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

Abstract

[1] 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, Fe T , Mg, Ca, and CO 2 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. 87 Sr/ 86 Sr 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.

Seafloor basalt alteration and chemical change in the ultra thinly sedimented South Pacific

“…Away from unit boundaries, replacement of primary mineralogy, veins, halos, and brecciation is less common. The style and intensity of alteration at Site U1365 is similar to uppermost crust from other sites [e.g., Staudigel et al ., ; Alt and Honnorez , ; Alt et al ., ; Laverne et al ., ; Paul et al ., ].…”

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).…”

The redox budget of subduction zones

“…Kerrick (2001) estimated that carbonate formation in MOR hydrothermal systems produces 3.5 x 10 12 moles year −1 of carbonate. A lower estimate is provided by Paul et al (2006), who used measurements from ODP Site 1224, which is less altered than most ocean crust segments. Paul et al (2006) where n is the number of samples in their study), and that gabbro has a an average Fe 3+ /Fe tot of 0.2 ± 0.08 and a total Fe content (wt %) of 5.8 ± 3.6 (n=130).…”