Secondary alteration with variable intensity affects pillow basalts from all nine sites. Although local heterogeneities are common in every site, even at the scale of the individual sample, the alteration intensity systematically increases from the younger HT sites to the older RB sites. This general trend is clearly evidenced by alteration indexes such as the LOI value, the FeO/FeOt ratio, and the modal abundance of secondary minerals (Fig. 3).
The LOI (Fig. 3A) increases progressively from about 0.5 to 2 wt% with distance from the ridge axis, although local negative anomalies are evident in pillow basalts from Sites 1025 and subordinately 1029. Nevertheless, these anomalies are related to bulk rock composition, because the pillows from these sites are highly fractionated ferrobasalts and, consequently, the LOI value is strongly influenced by the oxidation of ferrous iron.
The FeO/FeOt value decreases progressively from 0.78 at Site 1023 to 0.61 at Site 1027 (Fig. 3B), mirroring the LOI pattern (Fig. 3A, Fig. 3B). In this context Site 1028 basalts show a very low FeO/FeOt value and a weak positive anomaly of LOI (Fig. 3A, Fig. 3B), suggesting a higher degree of alteration in comparison to the basalts of the other BB sites.
The total amount of alteration, determined by point counting the secondary minerals in thin section, varies from 0% at Site 1023 to 27% at Site 1027, with average values respectively varying from 0.2% to 12.73% (Fig. 3C). This distribution confirms the general increase of alteration with the distance from the ridge axis and the anomalous degree of alteration of pillow basalts from Site 1028.
Other alteration indicators such as the degree of vesicle fillings and the degree of olivine and glass alteration agree well with the indexes discussed above.