ALTERATION AND DISSOLUTION OF FINE-GRAINED MAGNETITE AND ITS EFFECTS ON MAGNETIZATION OF THE OCEAN-FLOOR

Scanning and transmission electron microscopy (STEM) of ocean-floor pi llow basalts with ages between 0 and 70 Ma reveals progressive alterat ion of submicrometer titanomagnetite to phases such as goethite and cl ays. In contrast, lar er titanomagnetite grains (> 1 mu m) oxidize to titanomaghemite without apparent change in crystal morphology, Rock ma gnetic experiments are consistent with a selective removal of the subm icrometer grains as the basalts age, and show good correlations betwee n the ranges of grain sizes observed by STEM and those indicated by hy steresis properties. Remanence contributions from the larger (pseudo-s ingle domain and multi-domain) titanium-iron oxides are inferred to de crease only slightly as the ocean floor becomes older, whereas the ove rall remanence decays with age as the substantial contribution from st able, single-domain titanomagnetite grains diminishes greatly due to t heir alteration to other phases. Although more work on many more sampl es is required to verify our conclusions, the current data imply that this alteration is one of the reasons that amplitudes of marine magnet ic anomalies diminish with age over time scales of tens of millions of years. (C) 1997 Elsevier Science B.V.