Boomerang Seamount: the active expression of the Amsterdam-St. Paul hotspot, Southeast Indian Ridge

During a survey of the axis of the Southeast Indian Ridge (SEIR), we discov ered a 1100 m tall, volcanically active submarine volcano, Boomerang Seamou nt, near the spreading center on the Amsterdam-St. Paul (ASP) Plateau. The: summit of the volcano is 650 m below sea level and has a 200-m-deep, 2-km- wide circular caldera. Samples of very fresh volcanic glass, dated by the P o-210-Pb-210 technique at similar to5 months old, were collected from the f loor of the caldera in March 1996. The volcano is 18 km northeast of Amster dam Island near the intersection of a long spreading segment and the Boomer ang Transform Fault. It is built on the stationary Antarctic Plate, where w idely scattered volcanic activity thickens the crust, continuing to build t he plateau. Water column profiles reveal a 1.7 degreesC temperature anomaly and a 0.3 V stepped nephelometer (water column turbidity) anomaly within t he caldera, nearly an order of magnitude larger than other hydrothermal plu me anomalies we measured. These anomalies suggest hydrothermal activity wit hin the caldera. Volcanic glass compositions at two sample sites on the vol cano summit are similar to each other and to Amsterdam and St. Paul Island basalts, but have some important differences as well. K2O/TiO2 ratios in Bo omerang Seamount glasses are similar to St. Paul Island samples, but differ significantly from Amsterdam Island samples. Rare earth element patterns i n lavas from Boomerang, Amsterdam, and St. Paul are similar. Sr, Nd, and Pb isotope ratios in samples from the Boomerang Caldera floor are similar to samples from Amsterdam Island. However, another sample from Boomerang Seamo unt deviates from a SEIR-St. Paul-Amsterdam mixing trend and shows evidence of mixing with a Kerguelen-like source component. The geochemical complexi ty of these three closely spaced volcanic edifices on the ASP Plateau sugge sts that the Boomerang Seamount source is heterogeneous on a very small sca le. (C) 2000 Elsevier Science B.V. All rights reserved.