HR: 09:20h
AN: GP11B-06    [Abstracts]
TI: Selecting samples for paleointensity measurements with FORC diagrams
AU: * Carvallo, C
EM: czc@noc.soton.ac.uk
AF: National Oceanography Centre, European Way, Southampton, SO14 3ZH United Kingdom
AU: Roberts, A
GP11B-06 AF: National Oceanography Centre, European Way, Southampton, SO14 3ZH United Kingdom
AU: Leonhardt, R
GP11B-06 AF: Department of Earth and Ocean Sciences, Ludwig-Maximilians University, Munich, 80333 Germany
AU: Laj, C
GP11B-06 AF: Laboratoire des Sciences du Climat et de l'Environnement, Unite mixte CEA-CNRS, Gif-sur-Yvette, 91198 France
AU: Kissel, C
GP11B-06 AF: Laboratoire des Sciences du Climat et de l'Environnement, Unite mixte CEA-CNRS, Gif-sur-Yvette, 91198 France
AU: Perrin, M
GP11B-06 AF: Laboratoire Tectonophysique, UMR CNRS UM2-5568 Universite Montpellier 2, Montpellier, 34095 France
AU: Camps, P
GP11B-06 AF: Laboratoire Tectonophysique, UMR CNRS UM2-5568 Universite Montpellier 2, Montpellier, 34095 France
AB: Paleointensity data are vital for a better knowledge of deep-earth processes. The paleointensity database is limited by the high failure rate of paleointensity analysis. Excluding thermal alteration, failure is usually caused by the presence of multi-domain (MD) grains and interactions among grains, two properties that can be identified using first-order reversal curve (FORC) diagrams. We measured FORC diagrams on about 200 samples that had been used for Thellier paleointensity determinations and looked for discrimination criteria for samples that gave acceptable paleointensity results and for samples that failed the experiment. The three most discriminating criteria are the spread of the coercivity peak along the vertical axis of the FORC distribution (FWHM), the spread of the FORC distribution along the Hc = 0 axis (width), and the bulk coercivity Hc. Setting the thresholds at 132 mT for the width of the distribution and 29 mT for the FWHM maximizes the number of unsuccessful rejected samples and minimizes the number of successful rejected samples. Using an additional threshold of Hc = 5.4 mT, 35 unsuccessful samples (34%) could be rejected. Seven samples that satisfy the paleointensity selection criteria would also be rejected using these selection criteria, but these samples did not provide ideal paleointensity determinations. Most of the samples that fail the paleointensity experiment without being detected by our selection criteria show evidence of thermal alteration when heated repeatedly. Considering that thermal alteration is a major cause of failure for paleointensity determinations and that it cannot be detected in room- temperature FORC diagrams, being able to eliminate one third of unsuccessful samples using the FORC diagram-based pre-screening procedure should provide a significant improvement in efficiency of paleointensity measurement time.
DE: 1503 Archeomagnetism
DE: 1521 Paleointensity
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005