Drilling a complete deep crustal section has been a primary yet elusive goal since the inception of scientific ocean drilling. In situ ocean crustal sections would contribute enormously to our understanding of the formation and subsequent evolution of the ocean crust, in particular the interplay between magmatic, hydrothermal, and tectonic processes. Ocean Drilling Program (ODP) Leg 206 was the first of a multileg project to drill an in situ crustal section that penetrated the gabbroic rocks of the Cocos plate (6°44.2´N, 91°56.1´W), which formed ~15 m.y. ago on the East Pacific Rise during a period of superfast spreading (>200 mm/yr) (Wilson, Teagle, Acton, et al., 2003).
During Leg 206, the upper 500 m of basement was cored in Holes 1256C and 1256D with moderate to high recovery rates. The igneous rocks recovered are predominantly thin (10 cm to 3 m) basalt flows separated by chilled margins. There are also several massive flows (>3 m thick), although their abundance decreases with depth in Hole 1256D, as well as minor pillow basalts, hyaloclastites, and rare dikes. The lavas have been slightly (<10%) altered by low-temperature hydrothermal fluids, which resulted in pervasive dark gray background alteration and precipitation of saponite, pyrite, silica, celadonite, and calcium carbonate veins.
Calcium carbonate is a common secondary precipitate that fills veins and cements breccias in the upper basement as the ocean crust spreads away from the ridge axis. The carbonate veins record the compositions of the fluids from which they precipitate. Coggon et al. (2004) demonstrated how calcium carbonate veins that precipitated in the upper basement accurately predict the chemistry of ridge flank hydrothermal fluids, with increasing temperature, across the eastern flank of the Juan de Fuca Ridge. Here we present a geochemical analysis of the CaCO3 recovered from cores. The compositions of ridge flank fluids within superfast spreading crust will be determined from these data, following the approach of Hart et al. (1994), Yatabe et al. (2000), and Coggon et al. (2004).
Calcium carbonate was identified in only 2.3% of the 5839 veins recovered from Site 1256 basement cores and is concentrated in two intervals, 250–350 and 450–550 meters below seafloor (mbsf) (Wilson, Teagle, Acton, et al., 2003). CaCO3 typically occurs as a minor constituent of composite veins, forming the cores of 0.5- to 1-mm-wide saponite + CaCO3 ± pyrite veins, or 1-mm-wide rims to wider (2–4 mm) quartz veins. Occasionally, CaCO3 is the principal component of 1-mm-wide veins.
1Coggon, R.M., Teagle, D.A.H., Cooper, M.J., Hayes, T.E.F., and Green, D.R.H., 2006. Data report: compositions of calcium carbonate veins from superfast spreading rate crust, ODP Leg 206. In Teagle, D.A.H., Wilson, D.S., Acton, G.D., and Vanko, D.A. (Eds.), Proc. ODP, Sci. Results, 206: College Station, TX (Ocean Drilling Program), 1–6. doi:10.2973/odp.proc.sr.206.002.2006
2School of Ocean and Earth Science, National Oceanography Centre, Southampton, University of Southampton, European Way, Southampton SO14 3ZH, United Kingdom. Correspondence author: rmc01@noc.soton.ac.uk
3Challenger Division for Seafloor Processes, National Oceanography Centre, Southampton, European Way, Southampton SO14 3ZH, United Kingdom.
Initial receipt: 29 April 2005
Acceptance: 9 December 2005
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publication: 16 October 2006
Ms 206SR-002