Open Access238 U‐ 230 Th‐ 226 Ra‐ 210 Pb‐ 210 Po, 232 Th‐ 228 Ra, and 235 U‐ 231 Pa constraints on the ages and petrogenesis of Vailulu'u and Malumalu Lavas, Samoa
Author(s) -
Sims Kenneth W. W.,
Hart S. R.,
Reagan M. K.,
Blusztajn J.,
Staudigel H.,
Sohn R. A.,
Layne G. D.,
Ball L. A.,
Andrews J.
Publication year - 2008
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2007gc001651
Subject(s) - basalt , geology , geochemistry
We report 238 U‐ 230 Th‐ 226 Ra‐ 210 Pb‐ 210 Po, 232 Th‐ 228 Ra and 235 U‐ 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. 210 Pb and 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 230 Th, 231 Pa, and 226 Ra, we interpret their 238 U ‐ 230 Th, 235 U‐ 231 Pa and 230 Th ‐ 226 Ra disequilibria in terms of the magmatic processes occurring beneath the Samoan Islands. ( 230 Th/ 238 U) > 1 indicates that garnet is required as a residual phase in the magma sources for all these lavas. The large range of ( 238 U/ 232 Th) and ( 230 Th/ 232 Th) is attributed to long‐term source variation. The Samoan basalts are all alkaline basalts and show significant 230 Th and 231 Pa excesses but limited variability, indicating that they have been derived by small but similar extents of melting. Their ( 230 Th/ 238 U), ( 231 Pa/ 235 U) and Sm/Nd fractionation are consistent with correlations among other ocean island basalt suites (particularly Hawaii) which show that ( 230 Th/ 238 U) and ( 231 Pa/ 235 U) of many OIBS can be explained by simple time‐independent models. Interpretation of the 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 210 Pb deficits that reflect either shallow magmatic processes or continuous magma degassing. In both cases, decadal residence times are inferred from these 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.