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Peer Reviewed


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Journal/Book/Conference Title

G3: Geochemistry, Geophysics, Geosystems

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


U-series disequilibria, Samoa, eruption ages

Journal Article Version

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Published Article/Book Citation

G3, 9:4(2008), pp. 1-30.


Copyright 2008 by the American Geophysical Union. Posted by permission.

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