Low-Temperature Alteration of Very Young Basalts from ODP Hole 648B: Serocki Volcano, Mid-Atlantic Ridge

Abstract

Basalts in Hole 648B, located in the rift valley of the Mid-Atlantic Ridge at 23°N in crust estimated to be less than 100,000 years old, are mainly fresh, but small amounts of secondary phases are found on fracture surfaces and in alteration halos within the rocks. The halos are defined by dark bands 1-4 mm thick that have developed parallel to fracture surfaces or pillow margins and which in some cases have migrated some centimeters into the rock. The dark bands are the principal locus of secondary phases. The secondary phases are olive-green and yellow protoceladonites, of composition and structure intermediate between celadonite and iron-rich saponite, red (Mn-poor) to opaque (Mn-rich) iron oxyhydroxides, mixtures of protoceladonite and iron oxyhydroxide, and rare manganese oxides. These phases occur mainly as linings or fillings of open spaces in the basalt within the dark bands. Sulfides and intersertal glass are the only primary phases that can be seen to have been altered. Where dark bands have migrated into the rock, the rock behind the advancing band is almost devoid of secondary phases, implying redissolution. The potassium and magnesium in the secondary phases could have been supplied from ambient seawater. The aluminum in the protoceladonites must have been derived from local reaction of intergranular glass. The source of iron and silica could have been intergranular glass or low temperature mineralizing solutions of the type responsible for the formation of deposits of manganese oxides and iron oxyhydroxides and silicates on the seafioor.

Low-Temperature Alteration of Basalts from the Hawaiian Arch, Leg 136

“…Dark alteration halos containing celadonite or celadonite-smectite occur in otherwise unaltered very young (<l Ma) seafloor rocks (Humphris et al, 1980;Vivier, 1983: Adamson andRichards, 1990). The similarity of the mineralogy and occurrence of these alteration zones to the celadonite-bearing dark halos around veins in Hole 843B suggests that the latter similarly formed very early during the history of the site.…”

Ridge-Flank Alteration of Upper Ocean Crust in the Eastern Pacific: Synthesis of Results for Volcanic Rocks of Holes 504B and 896A

“…These fluids could be derived from local reactions (breakdown of glass and titanomagnetite), but could also be derived from deeper in the volcanic pile, or could even be distal, low-temperature hydrothermal fluids mixed with seawater. Given the early appearance of such black halos (< 10,000 yr; Adamson and Richards, 1990), it is also possible that the Fe-enriched solutions may result from reactions during initial cooling of hot lavas, or from water-rock interactions during diking events. Whatever the process, such alteration features form in the uppermost volcanic section in young crust, and are typical of DSDP/ODP drilled volcanic sections in MORB crust (e.g., Alt, 1995a).…”

Alteration of Upper Ocean Crust in a Ridge-Flank Hydrothermal Upflow Zone: Mineral, Chemical, and Isotopic Constraints from Hole 896A

“…Basalts from oceanic crust younger than Hole 896A provide some constraints on the absolute timing of seafloor alteration. Celadonite is the only secondary mineral present in the very youngest basalts (<IO ka;Adamson and Richards, 1990), celadonite + Fe-oxyhydroxides are present in alteration halos in rocks less than 1 Ma, and saponite is present with the other minerals in rocks >2 Ma (Humphris et al, 1980;Laverne and Vivier, 1983). All of the mineral zones, including late zeolites and carbonates, are present in 3.2-m.y.-old rocks (Andrews, 1977), suggesting that most alteration of the crust may be complete by about 3 Ma, although carbonate veins may continue to form for several millions of years (e.g., Hart et al, 1994).…”