Paleointensity of the Earth's magnetic field in Early Cretaceous time: The Paraná Basalt, Brazil

Abstract

Paleointensity experiments were carried out with the Coe and Thellier methods on Early Cretaceous basalt (133 My old) from the Paraná basin in southern Brazil. Paleointensity estimates could be obtained from only six lava flows: three of normal and three of reversed polarity, of the 71 sampled. Moreover, the quality of the determinations is fairly low, with q ratio usually less than 10, mainly as a result of chemical and/or physical changes of the magnetic oxides during heating. This study strongly underscores the difficulty of finding natural rocks suitable for paleointensity experiments even so the directional results previously obtained were good. The most reliable result give an intensity of 28±1 μT, while the others range from 20.8 μT to 37.7 μT. The corresponding virtual dipole moments (VDMs) range from 4.7×10 22 to 7.9×10 22 Am 2 . This dispersion is attributed mainly to inaccurate estimates of the ancient field rather than to secular variation. Although the number of cooling units for which paleointensity estimates could be obtained is limited, the results nevertheless indicate that the early Cretaceous VDM was significantly lower than that for the recent field but greater than has been reported for this portion of the Mesozoic Dipole Low period.