Temporal evolution and geochemical variability of the South Pacific superplume activity

We are presenting a new set of K/Ar ages and geochemical analyses obtained on deep-sea samples dredged in 1999 on several seamounts of the Cook-Austral volcanic chains in the Pacific Ocean. The new geochemical results, together with published data on island samples, allow us to reveal a time evolution of the mantle source composition as well as an increase in geochemical variability of the superplume responsible for the regional South Pacific Superswell. Three identified volcanic stages of 58-40, 33-20 and 20-0 Ma are identified with signatures of mantle reservoir composition varying from close to C to N-MORB-types and C/HIMU-type, respectively. Using a geodynamic reconstruction for the most recent volcanic period, from 20 Ma to present, three hotspot tracks are needed to explain the several volcanic episodes observed within the limited geographical area of the central part of the Cook-Austral chains. At the scale of a single volcano, different magmatic phases can also be identified with different ages and geochemical signatures, emphasizing the importance of structural control, either crustal or lithospheric, in the location of the magmatic outputs. These observations, taken together, are in good agreement with a model where each hotspot could sample a small volume of the large very heterogeneous plume responsible for the regional South Pacific Superswell.