Electron Microscopy of MORB: Linking Marine Magnetic Anomalies and Rock Magnetism

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 (z) of titanomagnetite in MORB increases only gradually with sample age as the following z-age pairs show: (z about 0; less than 20ka), (z less than 0.4; 2 Ma or less), (z up to 0.9; tens of millions of years). Large grains (up to tens of micrometers in diameter, called Group A grains) show minor, but systematic changes in Ti-content from core to the rim of pillows. In contrast, abundant interstitial glass contains submicron magnetite (Group B), which can have large gradations in Ti-content and which can remain protected by the glassy matrix 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 of samples with Group A titanomagnetite grains appears to decrease substantially with increasing degree of maghemitization and, hence, with age. This decrease resembles the envelope of the smoothed magnetic anomaly amplitudes for the last 30 million years, but predicts even lower amplitudes for older anomalies than observed. Maghemitization of the micrometer-sized and larger grains is probably only partly responsible, whereas a significant and more or less permanent contribution to NRM intensities is inferred from those single-domain Group B titanomagnetite grains that did not become oxidized.