INTRODUCTION

A large flux of both heat and seawater occurs through hydrothermal circulation in the igneous crust of midocean ridge flanks, playing an important role in the alteration of the crust (Sclater et al., 1976; Anderson et al., 1977; Alt et al., 1986; Purdy, 1987; Davis et al., 1992; Mottl and Wheat, 1994; Stein and Stein, 19

94). Chemical fluxes from low-temperature alteration of midocean ridge basalts (MORBs) could be important in crust as old as 10 Ma, or as long as significant ridge-flank fluid flow occurs (Hart and Staudigel, 1986; Mottl and Wheat, 1994). Ocean Drilling Program Leg 168 was dedicated to studying ridge-flank hydrothermal processes near the Juan de Fuca Ridge (Davis, Fisher, Firth, et al., 1997).

Off-axis hydrothermal circulation can be broadly divided into "open" and "restricted" systems, where the former are colder and seawater dominated, and the latter are warmer and involve more altered basement fluids (Teagle et al., 1996). Alteration of the shallow crust is dominated initially by low-temperature and high oxygen-fugacity conditions during a 1-m.y. period of open seawater circulation. This may be followed by up to 10 m.y. of alteration resulting from more closed hydrothermal circulation at warmer temperatures (yet still 100°C) and low oxygen fugacity. The mineralogical record of oxidizing alteration includes Mg-rich saponite and K-bearing, Fe³⁺-bearing celadonitic clays combined with complex iron oxyhydroxide (Fe[O,OH]_x) and clay mixtures (iddingsite). The mineralogic record of alteration under reducing conditions includes saponite and pyrite.

Basalts cored during Leg 168 on the eastern flank of the Juan de Fuca Ridge (JdFR) exhibit a variety of low-temperature alteration features, particularly clay minerals, Fe(O,OH)_x, and pyrite. Processes that formed saponite, celadonitic clay, and Fe(O,OH)_x are superimposed in a variety of combinations (see Marescotti et al., Chap. 10, this volume). This paper reports on the chemical composition of a representative sampling of secondary clays produced during the low-temperature hydrothermal alteration of MORB recovered during Leg 168 and demonstrates that the major element and trace element chemistry of saponites and celadonitic clays vary within narrow limits.