A detailed rock magnetic and opaque mineralogy study of the basalts from the Nazca Plate

Abstract

Summary. The penetration of igneous basement in the Nazca Plate during DSDP Legs 16 and 34 provided samples of both fine‐grained pillow‐basalt and coarse‐grained massive flow units. The magnetic mineral in these basalt samples is initially a titanomagnetite (Fe3—xTnxO4) with a narrow range of composition of x= 0.62 ± 0.05. Subsequent to formation, the titanomagnetite grains are generally subjected to low temperature oxidation to titanomaghemite with a corresponding rise in Curie temperature from the initial values of 120–150°C up to a maximum of 400°C. Both grain size and low‐temperature oxidation state play important, and interrelated, roles in controlling the intensity and stability of magnetic remanence and other magnetic properties. Overall grain size can, in some cases, be related to oxidation state since some sections of the relatively impermeable massive flows can remain unoxidized for as long as 40 Myr while pillow basalts are extensively oxidized within 1/2 Myr. Low‐temperature alteration in turn effects magnetic grain size since oxidation and subsequent Fe cation migration results in grain subdivision by the formation of shrinkage cracks. A five‐stage sequence of the microscopic changes that are associated with progressive low‐temperature oxidation is proposed and illustrated with photomicrographs from these basalt samples. A hierarchy in the intensity of magnetic remanence may exist with unoxidized pillow basalts having a much higher intensity and oxidized pillow basalts having a much lower intensity than the massive flow units. While pillow basalts are relatively immune to the addition of secondary components of magnetization, the coarse‐grained massive flows readily acquire components of viscous remanence. Although they oxidize much more slowly than pillows, when oxidation does take place, components of chemical remanence can be acquired by the multi‐domained grains in the massive flow units. Copyright © 1978, Wiley Blackwell. All rights reserved