HR: 1330h
AN: V32C-1025    [PDF]
TI: $^{210}$Pb-$^{226}$Ra and Other U-Series Disequilibria in Very Young MORB and Loihi Tholeiites
AU: * van der Zander, I
EM: izander@hawaii.edu
AF: Univ. Hawaii, Geology \& Geophysics, Honolulu, HI 96822 United States
AU: Rubin, K H
EM: krubin@hawaii.edu
AF: Univ. Hawaii, Geology \& Geophysics, Honolulu, HI 96822 United States
AU: Smith, M
EM: Phelsnorian@geology.ufl.edu
AF: Univ. Florida, Geological Sciences, Gainesville, FL 32611 United States
AU: Perfit, M
EM: perfit@geology.ufl.edu
AF: Univ. Florida, Geological Sciences, Gainesville, FL 32611 United States
AU: Bergmanis, E C
EM: bergmani@hawaii.edu
AF: Univ. Hawaii, Geology \& Geophysics, Honolulu, HI 96822 United States
AB: Direct observations of submarine volcanic eruptions are very sparse. Radiometric age constraints on submarine lava flows are thus an essential component for understanding even the most recent histories of oceanic crust formation. Chronometers in the decadal to century time frame have heretofore been lacking. This study focuses on the development and application of $^{210}$Pb-$^{226}$Ra disequilibria as a geochronometer to provide quantitative eruption age constraints over the past 100 years, using submersible-collected samples from the North Cleft segment of the Juan de Fuca Ridge (JDFR) and adjacent Axial seamount, $9\deg$50'N East Pacific Rise (EPR) and Loihi (Hawaii), areas with known stratigraphic field relations between mapped lava flows. The data set provides a unique opportunity to calibrate the $^{210}$Pb-$^{226}$Ra geochronometer because it represents a broad selection of "zero age" ($^{210}$Po-$^{210}$Pb dated) and near-zero-age submarine lavas (glasses). $^{238}$U-$^{230}$Th-$^{226}$Ra-$^{210}$Pb radioactive disequilibria in these samples will be discussed to rationalize the range of conditions responsible for producing $^{210}$Pb-$^{226}$Ra disequilibria (effective half life = 22 yrs) as an initial step towards using this signature to constrain eruption ages and petrogenetic time scales. We will also investigate the temporal aspects of petrogenetic conditions responsible for producing the other disequilibria in these rocks, without the uncertainty imposed by decay corrections for rocks of unknown age. These data augment those we previously reported from the Aldo-Kihi and neighboring lava flows at $17\deg$26'S EPR (Rubin et al., EOS, 82, F1279, 2001). Systematic differences between and within study areas exist: most normal zero age MORB display modest $^{210}$Pb deficits (3-10%); older MORB (the N-cleft sheet flow) have smaller deficits (0-3%); rocks erupted in 1998 from Axial seamount have 8-15% $^{210}$Pb excesses; and, rocks erupted in 1996 at Loihi are in equilibrium within error. Disequilibria amongst other nuclides are all within the range of previously published values. Data broadly fall within the negatively correlated array of ($^{226}$Ra/$^{230}$Th) and ($^{230}$Th/$^{238}$U) (Sims et al., GCA, 66, 2002), yet within each geographic locale data are positively correlated. Although ($^{210}$Pb/$^{226}$Ra) covaries with some major/trace element signatures of low pressure crystal fractionation, it also covaries negatively with ($^{226}$Ra/$^{230}$Th) in all but the Loihi samples, raising the intriguing possibility that some or all of the $^{210}$Pb-$^{226}$Ra disequilibria arises deeper within the mantle (i.e., during melting or transport. This would shorten dramatically the time scales over which those processes occur). The suite of petrogenetic variations will be examined in the context of existing models for basaltic melt genesis.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 3035 Midocean ridge processes
DE: 9355 Pacific Ocean
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting