Amsterdam-St Paul Hotspot and Plateau: Geochronological Constraints and Geochemical Characteristics

The construction of the Amsterdam-St. Paul (ASP) plateau is attributed to the interaction between the Southeast Indian Ridge (SEIR) and ASP hotspot. During the PLURIEL cruise (2006) lava samples were collected by dredging the plateau and the chain of seamounts between 39°S-76°E and 33°S-83°E. New K/Ar dating seems to indicate that previous age determinations of the hotspot activity, mostly based on plates kinematics, were overestimated. The first real seamount off the limits of the plateau had its most "recent" activity, dated at 1.04 ± 0.02 My. The geochemical composition of ASP seamounts is highly alkaline due to very low partial melting degrees. Its evolution to a transitional tholeiitic volcanism is related to the interaction with the SEIR. Prior to the interaction, ASP seamount lavas are very enriched in Rare Earth Elements with La/Sm = 2.8-4.6 and Sm/Yb reaching 3.3. The enrichment level of the plateau lavas is intermediate between that of the hotspot track seamounts and the SEIR itself. The SEIR lavas where the ridge rifted the plateau are also enriched resulting from the hotspot influence.