Unintended local temperature enhancement by excitation laser might change Raman spectral features and potentially lead to misinterpretation of the data. To evaluate robustness of Raman CO2 densimeters in the presence of laser heating, we investigate the relation between temperature (T, °C), density (ρ, g/cm3), and Fermi diad split (Δ, cm−1) using a high‐pressure optical cell at 23°C to 200°C and 7.2–248.7 MPa. Results indicate that Δ decreases concomitantly with increasing temperature for a constant density in all density regions investigated. This result suggests that the density estimated based on Δ might be underestimated if the fluid is heated locally by the laser. Combining results of earlier studies with those of the present study indicates that the temperature dependence of Δ (|(∂Δ/∂T)ρ|) has a maximum value around 0.6–0.7 g/cm3. Consequently, at very high densities such as 1.1–1.2 g/cm3, |(∂Δ/∂T)ρ| is small. Thus, Δ at such densities is less affected by laser heating. However, at densities below approximately 0.7 g/cm3, although |(∂Δ/∂T)ρ| becomes smaller at lower densities, the relative density decrease becomes larger even for a small density decrease because the density itself becomes smaller. