Abstract:
Scanning and transmission electron microscopy (STEM) of ocean-floor pillow basalts with ages between 0 and 70 Ma reveals progressive alteration of submicrometer titanomagnetite to phases such as goethite and clays. In contrast, larger titanomagnetite grains (> 1 μm) oxidize to titanomaghemite without apparent change in crystal morphology. Rock magnetic experiments are consistent with a selective removal of the submicrometer grains as the basalts age, and show good correlations between the ranges of grain sizes observed by STEM and those indicated by hysteresis properties. Remanence contributions from the larger (pseudo-single domain and multi-domain) titanium-iron oxides are inferred to decrease only slightly as the ocean floor becomes older, whereas the overall remanence decays with age as the substantial contribution from stable, single-domain titanomagnetite grains diminishes greatly due to their alteration to other phases. Although more work on many more samples is required to verify our conclusions, the current data imply that this alteration is one of the reasons that amplitudes of marine magnetic anomalies diminish with age over time scales of tens of millions of years.
