Small-scale sublithospheric convection reconciles geochemistry and geochronology of 'Superplume' volcanism in the western and south Pacific

摘要

Cretaceous volcanism in the West Pacific Seamount Province (WPSP), and Tertiary volcanism along the Cook-Australs in the South Pacific are associated with the same broad thermochemical anomaly in the asthenosphere perhaps related to the Pacific 'Superplume.' Abundant volcanism has usually been attributed to secondary plumelets rising from the roof of the Superplume. The Cook-Australs display distinct geochemical trends that appear to geographically project, backward in time, to corresponding trends in the WPSP. However, the implied close proximity of source regions (i.e., similar to 1000 km) with very different geochemical fingerprints and their longevity over geological time (> 100 Myrs) appear to be at odds with the secondary plumelet hypothesis, a mechanism with a typical timescale of similar to 30 Myrs. Moreover, ages sampled along the individual volcano chains of the Cook-Australs, and of the WPSP violate the predictions of the plumelet hypothesis in terms of linear age-distance relationships. Our numerical models indicate that small-scale sublithospheric convection (SSC) as likely triggered by the thermochemical anomaly of the 'Superplume' instead reconciles complex age-distance relationships, because related volcanism occurs above elongated melting anomalies parallel to plate motion ('hot lines'). Furthermore, SSC-melting of a mantle source that consists of pyroxenite veins and enriched peridotite blobs in a matrix of depleted peridotite creates systematic geochemical trends over seafloor age during volcanism. These trends arise from variations in the amount of pyroxenite-derived lavas relative to peridotite-derived lavas along a 'hot line,' therefore stretching between the geochemical end-members HIMU and EMI. These predicted trends are consistent with observed trends in radiogenic isotopic composition from the Wakes, Marshalls, Gilberts (i.e., the individual volcano groups of the WPSP) and the Cook-Australs. For increasing mantle temperatures. volcanism is further predicted to occur at greater seafloor ages and with a more EMI-like signature, a result that can explain many of the observed systematics. Thus, SSC explains many of the geochemical observations with long-term temporal variations in mantle temperature, instead of persistent intermediate-scale (similar to 1000 km) compositional heterogeneity.