HR: 1340h
AN: T13B-1360    [Abstracts]
TI: Spatial and Geochemical Variations of Lavas Exposed Along a Crustal Section in the Blanco Transform: Insights into Accretion of the Upper Oceanic Crust at the Southern Juan de Fuca Ridge
AU: * Pollock, M A
EM: map16@duke.edu
AF: Duke University, Earth and Ocean Sci., Durham, NC 27708 United States
AU: Klein, E M
EM: ek4@duke.edu
AF: Duke University, Earth and Ocean Sci., Durham, NC 27708 United States
AU: Karson, J A
EM: jkarson@duke.edu
AF: Duke University, Earth and Ocean Sci., Durham, NC 27708 United States
AU: Tivey, M A
EM: mtivey@whoi.edu
AF: Woods Hole Oeanographic Inst., Geology and Geophysics, Woods Hole, MA 02543 United States
AB: Opportunities to examine the architecture of the upper oceanic crust are limited to rare tectonic windows, ocean drilling and ophiolites. Studies at the Hess Deep Rift, which exposes fast-spread crust, suggest a 4-D model for accretion in which dikes transport magma along-axis and construction of the extrusive unit continues off-axis. Additionally, density filtering restricts lava compositions to those of lowest density while higher density magmas represented in the dike unit never reach the surface. Within this context, we examined a suite of lavas exposed along the Blanco Transform (BT). The western portion of the BT consists of a deep trough and a steep northern scarp, providing a window into upper crust generated at the southern end of the intermediate-spreading Juan de Fuca Ridge. During the 1995 Blancovin dive program, 53 samples of lavas were collected along an $\sim$11 km transect of the north wall. Whole rocks were analyzed for major and high abundance trace elements by DCP, and for low-abundance trace elements by ICP-MS. All of the samples are incompatible-element depleted N-MORB, with variations in trace element ratios (e.g., La/Sm, Zr/Y) suggesting minor mixing between a depleted and enriched source. As a whole, the lavas exhibit a large range in extent of crystal fractionation, extending from relatively primitive MORB (Mg\# 64) to evolved FeTi basalts (Mg\# 40, 15wt% Fe$_{2}$O&_{3}$, 3wt% TiO$_{2}$). The diversity of compositions among the BT lavas contrasts with the more limited range in lava compositions found at Hess Deep and Hole 504B. In terms of spatial variations, five vertical dives located $\sim$1 km apart in the center of the study area show systematic variations in extent of crystallization with depth. Primitive upper lavas (avg. Mg\# 60) grade downward into lower evolved lavas (avg. Mg\# 48). This finding contrasts with a simple view of lava eruption during which an eruptive cycle begins with the emplacement of primitive (recently replenished) magmas, forming the base of the lava sequence, followed by continued cooling and crystallization, leading to more evolved lavas toward the top of the unit. Calculated density relations for these lavas reveal, however, that the highest density lavas (low MgO, high FeO) form the base of the lava unit and are overlain by progressively lower density magmas. Density variations both between dikes and lavas and within the lava unit may be key to understanding upper crustal architecture and accretion in space and time.
DE: 3655 Major element composition
DE: 3670 Minor and trace element composition
DE: 3035 Midocean ridge processes
DE: 1020 Composition of the crust
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting