HR: 1340h
AN: V13B-0554 [Abstracts]
TI: U-series Disequilibria in Continental Arcs: NE Japan Case
AU: * Tepley, F J
EM: ftepley@coas.oregonstate.edu
AF: Oregon State University, College of Oceanic and Atmospheric Sciences, 104 COAS Admin Bldg., Corvallis,
OR 97331
United States
AU: Gill, J B
EM: jgill@pmc.ucsc.edu
AF: University of California, Santa Cruz, Earth Sciences Department, 1156 High St., Santa Cruz, CA 95064
United States
AU: Williams, R W
EM: williams141@llnl.gov
AF: Lawrence Livermore National Laboratory, Chemical Biology & Nuclear Science Division, L-231, Livermore,
CA 94551
United States
AB:
Basalts and andesites from continental arcs typically are close to equilibrium between (238U) and (230Th), and have
subdued excesses of 226Ra compared to oceanic arcs (Turner et al., 2003). There is ambiguity whether these and other
geochemical features derive from subducted sediment, subcontinental lithosphere, or the crust. We report new
238U-234U-230Th-226Ra data for 20 historical to Holocene samples from Asama in the south of NE Japan to
Tarumai in the north. Most straddle the equiline with <5% excess U or Th, yet have 10-60% excess 226Ra. The only
significant excess U occurs at Usu and Iwaki in the north where it correlates with higher (230Th)/(232Th) as in
other arcs (e.g., Mariana). Otherwise (230Th)/(232Th) ratios are 0.75-1.05 which is less than in the surrounding
Izu and Kurile oceanic arcs. Most ratios <0.95 are associated with trace element evidence of crustal contamination that
drags samples down the equiline. The exception is Iwate where Th/U ratios are ~4.0 even in depleted basalts. Excess
226Ra is greater in basalts than andesites. The excess 226Ra, plus elevated 10Be and Th, is attributed to
ubiquitous slab inputs that are overprinted by even higher Th contents from a crustal source with Th/U >4.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1040 Radiogenic isotope geochemistry
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005