ODP Site 1224: A missing link in the investigation of seafloor weathering

Abstract

A section of Eocene (46 Ma) upper oceanic crust was recovered at Ocean Drilling Program (ODP) Site 1224 in the northeast Pacific basin. The secondary mineralogy and geochemistry of altered basalts and isotopic composition of Ca‐carbonate were studied to determine the extent and nature of alteration at this site. Most basalts are <5% altered to secondary mineral assemblages of Fe‐oxyhydroxides, celadonite, saponite, Ca‐carbonate, trace pyrite, and rare phillipsite and quartz. Alteration is concentrated within brown and dark gray haloes, which formed adjacent to veins and fractures. Haloes comprise 10% of the core (9% dark gray; 1% brown); veins comprise <1% of the core, with an average of 18 veins per meter of recovered cores. Calculated chemical fluxes, which account for groundmass alteration and the composition and abundance of secondary minerals associated with veins, vesicles, and haloes, indicate significant additions of Si, FeT, Mg, Ca, and CO2 and minor additions of Al, Mn, and Na to the crust. The magnitude of these fluxes is low relative to other drilled volcanic sections such as Sites 504, 896, 417, and 418. 87Sr/86Sr ratios (0.70776 to 0.70824) and trace element ratios (Mg/Ca) of calcite imply that Ca‐carbonate formed within 20 Ma of crustal formation from relatively unaltered seawater. Oxygen isotope data yield Ca‐carbonate formation temperatures of 4 to 11°C. The relative freshness of the Site 1224 lavas is attributed to the capping of the volcanic section by massive flows.