Abstract
Abstract Definition of the long-term variation of the geomagnetic virtual dipole moment requires more reliable paleointensity results. Here, we applied a multisample protocol to the study of the 130.5 Ma Ponta Grossa basaltic dikes (southern Brazil) that carry a very stable dual-polarity magnetic component. The magnetic stability of the samples was checked using thermomagnetic curves and by monitoring the magnetic susceptibility evolution through the paleointensity experiments. Twelve sites containing the least alterable samples were chosen for the paleointensity measurements. Although these rocks failed stepwise double-heating experiments, they yielded coherent results in the multisample method for all sites but one. The coherent sites show low to moderate field intensities between 5.7±0.2 and 26.4±0.7 μT (average 13.4±1.9 μT). Virtual dipole moments for these sites range from 1.3±0.04 to 6.0±0.2 × 1022 A m2 (average 2.9±0.5 × 1022 A m2). Our results agree with the tendency for low dipole moments during the Early Cretaceous, immediately prior to the Cretaceous Normal Superchron (CNS). The available paleointensity database shows a strong variability of the field between 80 and 160 Ma. There seems to be no firm evidence for a Mesozoic Dipole Low, but a long-term tendency does emerge from the data with the highest dipole moments occurring at the middle of the CNS.
Highlights
Definition of the long-term variation of the geomagnetic virtual dipole moment requires more reliable paleointensity results
Our results agree with the tendency for low dipole moments during the Early Cretaceous, immediately prior to the Cretaceous Normal Superchron (CNS)
There seems to be no firm evidence for a Mesozoic Dipole Low, but a long-term tendency does emerge from the data with the highest dipole moments occurring at the middle of the CNS
Summary
The Paraná-Etendeka Magmatic Province (PEMP) is one of the largest known continental flood volcanic province (Fig. 1). Paleomagnetic studies were performed by several authors on Serra Geral volcanics and the dikes of Ponta Grossa and Florianopolis (e.g., Ernesto et al, 1990, 1999; Raposo and Ernesto, 1995; Raposo et al, 1998; Alva-Valdivia et al., 2003). All of these studies revealed very stable two-polarity characteristic magnetic components carried by titanomagnetite. All analyzed samples were cut from the same hand-sample
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