238U-Th-230-Ra-226-Pb-210-Po-210, Th-232-Ra-228, and U-235-Pa-231 constraints on the ages and petrogenesis of Vailulu'u and Malumalu Lavas, Samoa

We report U-238-Th-230-Ra-226-Pb-210- Po-210, Th-232-Ra-228 and U-235-Pa-231 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. Pb-210 and Po-210 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 Th-230, Pa-231, and Ra-226, we interpret their U-238-Th-230, U-235-Pa-231 and Th-230 - Ra-226 disequilibria in terms of the magmatic processes occurring beneath the Samoan Islands. (Th-230/U-238) > 1 indicates that garnet is required as a residual phase in the magma sources for all these lavas. The large range of (U-238/Th-232) and (Th-230/Th-232) is attributed to long-term source variation. The Samoan basalts are all alkaline basalts and show significant Th-230 and Pa-231 excesses but limited variability, indicating that they have been derived by small but similar extents of melting. Their (Th-230/U-238), (Pa-231/U-235) and Sm/Nd fractionation are consistent with correlations among other ocean island basalt suites ( particularly Hawaii) which show that (Th-230/U-238) and (Pa-231/U-235) of many OIBS can be explained by simple time-independent models. Interpretation of the Ra-226 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 Pb-210 deficits that reflect either shallow magmatic processes or continuous magma degassing. In both cases, decadal residence times are inferred from these Pb-210 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.