Drilled petit-spot basalts reveal asthenospheric mantle heterogeneity before subduction

Norikatsu Akizawa , Kazuto Mikuni, Jennifer M. Lington, Christopher Yen, Takashi Miyazaki, Francesco Narduzzi, Takanori Kagoshima, Katie Preece , Hayato Ueda, Paola Vannucchi, Lottie Atton, Hirano Naoto, Shiki Machida, Takeshi Hanyu, Tomoki Sato, Bogdan S. Vaglarov, Qing Chang, Yuto Sato, Kenji Shimizu, Takayuki Ushikubo, Asuka Yamaguchi, Hiroko Kitajima , Natsumi Okutsu, Yuya Akamatsu , Olivier Fabbri, Matt J. Ikari, Taro Kido, Rachel Lauer, Kenji M. Matsuzaki, Ayumu Miyakawa, Yasuyuki Nakamura, Hanaya Okuda , Andrew P. Roberts, Louisa H. Stokes, Fuka Takuwa, Taizo Uchida, Edoardo Barbero, Chin-Yeh Chen, Jose-Abel Flores, Mila Huebsch, Yuji Kato, Eun Young Lee, Jason P. Morgan, Sebastien Pilet

Submarine petit-spot volcanoes form where oceanic plates bend and flex before sinking into subduction zones, providing a rare opportunity to sample mantle-derived melts before subduction. Here we report results from the first scientific drilling of a sediment-buried petit-spot volcano during International Ocean Drilling Programme Expedition 502, recovering basalts from a previously inaccessible interior of a petit-spot magmatic system. The recovered basalts exhibit a distinctive geochemical signature among oceanic intraplate magmas, combining Ni-rich olivine phenocrysts with unusually low Mg/(Mg + Fe2+), SiO2-rich bulk compositions, pronounced Ba enrichment and Th–U–Nb–Ta depletion, and the most enriched mantle 1 (EM1)-like Sr–Nd isotopic compositions yet recognized in petit-spot basalts. Thermodynamic modelling rules out fractional crystallization of dry or carbonated peridotite-derived parent melts and requires a contribution from enriched SiO2-excess pyroxenite. Fresh glasses further record CO2 depletion and low vesicularity relative to exposed petit-spot basalts. These findings reveal the petrological complexity of known EM1-type mantle heterogeneity, with isotopic signatures that can mask diverse mantle lithologies and melting processes that remain hidden from conventional seafloor sampling.