HR: 0830h
AN: V51H-0378    [PDF]
TI: Geochemical and Paleomagnetic Implications of Iron-Sulfide Formation in Clayey Sediments from the Upper Pliocene Valle Ricca Section, Italy
AU: * Jiang, W
EM: atwtj@mail.ncku.edu.tw
AF: National Cheng Kung University, Department of Earth Sciences, Tainan, 701 Taiwan
AU: Huang, C
AF: National Cheng Kung University, Department of Earth Sciences, Tainan, 701 Taiwan
AU: Roberts, A P
EM: arob@mail.soc.soton.ac.uk
AF: University of Southampton, School of Ocean and Earth Science, Southampton Oceanography Centre, Southampton, SO14 3ZH United Kingdom
AU: Horng, C
EM: cshorng@earth.sinica.edu.tw
AF: Academica Sinica, Institute of Earth Sciences, Taipei, 115 Taiwan
AB: Clayey upper Pliocene sediments from the Valle Ricca section (Rome, Italy) exhibit intriguing intergrowth features among pyrite, pyrrhotite, and greigite, which have implications for variations in sediment diagenesis and paleomagnetism. SEM data indicate that pyrite occurs as octahedral crystals (sub- to several microns in size) that form framboids or irregular aggregates locally surrounding greigite and other pyrite framboids. Individual euhedral crystals and framboids or aggregates of pyrite are present in the matrix, occasionally enclosed within foraminiferal skeletons. Greigite typically occurs as grains less than 0.5 micron in size. The greigite forms framboids several microns in size or small irregular aggregates that occupy space between pyrite crystals or as framboids within pyrite aggregates, or as dispersed grains or aggregates intermixed with randomly-oriented subhedral pyrrhotite crystals forming nodules several mm in size. Pyrrhotite occurs with 3 types of microstructure: (1) banded aggregates up to several hundred microns thick, consisting of sub-parallel to parallel platy crystals of hexagonal shape up to 30 microns wide and 5 microns thick, associated with and locally enclosing pyrite and greigite framboids; (2) aggregates of randomly-oriented platy crystals forming massive pyrrhotite-greigite nodules, with pyrrhotite enriched in border areas, and (3) small clusters of platy crystals, up to several tens of microns in size occurring in matrix or within phyllosilicate \{001\} cleavages. The complex growth features of iron-sulfide minerals collectively suggest authigenic neoformation following the sequence of pyrite, greigite, and pyrrhotite, implying a change from early moderate Eh and high total dissolved S to highly reducing and relatively low dissolved S environments during diagenesis. The petrographic evidence for post-depositional formation of greigite is supportive of previously identified low-temperature normal polarity that is opposite to that recorded by detrital magnetite.
DE: 1045 Low-temperature geochemistry
DE: 1519 Magnetic mineralogy and petrology
DE: 1714 Geomagnetism and paleomagnetism
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3665 Mineral occurrences and deposits
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting