A Detailed Opaque Petrological and Magnetic Investigation of a Single Tertiary Lava Flow from Skye, Scotland?III.

A selection of samples representing extremes of magnetic and opaque petrological properties represented in a single lava has been used to investigate the possibilities of obtaining the intensity of the ambient field (FANC) at the time of extrusion of the flow. Two methods have been used to obtain values of FANC; the first using a version of Thellier's method of comparison of partial natural remanences (PNRMs) and partial thermoremanences for several temperature intervals and the second using Van Zijl's method of comparison of progressive a.c. demagnetization of the NRM and of a laboratory TRM. Initial assessment of the results was based on the similarity of form of natural and laboratory weak field remanence demagnetization curves together with the internal consistency of apparent FANC values. Using these criteria, results from a number of samples give acceptable values for FANC. While mean values for these samples from the two methods are not significantly different, the a.c. method gives the highest internal consistency; the RMS error of the mean value being ±3 per cent against 8 per cent for the PNRM-PTRM method. The overall mean value for FANC was 0·38 Oe corresponding to a VDM (virtual dipole moment) of 7·2 × 1025 c.g.s. units. Clear relationships were found to exist between the strong field magnetic and opaque petrological properties on the one hand, and their suitability for FANC measurement on the other. Suitable samples were characterized by reversible strong field heating curves with single high Curie points, high titanomagnetite deuteric oxidation (magnetite oxidation numbers of 3·3 to 4·4, the highest value found in the flow) and the absence of titanomagnetite granulation. Unsuitable samples were characterized by double Curie points, magnetite oxidation numbers of less than 3·3 and, in four of five samples, titanomagnetite granulation.