Variation of magnetic properties in a single deep-sea pillow basalt

Abstract

A single 2.5 cm diameter core of 30 cm length has been taken normal to the cooling surface of a single submarine alkalic pillow basalt, recovered from a seamount in the Scotia Sea. The core traverses from a glassy rim, through a variolitic zone, to an aphanitic interior, and back into a mixed glassy/variolitic segment. The silicate petrology and textures, and the magnetic and opaque mineralogical parameters have been determined at intervals of 2.2 cm along the core. Considerable systematic variations in the magnetic properties are due to quenching rate differences, except towards the lower-center of the pillow, where an optically-undetectable slight increase in oxidation is expressed by change in magnetic stability, Curie point, and sulfide content. It is suggested that very low oxidation state titanomagnetites, abundant fine sulfides, and low Curie points may characterize quenched submarine basalts, probably because cooling rates exceed oxidation rates. Because of the considerable variation in magnetic parameters (which has also been detected in seven tholeiitic pillow basalts) it is suspected that representative data may not be readily obtained from submarine extrusives. Averaging of results from several specimens per sample, or establishment of a specimen rejection criterion is required. It is suggested that restriction of analyses to the variolitic segments of pillow basalts may minimize the effect of within-sample variations, when between-sample comparisons are made.