Noble gas and carbon isotopic compositions of petit-spot lavas from southeast of Marcus Island

Abstract

We measured noble gas isotopic compositions of quenched lavas sampled from seamounts, so-called petit-spot volcanoes, on the 160-million-year-old northwestern Pacific Plate. The samples 3 He/ 4 He and 40 Ar/ 36 Ar ratios were, respectively, 2.5-8.3 Ra and up to 1735, where Ra stands for atmospheric 3 He/ 4 He, which are analogous to or lower than those of MORB. Considering narrow sampling regions, a secondary effect might be responsible for variation of the data. During ascent and subsequent cooling of magma in the oceanic lithosphere, chemical components in the magma will be assimilated with those in the lithosphere. Correlation between CO 2 / 3 He ratios and carbon isotopic ratios suggests that carbon was affected by the incorporation of seafloor carbonate. The same would be true of noble gases. The mixing of noble gases among a mantle source, an atmospheric component dissolved in seawater and a radiogenic component can explain the data distribution. No 3 He/ 4 He ratio exceeds the MORB-like value. The mantle source of the petit-spot magma was likely to have had a MORB-like 3 He/ 4 He ratio originally. The eruption of petit-spot magma shows a close relation with the bending of subducting oceanic plates. The MORB-like 3 He/ 4 He ratio supports the hypothesis that the petit-spot magma is derived from the lithosphere-asthenosphere boundary.

Carbonatite-induced petit-spot melts squeezed upward from the asthenosphere beneath the Jurassic Pacific Plate

“…1c; Lu et al, 2019). Furthermore, the MORBlike to more depleted noble-gas isotopic compositions of the petit-spot knoll (6K#1203, 1206) suggested its upper mantle origin (Yamamoto et al, 2018). Along with the outer-rise bulge in front of the Mariana Trench detected through a positive gravitational anomaly (Hirano et al, 2019), these data suggested that the western Pacific petit-spot volcanoes could have erupted at ~0-3 Ma owing to the flexure of the subducting Pacific Plate into the Mariana and Ogasawara Trenches.…”

“…Such a subduction-related fore-bulge in front of the Mariana Trench has been numerically modeled and detected in satellite gravity maps despite crosscutting by several seamounts (Bellas et al, 2022;Hirano et al, 2019;Zhang et al, 2014Zhang et al, , 2020. The petrography, geochemistry, and geochronology of petitspot basalts and detrital zircons in peperites, which were collected from a knoll, suggested that petitspot magmas in this region ascend from the asthenosphere along the concavely flexed plate in response to subduction into the Mariana Trench at younger than ~ 3 Ma (Yamamoto et al, 2018;Hirano et al, 2019). Below the study area, low seismic velocity zone was observed under the lithosphere (Fig.…”

“…Intracontinental alkali basalts have been reported in northeastern China (e.g., Lei and Zhao, 2005;Ohtani and Zhao, 2009), and the North American Basin and Range province (Axen et al, 2018;Valentine and Hirano, 2010). Non-plume related intraoceanic alkali volcanoes, called petit-spot volcanoes, probably originate where nearby plate subduction causes plate flexures and upwelling of asthenospheric magma (Hirano et al, 2006;Hirano and Machida, 2022;Machida et al, 2015Machida et al, , 2017Yamamoto et al, 2014Yamamoto et al, , 2018Yamamoto et al, , 2020.…”

“…Considering that the mid ocean ridge basalt (MORB)-like noble-gas isotopic compositions and the multiphase saturation experiments of petit-spot lavas confirm the petit-spot melts originating from the asthenosphere, petit-spot volcanoes could be a key to elucidating the nature of the LAB, leading to an understanding of plate tectonics (Hirano et al, 2006;Machida et al, 2015Machida et al, , 2017Yamamoto et al, 2018). Recently, similar volcanic activities have been observed worldwide, implying the universal occurrence of petit-spot magmatism (Axen et al, 2018;Buchs et al, 2013;Falloon et al, 2022;Hirano et al, 2013Hirano et al, , 2016Hirano et al, , 2019Reinhard et al, 2019;Taneja et al, 2016;Uenzelmann-Neben et al, 2012;Yamamoto et al, 2018Yamamoto et al, , 2020Zhang et al, 2019).…”

“…Considering that the mid ocean ridge basalt (MORB)-like noble-gas isotopic compositions and the multiphase saturation experiments of petit-spot lavas confirm the petit-spot melts originating from the asthenosphere, petit-spot volcanoes could be a key to elucidating the nature of the LAB, leading to an understanding of plate tectonics (Hirano et al, 2006;Machida et al, 2015Machida et al, , 2017Yamamoto et al, 2018). Recently, similar volcanic activities have been observed worldwide, implying the universal occurrence of petit-spot magmatism (Axen et al, 2018;Buchs et al, 2013;Falloon et al, 2022;Hirano et al, 2013Hirano et al, , 2016Hirano et al, , 2019Reinhard et al, 2019;Taneja et al, 2016;Uenzelmann-Neben et al, 2012;Yamamoto et al, 2018Yamamoto et al, , 2020Zhang et al, 2019). Although there is still an open question of whether the LAB discontinuity is due to the differences in the physical properties of minerals (e.g., Hirth and Kohlstedt, 1996;Karato and Jung, 1998;Katsura and Fei, 2020;Stixrude and Lithgow-Bertelloni, 2005;Wang et al, 2006), presence of partial melts (e.g., Chantel et al, 2016;Conrad et al, 2011;Debayle et al, 2020;Kawakatsu et al, 2009;Mierdel et al, 2007;Sakamaki et al, 2013;Yoshino et al, 2006), or hybrid factor (e.g., Audhkhasi and Singh, 2022;Herath et al, 2022), the occurrence of petit-spot volcanism reveals the partial melting of the asthenospheric mantle of the region (Hirano et al, 2006;Hirano and Machida, 2022;Machida et al, 2015Machida et al, , 2017…”