The Effects of Alteration on the Natural Remanent Magnetization of Three Ophiolite Complexes: Possible Implications for the Oceanic Crust

Abstract

Magnetic properties are compared for the pillow basalts and sheeted dike complexes for the following three ophiolite suites: Macquarie Island (~27 million years), Troodos Massif, Cyprus (~ middle Cretaceous), and Smartville complex, California (~Jurassic). The magnetic properties of the pillow basalts strongly depend on their degree of alteration. From the least altered, low-temperature- oxidized, upper pillow basalts of Macquarie Island and the pillow basalts of the Troodos Massif (zeolite facies metamorphism) to the most altered pillow basalts of the Smartville complex (greenschist facies metamorphism) the average intensity of the natural remanent magnetization (NRM) decreases from a maximum of 80 × 10-4 to 0.50 × 10-4 gauss; the Koenigsberger ratio (Q) decreases from 6 to 0.3; the contribution due to viscous remanent magnetization (VRM) increases, and the NRM stability with respect to alternating fields decreases. For the most altered pillow basalts of the Smartville complex the NRM is probably predominantly a VRM, acquired during the present polarity epoch. Thermomagnetic analyses for the least altered pillow basalts exhibit the characteristic behavior of low-temperature-oxidized cation deficient titanomagnetites, while the more altered pillow basalts have reversible thermomagnetic curves with magnetite-like Curie points, probably caused in situ by thermally induced unmixing of the low-temperature-oxidized titanomagnetites. The sheeted dike complexes have NRM intensities of about 10×l0-4 gauss and Q values of about 0.8, and most of their magnetic properties do not vary much between the three ophiolite complexes.