Petit-spot, the fourth kind of universal volcanic activity on the Earth after island arcs, mid ocean ridges, and hotspots, is expected to originate from the lithosphere-asthenosphere boundary. Therefore, the study of petit-spot basalts potentially provides information about the nature of the asthenosphere as a fundamental theory of plate tectonics. In this study, the analysis of major and trace element compositions, Sr, Nd, and Pb isotopic ratios, and Ar-Ar datings are performed for western Pacific petit-spot volcanoes to investigate the chemical characteristics of upper mantle below the oldest portion of the Pacific Plate. We report the nature of melting source of the petit-spot magmas on the basis of their geochemistry and some modelled compositions of partial melt from carbonated materials.

Recently, some Paleogene Ar-Ar ages were recently reported at Minamitorishima Island, the Chuo Seamount on Ogasawara Plateau and Uyeda Ridge located in the Western Pacific Seamounts Province (WPSP) which is an area recognized as group of short-lived Cretaceous hotspot tracks. Although the Ogasawara Plateau and Minamitorishima Island were already formed in Early Cretaceous, it is impossible to recognize as a continuous eruption since Early Cretaceous because of their too long hiatus. In this study, we analyzed acid-leached Sr, Nd, Pb isotopic composition and calculated the eruptive region based on Ar-Ar ages of the Paleogene lava sample to discuss the origin of the magma. The samples from four seamounts of the Cretaceous hotspot seamount trails (N-Wake) were simultaneously analyzed as well. The Paleogene samples from Minamitorishima Island and Chuo Seamount are clearly discriminated from the Cretaceous hotspot seamounts in both isotopic composition and eruptive site .

Submarine petit-spot magmatisms induced by lithospheric flexure provide some injections into old lithosphere and submarine environment near trench prior to its subduction. Although Ar-Ar age estimates were poorly constrained for lava samples because of recoil and excess Ar, eight holocrystalline samples provide the good plateau ages. In a further attempt to determine the age of the lavas, we separated zircon crystals from peperite samples, which show upper limit of eruption age. Older zircons, moreover, were dated at various ages all younger than the crustal age, implying submarine sediments were annealed and highly disturbed due to the petit-spot magma. Our Ar-Ar and U-Pb age data of lavas and peperites provide a new understanding of the lithosphere and submarine environment of the NW Pacific Plate.

Petit-spot basalts are geochemically classified as oceanic island basalts (OIBs) influenced by the EM-1 component. The regional differences in trace element compositions of EM-1 OIBs provide clues to the heterogeneity of mantle and the conditions of origin of petit-spot magmas. In this study, the whole-rock trace element compositions of the world EM-1 OIBs are performed by principal component analysis to find characteristic elemental behaviors using the multidimensional elemental data. The selected principal components suggest that many hot spot volcanoes and some petit spot volcanoes are affected by different principal components. Petit-spot volcanoes on the NW Pacific Plate, Site C, were strongly influenced by both principal components. The relatively diverse compositions even in the same area suggest that petit-spot magmas originate from small-scale and heterogeneous mantle.

Thermalized neutrons arising at the surface of solar planets are produced from the interaction of cosmic rays with the nucleus consisting of surficial materials. The neutron energy spectrum in the range between thermal and epithermal regions at the surface of the material can be investigated based on the combination of the isotopic variations of Sm and Gd caused by the thermal neutron-capture reactions in our previous study, with those of Dy, Er, and Yb caused by the epithermal neutron-capture reactions.

Recently, Lee and Tanaka [1-2] developed a method to accurately measure the Eu isotope ratio using MC-ICP-MS. Lee and Tanaka [3] also reported an Eu isotope fractionation (¹⁵¹Eu enrichment compared to ¹⁵³Eu) in the highly fractionated igneous rocks with extremely large Eu negative, and proposed that Eu isotope fractionation in the highly fractionated igneous rocks might be related with anorthosite emplacement due to feldspar crystallization. The anorthosite at Sancheong-Hadong area in the southern part of Korean Peninsular is found as long belt. The Sancheong-Hadong anorthosites have a geochemical characteristic of extremely large Eu positive anomaly in the chondrite-normalized REE pattern. Therefore, in this study, we had performed an experiment to determine the Eu isotope ratio of the Korean anorthosite in order to confirm the idea by Lee and Tanaka [3]. Our results clearly showed a geochemical feature that the anorthosite was enriched in ¹⁵³Eu isotope enrichment compared to ¹⁵¹Eu isotope. In addition, the relationship between Eu anomaly in the chondrite-normalized REE pattern and Eu isotope ratio from igneous rock materials indicates that Eu isotope fractionation in igneous rocks should be derived from feldspar crystallization during magmatic differentiation [1] Lee, S-G. and Tanaka, T. (2019) Determination of Europium isotopic ratio by multi-collector inductively coupled plasma mass spectrometry using a Sm internal standard, Spectrochm. Acta Part B, v.156, 42-50.[2] Lee, S-G. and Tanaka, T. (2021) Gd matrix effects on Eu isotope fractionation using MC-ICP-MS: Optimizing Europium isotope ratio measurements in geological rock samples. Int.J.Mass.Spec.116668.[3] Lee, S-G. and Tanaka, T. (2021) Eu isotope fractionation in highly fractionated igneous rocks with large Eu negative anomaly Geochem. J. v.55, No.4, e9-e17.

