HR: 1340h
AN: GP53A-0955    [Abstracts]
TI: Assessing the Effects of Mineral Alteration on Palaeointensity Determinations
AU: * Davies, C J
EM: ceri.davies@liv.ac.uk
AF: University of Liverpool, Department of Earth and Ocean Sciences, Liverpool, L69 7ZE, United Kingdom
AU: Hill, M J
EM: m.hill@liv.ac.uk
AF: University of Liverpool, Department of Earth and Ocean Sciences, Liverpool, L69 7ZE, United Kingdom
AU: Shaw, J
EM: shaw@liv.ac.uk
AF: University of Liverpool, Department of Earth and Ocean Sciences, Liverpool, L69 7ZE, United Kingdom
AU: Prior, D
EM: davep@liv.ac.uk
AF: University of Liverpool, Department of Earth and Ocean Sciences, Liverpool, L69 7ZE, United Kingdom
AB: We have analysed Madagascan basaltic samples of Cretaceous Normal Superchron age. Microwave palaeointensity experiments have been carried out on all samples. Rock magnetic, microscopy, X-ray diffraction and Mossbauer spectroscopy analyses have been carried out to determine the magnetic carriers in the samples. It is important to distinguish between the primary thermal remanent magnetisation, TRM and that of a secondary chemical remanent magnetisation, CRM, acquired at a later date. With palaeointensities only viable if the signature is of primary origin, differentiating these two components is paramount. Thermomagnetic plots have the potential to identify the presence of titanomaghemite (inversion on heating), indicative of a CRM, yet can this be accredited as a method of identifying all secondary magnetisation? Scanning electron microscope, SEM analysis allows us to identify both high temperature ductile alteration; the direct formation of ilmenite trellis, lamellae and zones within the titanomagnetite host, and low temperature brittle alteration; the cracking of maghemitised regions. X-ray diffraction and Mossbauer spectroscopy allow us to quantify this alteration. Should this alteration occur above the Curie temperature, Tc, of the identified sample then the sample is considered suitable for palaeointensity investigation. With the Tc almost linearly dependent on the titanium present in the sample however, this is another variable factor that must be clarified. Drawing from all data it is possible to create a cooling history for the basalts, allowing for remanence identification and the attempted clarification of this history and its relationship with the palaeointensity recorded in the samples.
DE: 1519 Magnetic mineralogy and petrology
DE: 1521 Paleointensity
DE: 1540 Rock and mineral magnetism
DE: 3617 Alteration and weathering processes (1039)
DE: 3620 Mineral and crystal chemistry (1042)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting