HR: 0800h
AN: T31A-0483    [Abstracts]
TI: Endeavour basalt geology and petrology
AU: * Gill, J B
EM: jgill@pmc.ucsc.edu
AF: Earth Sciences Dept., University of California Santa Cruz, Santa Cruz, CA 95064 United States
AU: Stakes, J
EM: jkela@pmc.ucsc.edu
AF: Earth Sciences Dept., University of California Santa Cruz, Santa Cruz, CA 95064 United States
AU: Ramos, F
EM: ramos@Geology.cwu.EDU
AF: Dept. Geological Sciences, Central Washington University, Ellensburg, WA 98926 United States
AU: Michael, P
EM: pjm@utulsa.edu
AF: Dept. Geosciences, University of Tulsa, Tulsa, OK 74104
AU: Stakes, D
EM: debra@mbari.org
AF: MBARI, 7780 Sandholdt Rd., Moss Landing, CA 94039 United States
AB: We report major and trace element and isotope data from 250 basalt samples recently collected by submersible from the axial valley and flanks of the Endeavour segment of the Juan de Fuca Ridge. Off-axis volcanism is abundant on both flanks which are mirror images of one another geologically. Axial valley walls up to 1 km off axis appear to be steps of in tact but variably fractured sheet, lobate, and hackly lava flows similar to the youngest lavas seen in collapse features in the axis. Coverage by pillow terrane increases with distance off axis and coverage becomes complete after 1 km. The similarity of the two flanks suggests that the currently asymmetric axial magma chamber (van Ark et al., 2004) may be shorter-lived than the off-axis volcanism. MgO contents range from 6.0-8.5% and generally are lower on the flanks consistent with consistently cooler chamber edges there. La/Yb ratios vary 3-fold within 100 m in the axial valley, with normalized La/Sm = 0.8-2.5 in contrast to constant Sr and Nd isotopes. However, Th/U and 230Th/232Th ratios vary only slightly in the axial valley, which may enable dating of off-axis samples. H2O/Ce is less than 170, typical of values throughout much of the Pacific. Variations in depth and degree of melting, and in source composition, are implied. At times, these heterogeneities escaped homogenization in axial magma chambers. Cl concentrations and Cl/K ratios are surprisingly low considering the active hydrothermal systems in close proximity and the potential for brine incorporation into the magma chamber.
DE: 3619 Magma genesis and partial melting (1037)
DE: 8178 Tectonics and magmatism
DE: 8416 Mid-oceanic ridge processes (1032, 3614)
SC: Tectonophysics [T]
MN: Fall Meeting 2005