No abstract

Basalts from the upper 70 meters of the oceanic crust, at Site 396B of DSDP Leg 46 (about 13 my old) have been subjected to at least two main stages of low temperature alteration: (1) all of the samples studied have undergone a slight oxidation and hydration, often with introduction of calcite and smectites; (2) most of them are locally strongly oxidized and hydrated near cracks and other exposed surfaces, having lost their olivine and gained smectites, Fe-Mn oxides, phillipsite, and calcite; Mg and Si have been lost during this process. The most visible effect of the second phase of alteration, an abrupt change in rock color from gray to brown, suggests sharp chemical gradients. The least oxidized (gray) basalts essentially contain fine skeletal titanomagnetite crystals up to 5 µm, which always display evidences of oxidation to titanomaghemite. Larger euhedral crystals (up to 25 µm) of homogeneous titanomagnetite (unoxidized or only partly oxidized to titanomaghemite) are occasionally clustered around myarolitic voids. In the more oxidized (brown) samples, anhedral relicts of the skeletal phase sometimes remain, but they are often completely broken down. The other opaque minerals in both gray and brown samples are ubiquitous but quantitatively unimportant, and comprise one or more primary sulfide spherules, Cr-spinel, and rare hematite overgrowths on the larger titanomagnetite crystals. Comparisons of whole rock oxidation state (OS = Fe 3+ /Fe 2+ + Fe 3+) for pairs of adjacent gray and brown zone samples, with Curie temperature and saturation magnetization show that (1) Curie t° is rather high in all of the rocks, and slightly higher in the more oxidized zones; and (2) saturation magnetization is much lower in the more oxidized zones. The high Curie t° indicate that titanomagnetite has been converted to titanomaghemite in all of the samples, thus accounting for the slight oxidation (Ox-0.3) of the least altered rocks. This is confirmed by petrographic observation of the opaque phases and suggests that the alteration of titanomagnetite took place more readily than that of the magmatic silicate minerals. The marked decrease of the saturation magnetization in the more oxidized (brown) zones indicates that with the stronger oxidation stage (Ox = 0.6), part of the titanomaghemite has been lost.