Understanding the fractionation mechanisms of the stable isotopes of redox-active metals in terrestrial records is a prerequisite for refining the application of redox reconstruction. We measured the Ce concentration, oxidation state, and isotope composition of bauxites forming on the Columbia River Basalts (CRBs) to evaluate the impacts of redox and non-redox processes on the Ce isotopic variability on Earth’s surface. Bauxites recovered from drill cores show upward increasing Ce concentration (5.4–88.7 μg/g) and overall depletion of Ce (mass transfer coefficient, τCe,Nb: -0.96 to -0.52, i.e., net depletion as much as 52-96% relative to their parent CRBs). There is a wide range of the Ce anomaly (Ce/Ce*) from 0.4 to 6.3 and 0.6 to 1.4 in the Cowlitz and Columbia bauxites, respectively. The most positive Ce anomalies appear at the shallow regolith (2-5 m depth). Cerium primarily presents in its trivalent form in the dust and CRBs, and the downward increase in the fraction of Ce(IV) in the bauxites corresponds with Mn (III/IV) enrichment. Regolith δ142Ce ranges from -0.16±0.03 to -0.02±0.04‰ and -0.28 ±0.03 to -0.05±0.03‰ in the Cowlitz and Columbia profiles, respectively. There are negative Ce isotope shifts from the basaltic parents (CRBs, δ142Ce from -0.08±0.03 to -0.01±0.03‰) in the deep regolith (5 to 9 m), consistent with the enrichment of high-valence Ce and Mn. Regolith Ce isotope records primarily reflect the redox cycle of Ce and may be masked by dust addition on the top. The vertical difference between Ce/Ce* and δ142Ce may be explained by the oxidation-reduction cycle of Mn, leaving discrete CeO2 in the shallow regolith while transferring isotopically light Ce linked to Mn (III/IV) deposits down to the deep regolith. We conclude the combination of Ce isotopes and anomalies in terrestrial records probably records Ce redox fluctuation.

Ion-adsorption type rare earth elements (REEs) located in tropical and subtropical zones have abundant movable and bioavailable ion-exchangeable REEs and could be an environmental hazard. However, our understanding of their environmental risk in urban areas is limited. We aimed to determine whether ion-adsorption type REEs in Guangzhou represent a kind of potential “Chemical Time Bomb” (CTB) and assess the environmental risk. We conducted a comprehensive survey of REEs in 181 samples including regolith (n = 70), surface water (n = 55), sediment (n = 25), vegetables (n = 22) and rhizosphere soil (n = 9), collected from five regions around Guangzhou, as a representative city of ion-adsorption type REEs in tropical and subtropical zones. The existing environmental risk was assessed by calculating the estimated daily intake (EDI) of REEs through vegetable consumption, and leaching simulation experiments were used to discuss the factors affecting the long-term stability of REEs. The average REEs concentrations (ΣREEs) in the regolith and sediment were 458.5 and 218.6 μg·g ⁻¹, respectively, which were higher than the background values of regolith (197.3 μg·g ⁻¹) and sediment (173.3 μg·g ⁻¹), and large proportions of ion-exchangeable REEs were observed in regolith and sediment, indicating that ion-adsorption type REEs in Guangzhou are a kind of potential CTB. The average ΣREEs in surface water (3.9μg·L ⁻¹), rhizosphere soil (466.9 μg·g ⁻¹) and vegetables (25.0 μg·g ^-1 dw) suggest that REEs have migrated to the supergene environment even organisms. The average EDI (55.4 μg·kg−1 d−1) close to the safety limitation (70 μg·kg ⁻¹ d ⁻¹) suggests that the existing health risk is very worrisome Human factors, including acid rain, mining and farming, probably ignite the CTB, causing the release of REEs to the urban environment on a large scale. This prospective study demonstrated that REEs exposure problems in urban areas of ion-adsorption type REEs should not be ignored.