Abstract:
Hydrothermal systems developed within slow- and fast-spreading mid-ocean ridges vary considerably as a function of tectonic and magmatic processes. Variations in the depths and temperatures of crust- seawater interaction can be assessed using temperature-dependent compositions of the stable isotopes lithium and oxygen. Oxygen-, and to a lesser extent lithium-, isotopic compositions of the upper oceanic crust have been well-studied via ophiolites and deep-sea drill cores from slow-, intermediate-, and fast-spreading centers. However, an understanding of the depths and temperatures of alteration throughout an entire section of oceanic crust has been impeded by the difficulty associated with obtaining samples of the lower gabbroic section. Knowing whether lithium- and oxygen-isotopic signatures correlate with depth through the entire oceanic crust is important because these are two commonly-used geochemical indicators of a recycled crustal component. Furthermore, it is not clear whether the different structural, and therefore hydrothermal, regimes would influence such a correlation. For this study, we examine lower crustal gabbros from IODP Expedition 305 Hole U1309D, originating from an oceanic core complex, the Atlantis Massif, Mid-Atlantic Ridge, 30 degrees N. Oceanic core complexes form in slow-spreading mid-ocean ridge environments where the tectonic influence on crustal extension can be manifested as low-angle detachment faults, exposing lower crust at the surface. Samples were selected at 50-meter intervals over the total 1.4 km length of the core. Samples were prepared for petrographic analysis and digested for geochemical analyses using standard HF-HNO (sub 3) acid digestion techniques. Bulk-rock major and trace element chemical analysis is in progress utilizing the ICP-OES at Colby College. Lithium- and oxygen-isotope analyses by ICP-MS are in progress at the University of Maryland and the Carnegie Institution of Washington, respectively. Results from this study will be compared to published lithium- and oxygen-isotope values for lower crustal gabbros examined from ophiolites and other MOR localities.