HR: 1330h
AN: T12B-0461    [PDF]
TI: Constraining the Extent and Duration of Low Temperature Alteration in Pacific Ocean Crust
AU: * Paul, H J
EM: hjpaul@uvic.ca
AF: School of Earth and Ocean Sciences, University of Victoria, 3800 Finnerty Rd, Victoria, BC V8W 5C2 Canada
AU: Gillis, K M
EM: kgillis@uvic.ca
AF: School of Earth and Ocean Sciences, University of Victoria, 3800 Finnerty Rd, Victoria, BC V8W 5C2 Canada
AB: The generation, alteration, and subduction of oceanic lithosphere is one of the primary chemical cycles that occur on Earth. The circulation of seawater through ocean crust facilitates the exchange of elements and chemical compounds between the hydrosphere and lithosphere, ultimately having a profound effect on both the chemical composition of seawater and ocean crust. Our understanding of the chemical cycles that result from ocean crustal alteration and subsequent subduction is largely based on core recovered by the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP). Basement holes drilled in the Pacific Ocean basin however, poorly represent the span of crustal ages that exist in fast-spreading Pacific Ocean crust. Thus, several pressing questions concerning the evolution of seawater-rock interactions and the extent of chemical exchange remain unanswered. The 46 Ma igneous basement that was drilled during ODP Leg 200 at Hole 1224F fills a key age gap in the drill core collection that falls between Holes 597C ($\sim$29 Ma) and 843B ($\sim$110 Ma) enabling a better understanding of how ocean crustal alteration proceeds. The secondary mineral assemblages identified in Hole 1224 cores are typical of low temperature alteration, consisting of Fe-oxyhydroxides, saponite, celadonite, carbonate, minor pyrite and quartz, and rare phillipsite. When compared to adjacent fresh rock, haloed rock samples have increases in Si, Fe and Cr and minor increases in Ca, CO$_{2}$, K, Mg, Mn, P, Na and Al. A significant finding thus far is that the extent of primary mineral replacement and quantity of secondary minerals filling voids is surprisingly low; much less than that found in younger crust (e.g. 6.9 Ma, Sites 504, 896). This suggests that crustal age may not be a controlling factor in the extent of crustal alteration. Hole 1224F also provides a critical constraint for understanding the controls on carbonate uptake in the Pacific upper ocean crust through time. Preliminary calculations show that the carbon content of Hole 1224F cores is less than that contained in younger crust, suggesting that age is not a dominant factor in regulating carbon uptake by the ocean crust.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1030 Geochemical cycles (0330)
DE: 1045 Low-temperature geochemistry
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3655 Major element composition
SC: Tectonophysics [T]
MN: 2003 Fall Meeting