HR: 1340h
AN: GP23A-0165    [Abstracts]
TI: Titanomaghemite inversion and associated changes of remanence in oceanic basalts
AU: * Kr\'asa, D
EM: krasa@lmu.de
AF: Department of Earth and Environmental Sciences, University Munich, Theresienstr. 41, Munich, 80333 Germany
AU: Matzka, J
EM: matzka@geophysik.uni-muenchen.de
AF: Department of Earth and Environmental Sciences, University Munich, Theresienstr. 41, Munich, 80333 Germany
AU: Petersen, N
EM: petersen@geophysik.uni-muenchen.de
AF: Department of Earth and Environmental Sciences, University Munich, Theresienstr. 41, Munich, 80333 Germany
AU: Kunzmann, T
EM: kunzmann@min.uni-muenchen.de
AF: Department of Earth and Environmental Sciences, University Munich, Theresienstr. 41, Munich, 80333 Germany
AB: Oceanic basalts older than 10 Ma often show a complex remanence behavior during stepwise thermal demagnetization with maximum unblocking temperatures above 500$^{\circ}$C. At the same time, strong field thermomagnetic measurements reveal Curie temperatures of 300-360$^{\circ}$C for these basalts corresponding to a highly oxidized Ti-rich titanomaghemite (TMgh). In the past, this apparently paradoxical fact has been taken as evidence that the large TMgh grains, although dominating strong field thermomagnetic measurements, are in the MD grain size range and are not the principal remanence carriers. Instead, it has been proposed that small SD sized Ti-poor titanomagnetites are carrying the primary NRM. However, the Ti-rich TMgh phase dominating thermomagnetic curves is unstable and prone to inversion when heated above 300$^{\circ}$C. The result of this inversion is also Ti-poor titanomagnetite. Thus, the high unblocking temperatures might as well be an artifact of this inversion process. In order to resolve this ambiguity we conducted strong field thermomagnetic measurements as well as continuous thermal demagnetization experiments. By comparing the results we can demonstrate that the initial maximum unblocking temperature is well below 400$^{\circ}$C being in good agreement with the initial Curie temperature of the Ti-rich TMgh. Ti-poor titanomagnetite as the remanence carrier is not present during thermal demagnetisation runs up to 400$^{\circ}$C. Only during subsequent heating to higher temperatures causing the inversion of the TMgh, the maximum unblocking temperature is increasing to its final value above 500$^{\circ}$C. Surprisingly, the remanence direction remains constant during this process. This implies that the primary remanence is carried by the oxidized TMgh and is inherited by the Ti-poor titanomagnetite formed by inversion.
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting