The relationship of cooling history and stability in two suites of igneous rocks

Abstract

In field sampling for paleomagnetic study, one always hopes to choose the most stable samples available in order to obtain reliable information about the earth's ancient magnetic field. If the remanence is unstable, of course, the direction and intensity of the magnetization may have changed through time, thus diminishing the value of the field effort, its time and expense. It was our hope in this study to find reliable methods to choose samples in the field by studying the position of a sample in the sill or flow versus its cooling history, and then the relationship of cooling history to stability. AC demagnetization was used to evaluate the stability of the samples. Petrographic examination under reflected light determined the composition and oxidation state of the opaque minerals. Recent work has indicated that oxidation states and stability are closely related (Watkins and Haggerty, 1967, Ade-Hall et al., 1968). We hoped to extend the generality of that relationship and examine its connection to cooling history.