HR: 1330h
AN: B12A-0751    [PDF]
TI: Results of New High-Resolution Geological Mapping and Geochemistry of the Endeavour Segment, Juan de Fuca Ridge
AU: * Woodcock, J P
EM: jason@mbari.org
AF: MBARI, 7700 Sandholt Road, Moss Landing, CA 95039-9644 United States
AU: * Woodcock, J P
EM: jason@mbari.org
AF: University of California, Santa Cruz, Department of Earth Sciences, Santa Cruz, CA 95060 United States
AU: Stakes, D S
EM: debra@mbari.org
AF: MBARI, 7700 Sandholt Road, Moss Landing, CA 95039-9644 United States
AU: Gill, J
EM: jgill@es.ucsc.edu
AF: University of California, Santa Cruz, Department of Earth Sciences, Santa Cruz, CA 95060 United States
AU: Kela, J M
EM: keje@mbari.org
AF: MBARI, 7700 Sandholt Road, Moss Landing, CA 95039-9644 United States
AB: Major and trace element concentrations, petrography, and volcanic stratigraphy of 40 mid-ocean ridge basalts (MORB) collected by ROV along the Endeavour Axial Ridge Volcano (EARV) have been analyzed to determine geochemical variations as a function of proximity to the spreading axis and potential sites of off-axis volcanism. The EARV ISS site is an 18 km long, 6 km wide pair of axial highs separated by a 1 km wide, ~200 m deep valley. At a half-spreading rate of 30 mm/yr, it is $<$50,000 years old. Lavas are encompassed by numerous fissures on the valley floor, including en echelon grabens between Mothra and Main Field. Geochemical and structural characteristics of this southern segment (between Mothra and Main Field) can best be explained by a series of axis-parallel dike injections with little crustal hydridization of magmas. This may be a result of the intensity of crustal fracturing and robust hydrothermal circulation. Large-offset faults uplift relatively intact terraces and define the valley walls. About 80% of the lavas have $<$2% plagioclase (An70-90) and olivine (Fo85) phenocrysts. More porphyritic lavas are restricted to one near-axis hornito that sits astride the western boundary fault. Most lavas are moderately differentiated (MgO=7-8 wt.%) with geochemical variations consistent with modest ol+pl+sp fractionation. Incompatible element ratios vary by a factor $>$1.6 within scores of meters in the rift valley (e.g., Nb/Zr (0.09-0.14), La/Yb (2.9-4.2), and K2O/TiO2 (0.20-0.32)) and usually are higher (more E-MORB) than off the EARV. This indicates that the AMC may be episodic, and that off-axis magmas bypass it altogether. Variations in enrichment seem more related to source heterogeneity than degree of melting.
DE: 3035 Midocean ridge processes
DE: 3045 Seafloor morphology and bottom photography
SC: Biogeosciences [B]
MN: 2003 Fall Meeting