Palaeomagnetic investigation of Tertiary lava from Barrington Tops, NSW, Australia, using thermal and microwave techniques

Tertiary lava (53 Ma) from Barrington Tops, New South Wales, Australia has been studied using conventional thermal and microwave techniques. Twenty-seven flows in two sections were sampled and their rock magnetic characteristics determined. On heating the majority of samples exhibited a single Curie temperature at around 200°C, indicating a titanium rich titanomagnetite. Some samples exhibited two magnetic phases and a few a single low titanium titanomagnetite phase. Thermal demagnetisation yielded a mean direction of D=189.5, I=63.6, α95=4.3, which corroborates the previous findings of Wellman et al. [Geophys. J. R. Astron. Soc. 18 (1969) 371–395]. A pilot conventional Thellier palaeointensity analysis was unsuccessful mainly due to the samples being highly susceptible to thermo-chemical alteration. Far greater success was achieved (58%) using the microwave palaeointensity technique, where heating of the bulk sample and hence alteration is vastly reduced. Palaeointensity estimates range from 3 to 28 μT (mean 11±5 μT), which, assuming that the remanence is a primary thermal remanent magnetisation, indicates a low field intensity (a fifth of the present day value) in the early Tertiary. This study demonstrates the applicability of the microwave palaeointensity technique to ancient lava.