Abstract

Vailulu'u Seamount is identified as an active volcano marking the current location of the Samoan hotspot. This seamount is located 45 km east of Ta'u Island, Samoa, at 169°03.5'W, 14°12.9'S. Vailulu'u defines the easternmost edge of the Samoan Swell, rising from the 5000-m ocean floor to a summit depth of 590 m and marked by a 400-m-deep and 2-km-wide summit crater. Its broad western rift and stellate morphology brand it as a juvenile progeny of Ta'u. Seven dredges, ranging from the summit to the SE Rift zone at 4200 m, recovered only alkali basalts and picrites. Isotopically, the volcano is strongly EM2 in character and clearly of Samoan pedigree (⁸⁷Sr/⁸⁶Sr: 0.7052-0.7067; ¹⁴³Nd/¹⁴⁴Nd: 0.51267-0.51277; ²⁰⁶Pb/²⁰⁴Pb: 19.19-19.40). The ²¹⁰Po-²¹⁰Pb data on two summit basalts indicate ages younger than 50 years; all of the recovered rocks are extremely fresh and veneered with glass. An earthquake swarm in early 1995 may attest to a recent eruption cycle. A detailed nephelometry survey of the water column shows clear evidence for hydrothermal plume activity in the summit crater. The water inside the crater is very turbid (nephelometric turbidity unit (NTU) values up to 1.4), and a halo of "smog" several hundred meters thick encircles and extends away from the summit for at least 7 km. The turbid waters are highly enriched in manganese (up to 7.3 nmol/kg), providing further evidence of hydrothermal activity. Vailulu'u is similar to Loihi (Hawaii) in being an active volcanic construct at the eastern end of a hotspot chain; it differs importantly from the Hawaiian model in its total lack of tholeiitic basalt compositions.