HR: 1340h
AN: V13B-0556    [Abstracts]
TI: U-series disequilibrium of basaltic rocks from Kick'em-Jenny submarine volcano, Lesser Antilles island arc
AU: * Huang, F
EM: fhuang1@uiuc.edu
AF: University of Illinois, Urbana-Champaign, Department of Geology, 245 Natural History Building, 1301 W. Green Street, Urbana, Il 61801 United States
AU: Lundstrom, C C
EM: lundstro@uiuc.edu
AF: University of Illinois, Urbana-Champaign, Department of Geology, 245 Natural History Building, 1301 W. Green Street, Urbana, Il 61801 United States
AB: Kick­_em Jenny (KEJ) submarine volcano located 9 km to the north of Grenada in the Lesser Antilles volcanic arc produces lavas ranging in composition from high MgO basalts to moderately evolved andesites. We have determined U-series disequilibria in 12 porphyritic lavas erupted from KEJ volcano by TIMS and MC-ICP-MS methods to constrain the timing and identify the processes creating the magma diversity observed. The SiO₂ contents of samples studied here vary from 47 to 55 wt.% SiO₂ while REE patterns evolve from slightly LREE enriched, MREE/HREE = 1 patterns to strongly LREE enriched, MREE depleted concave-up patterns. Separate dissolutions of sample KEJ100 indicate an external reproducibility (1s) of 0.7% for (²³⁰Th/²³⁸U) (n=4), 0.8% for (²³⁰Th/²³²Th) (n=4) and 0.6% for (²²⁶Ra/²³⁰Th) (n=3), respectively. For all sample, (²³⁴U/²³⁸U) lies within 0.7% of unity, suggesting that secondary alteration by seawater has not disturbed the U-series data significantly. Sample ages for these submarine erupted samples are unknown, resulting in uncertain values for initial (²²⁶Ra/²³⁰Th); however, 10 out of 12 of the measured (²²⁶Ra/²³⁰Th) range between 3.16 and 1.13 and are thus unequivocally young with respect to decay of ²³⁰Th and ²³¹Pa since eruption. The U (0.535 - 4.876 ppm) and Th (1.25 - 10.78 ppm) concentrations increase with SiO₂ contents. (²³⁰Th/²³²Th) has a restricted range, varying from 0.994 to 1.093 with the exception of one sample. (²³⁰Th/²³⁸U) ranges from 0.684 to 0.875 while (²³¹Pa/²³⁵U) ranges from 1.76 up to 2.84, among the highest ²³¹Pa excess in island arcs yet reported. These data confirm previous observations of the unusual behavior of KEJ lavas relative to global observations in having both large ²³⁸U and ²³¹Pa excesses. Combined with (²²⁶Ra/²³⁰Th), these disequilibria observations require that ²³⁸U excesses reflect more than solely fluid addition to the mantle wedge from the subducted oceanic slab.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005