INTRODUCTION

Pillow basalts, varying in age from around 0.8 to 3.5 Ma, were recovered during Leg 168 from nine sites (Sites 1023-1029 and 1031-1032), drilled across the eastern flank of the Juan de Fuca Ridge (JdFR). These sites are located from about 20 km to roughly 100 km east of the ridge axis and were subdivided into three main transects named the Hydrothermal Transition (HT, Sites 1023-1025), Buried Basement (BB, Sites 1028-1031), and Rough Basement (RB, Sites 1026 and 1027) transects, as described in the Leg 168 Initial Reports (Davis, Fisher, Firth, et al., 1997; Fig. 1).

As a consequence of the flat-lying sedimentary cover that onlaps the eastern flank of the JdFR, the present-day basement temperatures at the top of the volcanic sequence systematically increase from west to east along the transects, from 15°-38°C (HT), to 40°-59°C (BB), to 61°-64°C (RB).

The pillow basalts recovered from the three transects represent an ideal opportunity to study the effects of the low-temperature interaction between basalts and fluids ("normal" and "evolved" seawater) in contact with, or circulating within, the upper section of oceanic igneous crust.

With this study we document the petrography, mineralogy, and mineral paragenesis associated with the low-temperature alteration processes (<100°C) occurring in <3.5-Ma pillow basalts; the recovered massive units (Sites 1025, 1027, and 1032) and massive diabase (Site 1027) are not discussed in this manuscript. We give particular emphasis to the characterization of the transition from the oxidizing alteration style, which is related to open seawater circulation, to the reducing alteration style, which occurred under more restricted circulation (i.e., at lower water/rock ratios) and lower (oxygen fugacity).