HR: 14:55h
AN: T23F-06    [Abstracts]
TI: Geophysical Measurements of Age-dependent Alteration of Upper Oceanic Crust
AU: * Jarrard, R D
EM: jarrard@earth.utah.edu
AF: University of Utah, Department of Geology and Geophysics 135 S. 1460 East, Salt Lake City, UT 84112
AB: The hydrologic evolution of oceanic crust causes a corresponding evolution of geophysical properties. Previous studies of global datasets of DSDP and ODP core physical properties and downhole logs indicated that increasing crustal age reduces macroporosity (large-scale porosity such as cracks), matrix density, and intergranular velocity, but it increases large-scale velocity. Doubts remain, however, concerning the age (if any) at which these changes stop, and whether this cessation corresponds to termination of fluid interchange between seawater and the upper crust. I explore these questions by analyzing archive-half cores from the top 40 m of oceanic crust from 22 Pacific and Indian Ocean DSDP/ODP sites, ranging from 0.5 to 167 Ma. Hydration and smectite abundance were estimated using 1514 measurements of visible and near-infrared spectroscopy (VNIS). Potassium content was measured for 213 core sections using gamma ray spectroscopy. Both techniques indicate that crustal alteration increases with increasing crustal age, including significant ongoing alteration beyond at least 50 Ma. Unlike our previous analyses (e.g., matrix density), however, these data cannot distinguish between a linear vs. logarithmic waning of hydrothermal circulation with increasing age. The demonstration of ongoing potassium enrichment in crust beyond 50 Ma indicates that the continuing alteration involves fluxes between seawater and upper oceanic crust, not merely hydrothermal circulation confined within the crust.
DE: 1039 Alteration and weathering processes (3617)
DE: 3035 Midocean ridge processes
DE: 3617 Alteration and weathering processes (1039)
DE: 8135 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8424)
SC: Tectonophysics [T]
MN: Fall Meeting 2005