HR: 14:50h
AN: V33F-05    [Abstracts]
TI: The effect of assimilation and fractional crystallization on U-series disequilibria in arc lavas
AU: * Huang, F
EM: fhuang1@uiuc.edu
AF: Dept. of Geology, Univ. of Illinois at Urbana-Champaign, 245 NHB, 1301 W. Green St., Urbana, Il 61801, United States
AU: Gao, L
EM: liligao2@uiuc.edu
AF: Dept. of Geology, Univ. of Illinois at Urbana-Champaign, 245 NHB, 1301 W. Green St., Urbana, Il 61801, United States
AU: Lundstrom, C C
EM: lundstro@uiuc.edu
AF: Dept. of Geology, Univ. of Illinois at Urbana-Champaign, 245 NHB, 1301 W. Green St., Urbana, Il 61801, United States
AB: U-series disequilibria in young arc lavas provide critical temporal constraint on magmatism in arc settings. Most young arc lavas have (²³¹Pa/²³⁵U) and (²²⁶Ra/²³⁰Th) greater than unity. Although the majority of young arc lavas have ²³⁸U excess over ²³⁰Th, a significant amount of samples have (²³⁸U/²³⁰Th) < 1. Models involving recent addition of U-Ra rich fluids to the mantle wedge can explain the ²³⁸U and ²²⁶Ra excesses over ²³⁰Th, but it cannot explain ²³¹Pa excess or ²³⁰Th excess. In-growth melting models can produce ²³¹Pa excess and ²³⁰Th excess, but it cannot account for the overall positive correlation between ²²⁶Ra excess and Ba/Th or Sr/Th, which is interpreted as a typical feature of fluid addition. There are still heated debates on generation of U-series disequilibria in arc lavas and time-scale of magmatisms in convergent margins. Most arc lavas have experienced significant assimilation and fractional crystallization (AFC) during magma evolution. Although AFC processes can fundamentally change trace element and isotopic features of magma, its effect on U-series disequilibria has not been rigorously investigated despite its importance. It is not quite clear yet whether the correlations of U-series data with major-trace element compositions of arc lavas (e.g., (²²⁶Ra/²³⁰Th) vs. Sr/Th or Ba/Th) reflect the geochemical features of the fluid-metasomatised mantle wedge or result from magma differentiation processes. In this study, we use a numerical model to show that time-dependent AFC processes can significantly change U-series data and other geochemical features of arc lavas via fractional crystallization, decay of short-lived nuclides, and dilution by old crustal assimilant. The positive correlation between ²²⁶Ra excess and Ba/Th or Sr/Th can be produced by fractionation of plagioclase and amphibole with contemporaneous decay of ²²⁶Ra. Assimilation of old crustal materials may dilute the primary U-series disequilibria originating from melting of the metasomatised mantle. Our model can also reconcile the discrepancy of the time scales of arc magmatism indicated by U-Th-Pa-Ra and ¹⁰Be/Be systematics. Because correlations between parameters sensitive to fluid addition and U-series data could simply be produced by the AFC processes in a magma chamber within several thousands years, recent fluid addition and ultra-fast upwelling rates of magma may not be required for generation and preservation of ²²⁶Ra excess in young arc lavas. U-series disequilibria observed in arc settings might reflect in-growth melting in the mantle wedge plus magma differentiation processes in magma chamber at crustal depths.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1036 Magma chamber processes (3618)
DE: 1037 Magma genesis and partial melting (3619)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3618 Magma chamber processes (1036)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting