HR: 0830h
AN: GP11B-0256    [PDF]
TI: CO$_{2}$ Step-heating of Feldspar Crystals: A New Technique to Derive Paleomagnetic Directional and Paleointensity Data
AU: * Cottrell, R D
EM: rory@earth.rochester.edu
AF: University of Rochester, Department of Earth & Environmental Sciences 227 Hutchison Hall, Rochester, NY 14627 United States
AU: Tarduno, J A
AF: University of Rochester, Department of Earth & Environmental Sciences 227 Hutchison Hall, Rochester, NY 14627 United States
AB: Feldspar grains can contain minute magnetic inclusions (50-300 nm in size) that have been shown to faithfully record the modern magnetic field. Here we explore the possibility of using step heating techniques with a CO$_{2}$ laser to extract direction and paleointensity information from such crystals. Test samples were chosen from lavas recovered at Nintoku Seamount (Ocean Drilling Program Leg 197, Site 1205). Samples from lava units that displayed stable behavior during thermal demagnetization (Tarduno et al., 2003) were chosen for the experiments. Basalt groundmass was carefully etched away from thin sections so that only feldspar grains free of visible inclusions remained. Neither the rock chip used to make the thin section, nor the epoxy needed to hold the chip to the microscope slide required heating during the curing process, reducing the chance of the acquisition of spurious magnetizations. A mark was etched into the glass slide to maintain sample orientation. Slides were trimmed so that a single feldspar grain could be heated with a CO$_{2}$ laser inside a field free space (provided by shields). The feldspar grains examined ranged in size from 1 mm to several mm (all were approximately 1 mm thick). All remanence measurements were made with a 2G Enterprises SQUID magnetometer, with high resolution sensing coils. In initial experiments, an oriented feldspar grain was given an isothermal remanent magnetization (IRM). Step heating was carried out with a CO$_{2}$ laser with wattage calibrated against a similar sample attached to a thermocouple. The demagnetization of the IRM showed a simple decay to the origin in orthogonal vector plots. Preliminary experiments to investigate natural remanent magnetizations held by the Site 1205 feldspar grains show directions similar to those of the whole rock from which they were extracted.
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting