Self-reversed magnetization carried by titanomaghemite in oceanic basalts

Abstract

Self-reversal, the remarkable ability of some rocks to acquire a magnetization antiparallel to the Earth's magnetic field, was of great interest in the 1950s and 1960s, but it was subsequently found to be rare, limited to a few hemoilmenite-bearing igneous rocks. Here we show through thermal demagnetization experiments that pillow basalts from the northwestern Pacific Ocean lacking hemoilmenite contain two nearly antiparallel components of natural remanent magnetization. Partial thermoremanent magnetization experiments indicate that the component isolated at lower temperatures is a self-reversal of the primary magnetization. The self-reversal is carried by titanomaghemite formed by low temperature oxidation. Because maghemitization of oceanic crust is ubiquitous, self-reversals may be an important factor contributing to the decrease with age in marine magnetic anomaly amplitudes observed for crust formed during the last 30 million years.