HR: 0800h
AN: T31A-0496    [Abstracts]
TI: Discriminating Lava Flows From the EPR ISS (9 25'-9 55' N) Using Lava Morphology, Petrography, and Geochemistry
AU: * Hinds, J S
EM: jshinds@ufl.edu
AF: University of Florida, Department of Geological Sciences 241 Williamson Hall, Gainesville, FL 32611 United States
AU: Perfit, M R
EM: perfit@geology.ufl.edu
AF: University of Florida, Department of Geological Sciences 241 Williamson Hall, Gainesville, FL 32611 United States
AU: Soule, S A
EM: ssoule@whoi.edu
AF: Woods Hole Oceanigraphic, Clark South, 272C, MS#24, Woods Hole, MA 02543 United States
AU: Fornari, D J
EM: dfornari@whoi.edu
AF: Woods Hole Oceanigraphic, Clark South, 272C, MS#24, Woods Hole, MA 02543 United States
AU: Ridley, W I
T31A-0496 AF: United State Geological Survey, Denver Federal Center Box 25046, MS 973, Denver, CO 80225 United States
AB: The neovolcanic zone of the East Pacific Rise (EPR) from 9°25' to 9°55' N is dominantly comprised of overlapping lava lobes, 0.25-1 km in length and 1-2 km in width, that emanate from the axial summit trough (AST) and extend up to a few km down the ridge flanks. This volcanic terrain is interspersed with sinuous to linear channel systems that are oriented perpendicular to the AST and serve to distribute lava off-axis, as well as elongate ridge-parallel pillow mounds that may have erupted from off-axis fissures at the edge of the neovolcanic zone. In order to better understand the distribution and geochemical heterogeneity of lavas that comprise the neovolcanic zone of the EPR and to re-evaluate models of upper oceanic crust construction at fast-spreading mid-ocean ridges, we have evaluated the degree of geochemical heterogeneity, petrographic diversity, and morphological variation within carefully mapped and sampled lava flow units. In addition to steep pillow flow fronts and sheet and lobate flow tops that comprise the flow units and form the overlapping terrain near the AST, ALVIN also collected samples from distributary lava channels and pillow mounds erupted off-axis. Lavas from a series of flow-fronts on either side of AST at 9°50' N and two channels at ~9°29' N are mainly aphyric to sparsely plagioclase phyric (< 2%) basalts with normal incompatible element depletions (N-MORB). In contrast the more evolved (lower MgO) lavas from off-axis mounds exhibit greater depletion of the most incompatible elements and contain as much as 10% microphenocrysts suggesting origins distinct from the flows emanating from the axis. Overlapping flow units extending from the AST each exhibit a distinct trace element signature. Pillow basalts comprising the flow fronts of these eruptive units are consistently more evolved (0.2-0.77 lower wt.% MgO) than their associated lobate and sheet flows in the body of the flow and have slightly higher concentrations of incompatible elements. The quantities of microphenocrysts in pillow lavas at the flow front are ~1.0-2.4 vol.% greater than those in the body of the flow. Fractional crystallization models at low pressures (0.025 kbar) predict 4-14 vol.% crystallization of parental magmas are required to produce the observed changes in chemistry within individual flow units; at odds with the measured crystallinity variation. Furthermore, some mixing between primitive and evolved melts is required to explain all of the variations in major element and trace element characteristics of the flow units. The results of our study suggest that the majority of chemical imprinting of lavas precedes eruption onto the seafloor and that little variation occurs once magmas have been erupted.
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1065 Major and trace element geochemistry
DE: 1090 Field relationships (3690, 8486)
DE: 3614 Mid-oceanic ridge processes (1032, 8416)
DE: 8486 Field relationships (1090, 3690)
SC: Tectonophysics [T]
MN: Fall Meeting 2005