The phyllosilicates in Hole 735B range from smectite to chlorite/smectite to chlorite and record varying temperatures and fluid compositions. The low-grade mineral assemblages mainly represent subgreenschist conditions (i.e., zeolite facies and lower-temperature "seafloor weathering") (see review in Alt, 1999).
Conditions were generally reducing, as shown by the presence of pyrite and the trioctahedral phyllosilicates, which contain all Fe as the ferrous ion. Sr and O isotopes indicate generally rock-dominated solution compositions.
Phyllosilicates formed over a range of temperatures but may also reflect variable reaction progress (i.e., variable extent of fluid-rock reaction and release of Al to be incorporated into chlorite or chlorite/smectite) (see discussion in Alt, 1999). Talc and serpentine are stable over a wide range of temperatures but reflect low water/rock ratios and limited reaction of the rock. The lack of smectite below 1300 mbsf suggests alteration at slightly higher temperatures below this depth.
Carbonates formed late from cold seawater solutions. More oxidizing conditions are indicated by the association of iron oxides with carbonates and phyllosilicates in the upper 600 m of the hole and by the local presence of nontronite in this zone. Saponite having low Fe/Fe + Mg ratios could also be related to more oxidizing conditions (Andrews, 1980; see review in Alt, 1999). Macroscopic carbonate veins are restricted to the upper 600 m of the core, which suggests penetration of seawater generally downward related to fracturing and faulting in the uppermost portion of the uplifted crustal block.