New 40Ar/39Ar Ages for Savai'i Island Reinstate Samoa as a Hotspot Trail with a Linear Age Progression

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• Russell, J. A.
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• Staudigel, H.
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• Hart, S. R.

Abstract

The volcanic islands and seamounts of the Samoan Archipelago have long been considered problematic in terms of the hotspot hypothesis. Existing K/Ar and 40Ar/39Ar measurements on subaerial samples from the Samoan islands have consistently given ages that are too young by several Myrs, conflicting with the expected linear age progression model. Previous data from the volcanic series and cones on Savai'i Island gave a range of ages between 0.2 and 2.1 Ma. This is in contrast to an age of 5.2 Ma that a Pacific plate motion of 7.1 cm/yr would predict for the onset of the shield building stage on Savai'i (Workman et al. 2004). The oldest shield ages for the islands of Upolu (2.7 My) and Tutuila (1.6 My) young eastward, toward the volcanically active Vailulu'u seamount that marks the current location of the Samoan hotspot (Hart et al. 2000; Staudigel et al. 2006). However, these ages are younger than predicted by the plate-speed model with 4.4 My and 2.7 My, respectively. The omnipresence of only young post-erosional volcanism on Savai'i has lead to extensive discussions on the origin of Samoan volcanism, and is often used as an argument against a possible hotspot and mantle plume origin. We present new 40Ar/39Ar data on volcanic samples from the deep flanks and rifts of Savai'i and a group of Samoan seamounts that were dredged during the ALIA Expedition. Twelve ages from eight different dredge locations confirm the predicted 7.1 cm/yr age progression for the Samoan hotspot. Three different volcanic samples from dredge ALIA-115, on the deepest portion of the SW flank of Savai'i Island, give indistinguishable ages (2σ confidence level) ranging from 4.99 to 5.21 Ma. In addition, a sample from dredge ALIA-128, on the NE flank of Savai'i, gives an age of 4.74 Ma. These results clearly demonstrate that the onset of the shield-building stage on Savai'i occurred much earlier than the oldest volcanics (2.1 Ma) sampled subaerially on the island. Sr-Nd-Pb isotopes and trace element data show that dredge 115 and 128 samples have a typical Samoan shield pedigree, and are distinct from the subaerial rejuvenated volcanics on Savai'i and Upolu (see companion abstract by Jackson et al., this volume). Based on these new 40Ar/39Ar age dating results, the Samoan seamount chain now depicts a clear linear age progression for the earliest stages of shield building. This reinstates Samoa as a hotspot trail with similar characteristics and age systematics as the "primary" Hawaiian and Louisville hotspot trails. It eliminates the major argument against a plume origin for Samoa. Our results also show that the volcanic evolution of Savai'i has a prolonged history of at least 5 Myr. In this context, the post-erosional veneer that covers most of Savai'i (and part of Upolu) most likely reflects a strong rejuvenated stage of volcanism related to the rapid eastward encroachment of the northern Tonga subduction/transform zone.