HR: 0800h
AN: GP11C-0842    [Abstracts]
TI: Variations in magnetic properties over a thin lava flow profile: implications for palaeo-direction and palaeo-intensity recordings
AU: * Verard, C
EM: chris@geophysik.uni-muenchen.de
AF: Department of Geosciences, University of Munich, Theresienstr. 41, Muenchen, 80333 Germany
AU: Winklhofer, M
EM: michael@geophysik.uni-muenchen.de
AF: Department of Geosciences, University of Munich, Theresienstr. 41, Muenchen, 80333 Germany
AU: Leonhardt, R
EM: leon@geophysik.uni-muenchen.de
AF: Department of Geosciences, University of Munich, Theresienstr. 41, Muenchen, 80333 Germany
AU: Fabian, K
EM: karl.fabian@uni-bremen.de
AF: Department of Geosciences, University of Bremen, Postfach 330440, Bremen, 28334 Germany
AB: Reversals and excursions of the Earth's magnetic field have previously been reported to be recorded in an 14.1 Ma old lava sequence of the mid-Miocene shield basalts from Gran Canaria (Canary Islands). Earlier palaeomagnetic and rock magnetic investigations on standard cylindrical inch samples showed that the rocks are excellent recorders of the past Earth's magnetic field. We have resampled parts of the sequence by taking complete sections of thin lava flows ($ <1.5$ meters) in order to determine the influence of variations in rock magnetic properties within a flow on palaeo-direction and in particular on palaeo-intensity determinations. Preliminary results obtained on mini-cores drilled every 0.5 cm in a 33 cm coherent lava section indicate significant variations in saturation magnetisation and coercivity over the profile. From bottom to top, M${}_{\rm s}$ and M${}_{\rm rs}$ increase moderately from $\sim 0.25$~Am${}^2$/kg and $\sim 6.0 \cdot 10^{-2}$ Am${}^2$/kg to $\sim 0.46$~Am${}^2$/kg and $\sim 8.8\cdot 10^{-2}$ Am${}^2$/kg, respectively from 0 to 14 cm over the profile. Between 14 and 16 cm, values are nearly tripled. M${}_{\rm s}$ and M${}_{\rm rs}$ then reach $\sim 1.3$~Am${}^2$/kg and $ \sim 0.25$~Am${}^2$/kg, before gently decreasing to $ \sim 1.0$~Am${}^2$/kg and 0.1~Am${}^2$/kg respectively from 16 to 33 cm. Variations in H${}_{\rm c}$ and H${}_{\rm cr}$ also exist but appear to be less pronounced, and the general trend is a decrease from bottom to top. Furthermore, the H${}_{\rm c}$/H${}_{\rm cr}$ ratio increases from $\sim$2.5 to $\sim$3.5 whereas M${}_{\rm rs}$/M${}_{\rm s}$ decreases from $\sim$2.0 to $\sim$1.0 up in the profile. The bulk mineralogical composition changes also notably, resulting in a variation of Curie temperatures. This is confirmed by similar variations in unblocking temperature spectra measured both by standard palaeomagnetic thermal demagnetisation and by continuous thermal demagnetisation. In particular, alteration processes appear to be more prominent near the base of the flow. The influence of such changes on palaeo-direction and palaeo-intensity determinations are finally addressed in this poster.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1513 Geomagnetic excursions
DE: 1519 Magnetic mineralogy and petrology
DE: 1521 Paleointensity
DE: 1530 Rapid time variations
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting