HR: 1340h
AN: V43B-1568    [Abstracts]
TI: Recent Alteration of 15-Ma Oceanic Crust, ODP Site 1256, Leg 206
AU: * Glass, J B
EM: jglass@u.washington.edu
AF: Earth and Space Sciences, 310 Condon Hall box 351310 University of Washington, Seattle, WA 98195 United States
AU: Cooper, K M
EM: kmcooper@u.washington.edu
AF: Earth and Space Sciences, 310 Condon Hall box 351310 University of Washington, Seattle, WA 98195 United States
AU: Alt, J C
EM: jalt@umich.edu
AF: Dept. Geological Sciences, University of Michigan, Ann Arbor, MI 48109 United States
AU: Elliott, T
EM: tim.elliott@bristol.ac.uk
AF: Dept Earth Sciences, University of Bristol, Bristol, BS8 1RJ United Kingdom
AU: Teagle, D A
EM: dat@soc.soton.ac.uk
AF: Southampton Oceanography Ctr., Univ Southampton, European Way, SO14 3ZH United Kingdom
AB: Hydrothermal alteration of fresh mid-ocean ridge basalt has been well-documented along the crests of mid-ocean ridges, and fluid circulation and alteration can persist for tens of millions of years off-axis. We measured ²³⁴U-²³⁸U disequilibria in whole-rock and vein samples from core recovered from the upper 500m of basaltic basement at ODP Hole 1256D in the Guatemala Basin to assess the distribution and timing of off-axis alteration and U exchange between seawater and crust. The cored basalts were erupted ~15 Ma at the East Pacific Rise during a period of superfast spreading and have experienced slight to moderate alteration. U in most whole-rock samples is close to radioactive equilibrium (i.e., δ²³⁴U = 0) with some modest ²³⁴U-excesses (δ²³⁴U = 11-15 permil). Vein phyllosilicates are uniformly out of equilibrium and most have ²³⁴U-excesses of 10-20 permil, ranging up to 115 permil. One saponite vein sample shows a significant ²³⁴U deficit (-70 permil). Assuming that U in the altered basalts is a mixture of U from fresh MORB (in radioactive equilibrium) and seawater uranium (δ²³⁴U =146 permil), the interplay between the mass balance of mixing and decay can be explored. For the sample with the highest excess, the maximum age of (single-stage) U exchange with seawater is ~95 ka. If instead the disequilibria reflect ongoing exchange and decay, up to 80% of the uranium in the sample must have been derived from seawater in the end-member case where alteration occurred at present. The observation of ²³⁴U-excesses in most samples suggests that U is being added from or exchanged with seawater, consistent with previous observations of a net addition of U to the crust during alteration. However, the lack of correlation in our preliminary data between (²³⁴U/²³⁸U) and U concentration suggests a process of U exchange with seawater rather than simple bulk addition of U. The few samples with ²³⁴U deficits reflect a more complex history of alteration followed by decay and later partial leaching of U from the altered basalt.
DE: 1021 Composition of the oceanic crust
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 1039 Alteration and weathering processes (3617)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005