The mantle structure below petit-spot volcanoes

Abstract

Petit-spot volcanoes provide a unique opportunity to directly examine the structure and geochemical composition of the lithospheric and asthenospheric mantle beneath the subducting oceanic crust. Currently, petit-spot volcanos—formed in response to the flexure of subducting oceanic plates—are known to exist off the Japan, Java, Chile and Mariana trenches, however, the difficulties associated with detecting them leaves the possibility that they may be more widespread along outer rise of plates. Investigation of the lavas and xenolithic material erupted from petit-spot volcanos has suggested that the suboceanic lithosphere is largely metasomatized by numerous ascending petit-spot melts. This somewhat contradicts previous understanding which indicated that the oceanic lithosphere has a depleted mantle composition. The ultimate source and geodynamic processes that lead to the formation of petit-spot melts remain ambiguous, however, possibilities include that the lavas originate from incipient asthenospheric carbonatitic or carbonated silicate melts that are characteristically depleted in zirconium and hafnium. Typical geochemical trends in petit-spot lava fields off Japan trench indicate elemental partitioning through the immiscible separation of carbonate and silicate melts.

The compositional fingerprint of petit-spot magmas represents an enriched mantle source and carbonatitic or carbonated silicate melts below the oceanic lithosphere, suggests a synthesis of geochemical analyses on petit-spot volcanism.