Low‐temperature oxidation in MORB of titanomagnetite to titanomaghemite: A gradual process with implications for marine magnetic anomaly amplitudes

Abstract

Mid‐ocean ridge basalt (MORB) samples, varying in age from Recent to Jurassic, were selected for electron microscopic and rock magnetic studies. Our observations indicate that the degree of oxidation of titanomagnetite in MORB increases only gradually with sample age. The titanomagnetite in recent MORB (<20,000 years) shows no sign of alteration (z∼0). Quaternary samples near the ridge (<2 Ma) typically have z values of less than 0.35, indicating a low degree of oxidation, whereas samples with ages of tens of millions of years have z values of up to 0.9. Some older samples show lower z values, but the upper envelope of our observations in a z versus age plot can be represented by the function z = p + q log t, where p ≈ 0.38, q ≈ 0.38, and t is in millions of years (for t>100 ka). Both electron microscopic observations and rock magnetic data support the notion that low‐temperature oxidation of titanomagnetite to titanomaghemite in MORB is a gradual process. Moreover, the rate of maghemitization is controlled by many factors on both macroscopic and microscopic scales, including regional oceanic crustal structures, lithological features, grain size, and surrounding matrices. Pillow lavas and the tops of massive flows tend to have higher degrees of oxidation than the interiors of massive flows, owing to higher porosity and permeability of the former. In contrast, interstitial glass can protect fine titanomagnetite grains from alteration. The natural remanent magnetization (NRM) intensity of MORB varies as a function of age, magnetic granulometry, concentration of magnetic materials, and the degree of alteration. Fine‐grained MORB samples typically have higher NRM intensity. The NRM intensity appears to decrease substantially with increasing degree of maghemitization and, hence, with age. The envelope of the smoothed magnetic anomaly amplitudes resembles the change in NRM intensity for MORB samples of the last 30 million years, but the underlying assumption that the highest degree of maghemitization observed in the samples of a given age is entirely responsible for these intensity changes is not supported. Maghemitization of the micrometer‐sized and larger grains is probably only partly responsible, whereas a significant contribution to NRM intensities is inferred from those submicrometer‐sized titanomagnetite grains that remained protected from oxidation by the surrounding matrix of interstitial glass.