Low-temperature oxidation in MORE of titanomagnetite to titanomaghemite: Agradual process with implications for marine magnetic anomaly amplitudes

Mid-ocean ridge basalt (MORB) samples, varying in age from Recent to Jurass ic, were selected for electron microscopic and rock magnetic studies. Our o bservations indicate that the degree of oxidation of titanomagnetite in MOR E increases only gradually with sample age. The titanomagnetite in recent M ORE (< 20,000 years) shows no sign of alteration (z <similar to> 0). Quater nary 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 ten s 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 ag e plot can be represented by the function z = p + q log t, where p <approxi mate to> 0.38, q approximate to 0.38, and t is in millions of years (for r > 100 ka). Both electron microscopic observations and rock magnetic data su pport the notion that low-temperature oxidation of titanomagnetite to titan omaghemite in MORE is a gradual process. Moreover, the rate of maghemitizat ion is controlled by many factors on both macroscopic and microscopic scale s, including regional oceanic crustal structures, lithological features, gr ain size, and surrounding matrices. Pillow lavas and the tops of massive fl ows 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 alteratio n. The natural remanent magnetization (NRM) intensity of MORE varies as a f unction of age, magnetic granulometry, concentration of magnetic materials, and the degree of alteration. Fine-grained MORE samples typically have hig her 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 inten sity for MORE samples of the last 30 million years, but the underlying assu mption that the highest degree of maghemitization observed in the samples o f a given age is entirely responsible for these intensity changes is not su pported. Maghemitization of the micrometer-sized and larger grains is proba bly only partly responsible, whereas a significant contribution to NRM inte nsities is inferred from those submicrometer-sized titanomagnetite grains t hat remained protected from oxidation by the surrounding matrix of intersti tial glass.