HR: 1330h
AN: V42D-0377    [PDF]
TI: $^{234}$U-$^{238}$U in volcanic rocks
AU: * Rubin, K H
EM: krubin@hawaii.edu
AF: U. Hawaii, Geology \& Geophysics, Honolulu, HI 96822 United States
AB: New and literature ($^{234}$U/$^{238}$U) data on standards and rocks erupted from volcanoes in each of the major tectonic environments are analyzed to evaluate the significance, magnitudes and causes of ($^{234}$U/$^{238}$U) variations within the global data set. The data set is limited to high precision mass spectrometric (MS) data, which are required to resolve small disequilibria differences (0.2-0.3%) and to distinguish them from secular equilibrium. The principle finding is that statistically significant $^{234}$U-$^{238}$U disequilibrium exists in 10-15% of volcanic rocks (413 total samples) and it varies systematically by tectonic setting. This disequilibria causes the mean ($^{234}$U/$^{238}$U) of all volcanic rock data to be $>$1 outside of standard error (2$\sigma$$_{m}$) and outside of the limitations imposed by U standards data. This is true regardless of $^{234}$U decay constant ($\lambda$234U) used to convert MS atomic abundance ratio data to activity ratios. A subset of this data set also allows for an independent test of 4 different $\lambda$234U used in the literature: ($^{234}$U/$^{238}$U) =1.000 in intraplate subaerial volcanic (SV) rocks using the most recent $\lambda$234U (Cheng et al., Chem. Geo., 169, 2000) indicating that this decay constant is preferable to older, more commonly employed values. The SV data also indicate a closed melt system for U exchange in volcanoes of this tectonic setting. ($^{234}$U/$^{238}$U) $>$1 in many convergent margin lavas and mid-ocean ridge basalts (MORB) indicate addition of U with a marine isotopic fingerprint during or after petrogenesis (i.e., as alteration). $^{234}$U-$^{238}$U disequilibrium and $^{238}$U- $^{230}$Th disequilibrium are strongly correlated in convergent zone data but not in MORB, indicating different controls on both ratios in the two environments. This and other major and trace element covariations with ($^{234}$U/$^{238}$U) are perhaps best interpreted as predominantly reflecting AFC of young altered crust at ocean ridges, and a combination of crustal AFC and deeper marine-derived fluid addition to the convergent zone lavas when ($^{234}$U/$^{238}$U) $>$1.
DE: 1020 Composition of the crust
DE: 1040 Isotopic composition/chemistry
DE: 3035 Midocean ridge processes
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting