HR: 1340h
AN: GP43B-1217    [Abstracts]
TI: Direct dating of remagnetizations in carbonate rocks: insights into the timing and mechanism from clay mineralogy and ⁴⁰Ar/³⁹Ar age analysis.
AU: * Tohver, E
EM: etohver@cyllene.uwa.edu.au
AF: School of Earth and Geographical Sciences, University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia
AU: Weil, A B
EM: aweil@brynmawr.edu
AF: Dept. of Geology, Bryn Mawr College, 101 North Merion Avenue, Bryn Mawr, PA 19010- 2899, United States
AU: Solum, J G
EM: ca_geologist@yahoo.com
AF: Dept. Geography and Geology, Sam Houston St University, Campus Box 2148, Huntsville, TX 77041, United States
AU: Hall, C M
EM: cmhall@umich.edu
AF: Dept. of Geological Sciences, University of Michigan, 35 C.C. Little Bldg, Ann Arbor, MI 48109, United States
AB: A significant problem for paleomagnetic study of carbonates is their propensity for remagnetization, especially during tectonic events. This phenomenon, recognized since the late 1980's, remains unexplained. We examined carbonate rocks of Devonian age from the Cantabrian arc of NW Spain, which are recognized to have become remagnetized during the Permo-Carboniferous amalgamation of Pangaea. Both whole rock samples and magnetic extracts have been investigated previously using conventional techniques of rock magnetism, demonstrating the presence of both single domain and superparamagnetic magnetite. X-ray diffraction of the insoluble, non-carbonate material in these rocks reveals abundant illite, with quartz and magnetite also identified. Smectite, in the form of interlayered illite-smectite or discrete smectite, is notably absent. Quantitative modeling of different grain size fractions (<0.05 μm, 0.05-2 μm, >2 μm, and ≫2 μm) indicates that the proportion of the detrital illite (polytype 2M₁) is commensurate with grain size, with authigenic illite (polytype 1M_d) predominating in the finest grain fractions. ⁴⁰Ar/³⁹Ar age dating of these different grain size fractions confirms the "younging" trend for the authigenic clay population. Extrapolating from the mixing line produced by the four size fractions to a 100% authigenic component yields a radiogenic age that overlaps with the known timing of remagnetization. We propose a mechanism for carbonate remagnetization in which Fe- rich smectite is transformed to Fe-poor illite + magnetite. The age of remagnetization (i.e., authigenic clay and the remanence-bearing magnetite) can be dated by ⁴⁰Ar/³⁹Ar analysis of the clay products.
DE: 1115 Radioisotope geochronology
DE: 1533 Remagnetization
DE: 1540 Rock and mineral magnetism
DE: 3675 Sedimentary petrology
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting