Geochronological and geochemical constraints on the petrogenesis and geodynamic process of Hemler, Vlinder, and Il’ichev seamount lavas in NW Pacific

Abstract

Oceanic intraplate volcanoes with linear age progressions are usually accepted to be derived from melting of an upwelling mantle plume. Several seamount groups in NW Pacific, however, show complex age-distance relationships that are difficult to explain using the classic “mantle plume hypothesis”, and thus their origins are controversial. In this study, we present ⁴⁰Ar-³⁹Ar age, geochemical, and Sr-Nd-Pb-Hf isotopic data of lavas from Hemler, Vlinder, and Il’ichev seamounts in NW Pacific, to elucidate their petrogenesis and geodynamic process. The lavas from Hemler, Vlinder, and Il’ichev seamounts are classified as alkali basalt, basanite/nephelinite, and trachyte. Lavas with MgO>8 wt.% exhibit high contents of CaO, FeO^T, and TiO₂, similar to the composition of melts formed from reaction between carbonated eclogite-derived melts and fertile peridotite. These lavas have elevated Zr/Hf ratios (40.6–45.2) and negative Zr and Hf anomalies, indicating the presence of a carbonate component in the mantle source. They are enriched in incompatible trace elements and have enriched mantle 1 (EM1)-like Sr-Nd-Pb-Hf isotopic compositions. The isotopic compositions of Vlinder, Il’ichev basanite, and Hemler lavas in this study are similar to the Rarotonga hotspot. Although occurring at the same seamount, the Il’ichev alkali basalts display more depleted Sr-Nd-Hf isotopic compositions compared to Il’ichev basanite. According to plate tectonic reconstruction results, the ages of Hemler (100.1 Ma), Vlinder pre- (100.2 Ma) and post-shield (87.5 Ma), and Il’ichev (56.4 Ma) lavas clearly deviate from the Macdonald, Arago, Rarotonga, and Samoa hotspot tracks, indicating that they cannot directly originate from mantle plumes. We propose that in the mid-Cretaceous, when the Pacific plate passed over Rarotonga hotspot, melting of Rarotonga plume formed the Vlinder (main-shield stage), Pako, and Ioah seamounts. The Rarotonga (and possibly Samoa) plume materials would have been dispersed into the surrounding asthenosphere by mantle convection. These diffuse plume materials would undergo decompression melting beneath lithosphere fractures that are widely distributed in the Magellan area, generating non-hotspot related Hemler and pre- and post-shield Vlinder lavas. The Il’ichev alkali basalts and basanite probably result from lithospheric fracture-induced melting of heterogeneous enriched components randomly distributed in the asthenosphere.