HR: 1330h
AN: V13B-09    [Abstracts]
TI: Geochemical Heterogeneity Within Lava Flows From 9°25'-9°51'N EPR
AU: * Hinds, J S
EM: jshinds@ufl.edu
AF: Department of Geological Sciences, University of Florida, Gainesville, FL 32611 United States
AU: Perfit, M R
EM: perfit@geology.ufl.edu
AF: Department of Geological Sciences, University of Florida, Gainesville, FL 32611 United States
AU: Soule, A
EM: ssoule@whoi.edu
AF: Geology and Geophysics Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Fornari, D J
EM: dfornari@whoi.edu
AF: Geology and Geophysics Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AB: We evaluated the degree of geochemical heterogeneity and morphological variation within carefully mapped and sampled lava flow units collected by ALVIN from 9 25'-55'N along the EPR to better understand the distribution of lavas that comprise the neovolcanic zone of the East Pacific Rise (EPR) and how the upper oceanic crust is formed at fast-spreading mid-ocean ridges (MORs). The selection of sample sites was guided by microbathymetry and sidescan sonar images in areas where discrete flow units appear to emanate from within or near the axial summit trough (AST) and extend up to a few kilometers off-axis. Pillow mounds with possible off-axis origins were also mapped and sampled. While a range of lava morphologies including pillow, sheet, and lobate flows were recovered, preliminary observations indicate that a variety of morphologic facies are present in each individual lava flow unit and the morphologies can be related to flow dynamics. More than 25 individual flows were directly observed and/or sampled along the crestal plateau where ABE microbathymetry and DSL-120A side scan sonar images show acoustically reflective scalloped morphologies that are observed to be overlapping flow fronts 100's of meters in length. At many sites, lava samples were collected from the pillows comprising flow fronts as well as lobate flows that form the tops of eruptive units as well as underlying units. Non-reflective, dendritic, sinuous to linear features that are largely perpendicular to the AST represent distributary channels that serve to move lavas off-axis. Seven sites in five separate channel systems were examined and sampled in detail. While there is only slight chemical variability (usually within analytical uncertainty) in glasses from most of the individual lava flows, two complimentary dives on either side of the EPR axis near 9 50'N sampled lavas with significant major element differences between flows proximal to the AST and those farther away. In each case, the most primitive samples are from the first few flow units emanating from the AST, while those now located up to several kilometers away from the AST are significantly more evolved. The observed inter-flow or intra-flow major element variations cannot be explained by low-pressure fractional crystallization; which might be expected if cooling and crystallization proceeded as flows moved off-axis. Instead, the data suggest the near- and off-axis samples are not directly related and that other magmatic processes have affected interflow geochemical variation. Higher pressure crystallization (4 to 4.5 kbars) is required to explain the chemistry of some samples as well as mixing between more evolved and more primitive melts; both unlikely to happen during seafloor eruptive and transport processes. In addition, ~30% crystallization is required to explain the interflow variation for each dive while no greater than 10% crystals are present in any samples from this section of the EPR.
DE: 3035 Midocean ridge processes
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 8429 Lava rheology and morphology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly