogenic silica and/or silicic volcanic ash. These depletions in magnetization are greater than can be accounted for by dilution with magnetite-poor sediments and suggest that post-depositional destruction of magnetite has occurred. Biosiliceous sediments usually also contain elevated concentrations of organic carbon, which makes it difficult to separate organic-carbon-related magnetite dissolution from other possible mechanisms for magnetite dissolution. However, the extent of magnetite dissolution in the sedimentary sequences that we have studied is not obviously related to the redox-state of the environment. This suggests that other mechanisms might have given rise to magnetite dissolution in these siliceous sediments. Thermodynamic calculations indicate that magnetite is unstable under conditions of elevated dissolved silica concentrations (and appropriate Eh-pH conditions) and predict that magnetite will break down to produce iron-bearing smectite. A survey of magnetic susceptibility and pore water geochemical data from widely distributed Ocean Drilling Program sites supports this observed link between high dissolved silica concentrations and low magnetic susceptibilities. This observed link also holds for environments with low biogenic silica productivity (and low organic carbon content), but with high interstitial silica concentrations due to dissolution of silicic volcanic ashes. Dissolution of magnetite is therefore predicted to be a common feature of siliceous sedimentary environments.
DE: 1045 Low-temperature geochemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1512 Environmental magnetism
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting