Late-stage in situ or intersertal crystallization in basalts at Site 1224 resulted in iron-rich differentiates and eventual formation of high-SiO2 immiscible melt that precipitated quartz and sodic plagioclase at temperatures <970°C. In the case of the seafloor basalts of Site 1224, the residual liquids were not mobile; they did not concentrate into veins. Because of degassing, they were probably significantly less hydrous than residual liquids in the gabbro suites. At no stage, for example, did magmatic amphibole crystallize, whereas titanian pargasite is a common, even abundant, mineral in abyssal gabbros (Stakes et al., 1991; Tribuzio et al., 2000; Gillis and Meyer, 2001).
Some titanomagnetite was strongly affected by alteration, actually losing Fe2O3 to circulating hydrothermal fluids, which perhaps contributed to the ubiquitous and widespread infilling of fractures and veins with iron oxhydroxides. This contrasts with the more typical loss of FeO from titanomagnetite in abyssal-tholeiite dikes and gabbros during oxyexsolution in the middle to lower ocean crust. Potassic and iron-rich smectite also precipitated from similar fluids in vein assemblages. Basaltic glass near the top of the section is partly altered to palagonite, which appears to be another consequence of reaction with low-temperature hydrothermal fluids carrying abundant trivalent iron and potassium in solution. The composite alteration assemblage of veins with iron oxyhydroxides and palagonite enriched in iron can be viewed as a consequence of diffuse fluid flux directed upward from deeper and hotter reaction zones in the lower crust, but also outward from and much more slowly than high-temperature vents at the ridge axis, where iron in fluids is generally reduced and sulfides precipitate in chimneys.