Uranium-series geochemistry of altered ocean crust at ODP/IODP site 1256

Hydrothermal circulation at mid-ocean ridges is an important process in chemical exchange between the lithosphere and thee oceans, but the timescales of ridge flank alteration processes are poorly constrained. To delimit the timing and extent of off-axis alteration in superfast-spreading crust, I measured 234U-238U disequilibria and U concentrations in 15 Ma crust drilled at ODP/IODP Site 1256D. Water-rock interaction in the ocean crust imparts elevated 234U- 238U to altered crust, and this 234U-238U disequilibria signature can constrain timescales of mineral precipitation or fluid-rock interactions that have occurred within the past ∼1.25 Myr. Disequilibria in whole rocks at 1256D shifts from modest 234U excesses in volcanic units to delta234U greater than 50‰ deeper in the sheeted dike section. The pattern of disequilibria is not related to the macroscopic alteration; altered rocks do not have elevated 234U excess relative to their fresh complements. The concentration of U decreases with depth, and is negatively correlated to the 234 U excess. The profile of 87Sr/86Sr with depth reflects circulation of a seawater derived hydrothermal fluid in the sheeted dikes that may also be independent of alteration mineralogy. Because 234U excess is not related to the alteration mineralogy or to the delta 18O profile, the fluids imparting 234U disequilibria likely follow different pathways than the high temperature fluids at the ridge axis. The higher U concentrations in the volcanic section are related to oxidizing low-temperature interaction with seawater that results in net addition of U to the crust, but 234U-238U analyses suggest that these processes are no longer active in the upper part of the section. The pattern of 234U excess and 87Sr/ 86Sr indicates that there is recent fluid circulation in the sheeted dike section of 1256D, which is consistent with hydrologic models of hydrothermal discharge in this area of the Cocos plate.