HR: 17:15h
AN: V52F-06    [PDF]
TI: $^{238}$U and $^{230}$Th Excesses in Kolbeinsey Ridge Basalts: Implications for Melt Generation and Transport Beneath a Slow Spreading Ridge
AU: * Sims, K W
EM: ksims@whoi.edu
AF: Dept of Geology and Geophysics, Woods Hole Ocanographic Institution, Woods Hole, MA 02543 United States
AU: Mattielli, N
EM: nmattiel@ulb.ac.be
AF: Department of Earth and Environmental Sciences, Universite de Bruxelles, CP160/02 VLB 50 av F D Roosevelt, Brussels, Bel 1050 Belgium
AU: Elliott, T
EM: tim.elliott@bristol.ac.uk
AF: Department of Earth Sciences,, Wills Memorial Building, Queens Road, University of Bristol, Bristol, BS8 1RJ United Kingdom
AU: Mertz, D
EM: mertz@mail.uni-mainz.de
AF: Institut fuer Geowissenschaften Universitaet Mainz, Becherweg 21, Mainz, 55099 Germany
AU: Kelemen, P
EM: pkelemen@whoi.edu
AF: Dept of Geology and Geophysics, Woods Hole Ocanographic Institution, Woods Hole, MA 02543 United States
AU: Devey, C
EM: cwdevey@uni-bremen.de
AF: Fachgebiet "Petrologie der Ozeankruste" University of Bremen, Fachbereich 5 - Geowissenschaften Postfach 330 440, Bremen, 28334 Germany
AU: Speigelman, M
AF: Lamont-Doherty Earth Observatory & Dept. of Applied Physics/Applied Mathematics Columbia University, Rte 9W, Palisades, NY 10964 United States
AU: Murrell, M
EM: mmurrell@lanl.gov
AF: Los Alamos Natl Laboratory, MS J514, Los Alamos, NM 87545 United States
AB: Here we report measurements of U-Th disequilibria, Nd, and Sr isotopes and major- and trace-element compositions for 8 samples, from two suites of basalts from the slow-spreading Kolbeinsey Ridge (approx. 1 cm/yr. half-spreading rate; 67\deg$05' - 70 \deg$26'N), north of Iceland. These basalts show both significant $^{238}$U excesses [($^{230}$Th /$^{238}$U)$<$1] and $^{230}$Th excesses [($^{230}$Th /$^{238}$U) $>$ 1], with ($^{230}$Th /$^{238}$U) ranging from 0.95 to 1.24. While the samples ($^{230}$Th/$^{232}$Th) activities are essentially constant (1.25 to 1.29), there is a large range in ($^{238}$U /$^{232}$Th) activities (1.00 to 1.32); therefore these data lie on a horizontal array on a ($^{230}$Th/$^{232}$Th)- ($^{238}$U /$^{232}$Th) isochron plot. $^{226}$Ra has not been measured for these samples; however, the fact that they form a horizontal array, which can be interpreted as a zero-age slope, provides strong evidence in support of the young age of these samples. Another independent geological constraint on these samples' young ages comes from the observation that the cruise dredges recovered basaltic glass, rather than glacial wash-out, suggesting their ages are $<$10ka. Normalized trace element abundances indicate that all samples are highly depleted [e.g. (La/Yb)n $<$ 0.4]. The samples with $^{238}$U excesses have the lowest Th concentrations, which is compatible with either shallow mantle melting or shallow mantle melt-rock reaction. Nd and Sr isotopic compositions of these samples are not correlated with ($^{230}$Th /$^{238}$U) and fall into two groups relative to their proximity to Iceland: samples south of the Spar fracture Zone generally are isotopically enriched, while samples north of the Spar fracture zone are isotopically depleted, reflecting a change in mantle source composition north and south of the fracture zone . The lack of a correlation between the Nd and Sr isotopic compositions and ($^{230}$Th /$^{238}$U) suggests that the variability in U/Th and ($^{230}$Th /$^{238}$U) in the Kolbeinsey samples is best explained by melting processes and not the mixing of melts from enriched (pyroxenitic) and depleted (peridotitic) sources. Comparing this data with our recently published data suite from the fast spreading East Pacific Rise (as well as other samples from the global MORB data base) enables us to further define the relationships between U-series disequilibria, axial depth and axial spreading rate.
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1749 Volcanology, geochemistry, and petrology
DE: 3035 Midocean ridge processes
DE: 8434 Magma migration
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting