Title: Alteration
of magnetic mineralogy at the sulfate-methane transition: Analysis of
sediments from the Argentine continental slope
Source: PHYSICS
OF THE EARTH AND PLANETARY INTERIORS 151 (3-4): 290-308 AUG 15 2005
Document Type: Article
Language: English
Abstract: On
the Argentine continental slope off the Rio de la Plata estuary, the
sulfate-methane transition (SMT) has been encountered at shallow depths
of a few meters below the seafloor. At around this horizon, where
sulfate diffusing downward from the bottom water is met and reduced by
methane rising from deeper in the sediment column, intense alteration
affects the detrital magnetic mineral assemblage. Less than 10% of the
dominant primary low coercivity ferrimagnetic (titano-) magnetite
remains after alteration. In the upper part of the suboxic environment,
underlying the iron redox boundary, which is located at a depth of
similar to 0.1 m, approximately 60% of the finer grained detrital
fraction is already dissolved. While the high coercivity minerals are
relatively unaffected in the suboxic environment, large portions (>
40%) are diagenetically dissolved in the sulfidic SMT zone.
Nevertheless, the characteristics of the magnetic residue are entirely
controlled by a high coercivity mineral assemblage. Unlike common
observations, that diagenetic alteration produces coarser magnetic
grain-sizes in suboxic milieus, a distinct overall fining is found in
the sulfidic zone. Different factors should contribute to this effect.
Scanning electron microscope analysis, combined with X-ray
microanalysis, identified fine grained (titano-) magnetite preserved as
inclusions in silicates and between high Ti titanohematite lamellae,
and possibly of prime importance, a comprehensive fragmentation of
larger grains in the course of maghemitization. The only secondary iron
sulfide mineral detected is pyrite, which is present as clusters of
euhedral crystals or directly replaces (titano-) magnetite. The
thermomagnetic measurements did not provide evidence for the presence
of ferrimagnetic sulfides such as greigite. Different from other
studies reporting a marked magnetic enhancement at around the SMT due
to the precipitation and preservation of such metastable ferrimagnetic
sulfides, a complete pyritization process will cause a distinct
magnetic depletion, like in the present case. (c) 2005 Elsevier B.V.
All rights reserved.
Author Keywords: diagenesis;
sulfate reduction; rock magnetism; scanning electron microscope
analysis
KeyWords Plus: ISOTHERMAL
REMANENT MAGNETIZATION; EASTERN MEDITERRANEAN SEA; MARINE-SEDIMENTS;
ACQUISITION CURVES; SOUTH-ATLANTIC; DIAGENETIC PROCESSES; EQUATORIAL
ATLANTIC; ANAEROBIC OXIDATION; PELAGIC SEDIMENTS; PYRITE FORMATION
Addresses: Garming
JFL (reprint author), Univ Bremen, Fachbereich Geowissenschaften,
Klagenfurter Str, Bremen, D-28359 Germany
Univ Bremen, Fachbereich Geowissenschaften, Bremen, D-28359 Germany
Publisher: ELSEVIER
SCIENCE BV, PO BOX 211, 1000 AE AMSTERDAM, NETHERLANDS
Subject Category: GEOCHEMISTRY
& GEOPHYSICS
IDS Number: 953LP
ISSN: 0031-9201