Low‐temperature alteration of mesozoic oceanic crust, Ocean Drilling Program Leg 185

Abstract

[1] Basalts drilled from the Mesozoic crust at Ocean Drilling Program (ODP) Sites 801 and 1149 during Leg 185 have been affected by a succession of submarine, variably oxidative alteration processes by cold seawater. The low-temperature alteration resulted in various combinations of secondary minerals both as replacements of igneous phases, and as void and crack fillings. Mg-smectite, calcite, celadonite and Feoxyhydroxide are the dominant secondary minerals. Quartz, chalcedony and phillipsite are always very minor phases that fill open spaces. The rocks generally exhibit concentric haloes, 0.5 to 1 cm in thickness, around gray inner cores and usually adjacent to exposed surfaces. Color zonations result from the abundance of various secondary mineral assemblages present in each zone. The green rocks are dominated by celadonitic phyllosilicates, whereas the dusky red haloes are characterized by abundant iddingsite and Fe-oxyhydroxides replacing the fine clinopyroxene. Finally Mg-smectite ± calcite, are the most common secondary phases in the inner gray cores. The most unusual effects of the low-temperature alteration of the study rocks are the pervasive, pale green to gray green color and the relatively abundant dusky red haloes surrounding inner reddish gray cores. Alteration of Mesozoic crust cored during Leg 185 is comparable with low-temperature alteration of 310 m thick uppermost volcanic section of the 5.9 My old crust of Hole 504B. Except the renewed precipitation of calcite in veins, the aging of oceanic crust does not depend on time probably because it occurs during the first few millions of years after the accretion, and ceases when the crust is sealed off.Components: 13,976 words, 12 figures, 9 tables.

The geochemical composition of the terrestrial surface (without soils) and comparison with the upper continental crust

“…A significant proportion of oceanic dissolved Mg is fixed in the oceanic crust by cation exchange (predominantly with Ca) at mid-ocean ridges or submarine basalts in general (Albarede and Michard 1986;Albarede 1998;Talbi and Honnorez 2003;Paul et al 2006). From there it is removed via subduction of the altered oceanic crust to the mantle (Lee et al 2008).…”

Sr‐Nd isotope systematics in 14–28 Ma low‐temperature altered mid‐ocean ridge basalt from the Australian Antarctic Discordance, Ocean Drilling Program Leg 187

“…Celadonite, a very common secondary mineral that is often described in other low‐temperature alteration studies [e.g., Marescotti et al , 2000], was not identified in Leg 187 samples by standard petrographic methods applied here. Similarly, characteristic dark black halos described elsewhere [e.g., Talbi and Honnorez , 2003] were also not present in our sample collection. Brief petrographic descriptions of Leg 187 samples used in this study are listed in Table 2.…”

“…Numerous studies based on physical properties of basalts suggest that low‐temperature alteration of the oceanic crust is progressive and age‐dependent [e.g., Johnson and Semyan , 1994; Zhou et al , 2001]. On the other hand, some mineralogical [ Talbi and Honnorez , 2003] and isotopic [ Hauff et al , 2003] investigations imply that low‐temperature alteration primarily occurs during the first few million years after crust formation and that only precipitation of carbonates has a major effect on the composition of the crust throughout its history on the seafloor [ Alt and Teagle , 1999].…”

Geochemical Constraints on Potential Biomass Sustained by Subseafloor Water–Rock Interactions

“…Within the oceanic crust aquifer, basaltic rocks react with oxygenated deep-sea water to form secondary minerals, replacing primary phases (e.g., glass, olivine, and metal sulfides) and/or filling fractures and void spaces (Alt et al 1996a, b;Teagle et al 1996;Talbi and Honnorez 2003). This process changes the composition of both the seawater and the oceanic crust (Hart and Staudigel 1986;Alt et al 1996a, b;Staudigel et al 1996;Elderfield et al 1999;Nakamura et al 2007).…”