Six holes were drilled at Site 1224, but only four recovered volcanic basement (Fig. F1): Hole 1224A (total depth [TD] = 32.2 meters below seafloor (mbsf); recovery = 5.2%), 1224D (TD = 64.7 mbsf; recovery = 46.7%), 1224E (TD = 36.7 mbsf; recovery = 51.9%), and 1224F (TD = 174.5 mbsf; recovery = 25.7%).
The basement consists of two massive basaltic flows overlying thinner flows and pillow fragments. In general, the cooling unit boundaries of pillows and flows are distinguished by the presence of glass or reductions in grain size. The basalts vary texturally from aphyric to very slightly porphyritic, with very rare plagioclase and clinopyroxene phenocrysts (<3 vol%) occurring in a few samples. The rarity of phenocrysts is typical of basalts from the East Pacific Rise (EPR) and contrast with basalts from more slowly spreading ridges (Bryan, 1983; Hékinian and Morel, 1980; Natland, 1980, 1991; Pan and Batiza, 2002); it has been attributed to the filtering action of the cumulus net of minerals in the column of lower crust that lies directly beneath the steady-state axial magma lens beneath the active portion of the EPR (Natland and Dick, 1996). In basalts from Site 1224, the grain size of groundmass phases is variable, depending on the distance from cooling unit boundaries, but it is generally very fine (<1 mm) except in the coarser interior portions of the two thick flows, which are fine to medium grained (~2 mm).
Except for the innermost portion of the massive flows, most of the rocks cored have been altered. Secondary minerals are mostly clays, but Fe-Ti oxyhydroxides, vein sulfides, and carbonates also occur, mostly along cracks and in small cavities. Some basalts experienced marked oxidative alteration that produced striking alteration halos adjacent to fractures (Paul et al., 2006). Nevertheless, Paul et al. (2006) emphasize that Site 1224 basalts are surprisingly unaltered despite their age, perhaps because the igneous basement is capped by massive flows (Fig. F1) that inhibited seawater-basalt interaction.