HR: 11:35h
AN: T12A-06    [Abstracts]
TI: The Influence of Ridge Geometry on Crustal Accretion: Insights From U-series Disequilibria on the SW Indian Ridge
AU: * Standish, J J
EM: jstandish@whoi.edu
AF: MIT/WHOI Joint Program, 266 Woods Hole Road MS 8, Woods Hole, MA 02543 United States
AU: Sims, K W
EM: ksims@whoi.edu
AF: Woods Hole Oceanographic Institution, 266 Woods Hole Road, Woods Hole, MA 02543 United States
AU: Dick, H J
EM: hdick@whoi.edu
AF: Woods Hole Oceanographic Institution, 266 Woods Hole Road, Woods Hole, MA 02543 United States
AB: Recent work has proposed that crustal accretion on ultra-slow spreading plate boundaries, both magmatic and amagmatic, is largely a function of ridge geometry, melt production, and the ensuing change in the lithospheric thermal regime. We present U-series data from the ultra-slow spreading SW Indian Ridge in an attempt to unravel some of the complexities embedded in the issues surrounding spreading geometry and crustal accretion. Initial 238U-230Th-226Ra disequilibria measurements on 12 dredged glasses from between 10-25$\deg$ E on the SW Indian Ridge show 238U and 230Th disequilibria. We interpret the 238U excesses (230Th/238U $<$ 1) and 230Th excesses (230Th/238U $>$ 1) to be primary based on measured 226Ra/230Th disequilibria, which indicate the lavas are young ($<$ 8 ka). The suite as a whole exhibits a large range in 230Th/232Th and 238U/232Th, covering nearly the entire extent of variability seen in the global U-series MORB database. Glasses from the oblique segment show the most enriched compositions (230Th/232Th = 0.8 and 238U/232Th = 0.76) of any MORB measured but have minor 230Th excesses ($<$9%) or small 238U excesses (3.5%). In contrast, 230Th excesses in lavas from the orthogonal spreading segment vary from 2 - 29% and are elevated relative to values on the oblique spreading segment. To a first order, this may suggest strikingly different melting regimes beneath each spreading segment. Contrary to most existing MORB U-series data, the majority of the oblique segment lavas do not maintain a negative correlation between 230Th/238U and depth, while the orthogonal segment lavas do. Our U-series data indicate that the change in ridge geometry from the orthogonal spreading segment to the oblique spreading segment directly affects the lithospheric thermal regime (e.g. thickening of the conductive cooling lid), which therein produces changes in the melting column, depth of melting, melt production, and melt transport. We will address these melting parameters as well as constrain the effective source composition (e.g. garnet peridotite versus garnet pyroxenite).
DE: 8434 Magma migration
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8125 Evolution of the Earth
DE: 8150 Plate boundary--general (3040)
DE: 1040 Isotopic composition/chemistry
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting