HR: 1340h
AN: B13A-0178    [Abstracts]
TI: $^{210}$Po-$^{210}$Pb Dating and U-Series Disequilibria of a Young Basaltic Flow at $10\deg$44'N EPR
AU: * van der Zander, I
EM: izander@hawaii.edu
AF: SOEST, 1680 East West Rd, Honolulu, HI 96822 United States
AU: Rubin, K H
AF: SOEST, 1680 East West Rd, Honolulu, HI 96822 United States
AU: Zierenberg, R A
AF: Dept. of Geology, University of California-Davis, Davis, Ca 95616 United States
AU: Voight, J R
AF: The Field Museum, 1400 S. Lake Shore Dr, Chicago, Il 60605 United States
AU: Von Damm, K
AF: EOS, University of New Hampshire, Durham, NH 03824 United States
AU: McClain, J
AF: Dept. of Geology, University of California-Davis, Davis, Ca 95616 United States
AB: Divers in the submersible Alvin observed and sampled a very young appearing lava flow at $10\deg$44'N on the East Pacific Rise (EPR) during a November 2003 biodiversity-funded dive series (DEB-0072695 to J.R. Voight) to the area. The occurrence of the glassy, unsedimented, low-lying graben filling lava flow was unexpected on this magma-starved ridge segment just north of the Clipperton Transform Fault and so 3 dives were used to investigate a 4 km long segment of the flow. Measurements (in progress) of $^{210}$Po-$^{210}$Pb disequilibrium in 4 samples of this lava flow confirm that it is likely extremely young (0-2 months old when the samples were collected). Such age dating is critical here because although this area of the EPR is covered by the SOSUS hydrophone array, data are not available in real time and will not be downloaded until November 2004. The $^{210}$Po-$^{210}$Pb geochronometer (T$_{1/2}$ =138.4 d) involves a time series of 3 or 4 $^{210}$Po analyses over the course of 1 year or more, so that our analyses are still ongoing (just 2 data per sample so far) and our results quite preliminary. By meeting time we will have better constrained eruption ages with the addition of more data to the ingrowth curves. Results of other chemical analyses on these same samples are being used to compositionally characterize the lavas and to constrain magmatic process timescales. All 13 of the lavas sampled in Nov 2003 are low MgO (6.5-6.7 wt%) basalts, displaying inter-sample compositional variations (lava flow heterogeneity) at the higher end of the known MORB flow catalogue, in contrast to mafic (8 wt% MgO) and compositionally homogeneous lava flows erupted at the nearby and well-studied $9\deg$50'N EPR site. $^{230}$Th-$^{238}$U disequilibria in the young $10\deg$44'N flow clusters around 14-15% $^{230}$Th-excesses, which overlaps the high end of the data range from "zero-age" MORB flows at $9\deg$50'N EPR and the Juan De Fuca Ridge (JDF). Slightly higher excesses, lower ($^{230}$Th/$^{232}$Th ), and higher Th/U ratios (2.5-2.6) distinguish our samples from $9\deg$50'N, suggesting a slightly different source composition (and perhaps melting rate) between the locales. The young flow is variably covered by thick, mucus-rich bacterial mats, sparsely populated by small animals, and had copious bacterial floc issuing from multiple diffuse flow vents ("snowblowers"). Focused high temperature flow vents were not observed (maximum measured fluid temperatures were just $10\deg$C), yet many of the active hydrothermal fluid flows were within lava collapses and were thus difficult to sample.Two hydrothermal vent fluid samples had elevated H$_{2}$S and Si above background levels, in spite of the near seawater Mg content of the fluids.
DE: 4815 Ecosystems, structure and dynamics
DE: 7218 Lithosphere and upper mantle
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 3035 Midocean ridge processes
DE: 1020 Composition of the crust
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting