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Super 238 U- super 230 Th- super 226 Ra- super 210 Pb- super 210 Po, super 232 Th- super 228 Ra, and super 235 U- super 231 Pa constraints on the ages and petrogenesis of Vailuluu and Malumalu lavas, Samoa



Super 238 U- super 230 Th- super 226 Ra- super 210 Pb- super 210 Po, super 232 Th- super 228 Ra, and super 235 U- super 231 Pa constraints on the ages and petrogenesis of Vailuluu and Malumalu lavas, Samoa



Geochemistry, Geophysics, Geosystems - G (super 3) 9(4): (Citation Q04003



We report (super 238) U- (super 230) Th- (super 226) Ra- (super 210) Pb- (super 210) Po, (super 232) Th- (super 228) Ra and (super 235) U- (super 231) Pa measurements for a suite of 14 geologically and geochemically well-characterized basaltic samples from the Samoan volcanoes Vailulu'u, Malumalu, and Savai'i. Maximum eruption ages based on the presence of parent-daughter disequilibria indicate that Vailulu'u is magmatically productive with young lavas (<8 Ka) resurfacing both its summit crater and lower flanks. (super 210) Pb and (super 210) Po measurements indicate that several flows have erupted within its summit crater in the past 100 years, with the newest observed flow being erupted in November of 2004. For lavas which have eruption ages that are demonstrably young, relative to the half-lives of (super 230) Th, (super 231) Pa, and (super 226) Ra, we interpret their (super 238) U - (super 230) Th, (super 235) U- (super 231) Pa and (super 230) Th - (super 226) Ra disequilibria in terms of the magmatic processes occurring beneath the Samoan Islands. ( (super 230) Th/ (super 238) U) > 1 indicates that garnet is required as a residual phase in the magma sources for all these lavas. The large range of ( (super 238) U/ (super 232) Th) and ( (super 230) Th/ (super 232) Th) is attributed to long-term source variation. The Samoan basalts are all alkaline basalts and show significant (super 230) Th and (super 231) Pa excesses but limited variability, indicating that they have been derived by small but similar extents of melting. Their ( (super 230) Th/ (super 238) U), ( (super 231) Pa/ (super 235) U) and Sm/Nd fractionation are consistent with correlations among other ocean island basalt suites (particularly Hawaii) which show that ( (super 230) Th/ (super 238) U) and ( (super 231) Pa/ (super 235) U) of many OIBS can be explained by simple time-independent models. Interpretation of the (super 226) Ra data requires time-dependent melting models. Both chromatographic porous flow and dynamic melting of a garnet peridotite source can adequately explain the combined U-Th-Ra and U-Pa data for these Samoan basalts. Several young samples from the Vailulu'u summit crater also exhibit significant (super 210) Pb deficits that reflect either shallow magmatic processes or continuous magma degassing. In both cases, decadal residence times are inferred from these (super 210) Pb deficits. The young coeval volcanism on Malumalu and Vailulu'u suggests the Samoa hot spot is currently migrating to the northeast due to dynamic interaction with the Tonga slab.

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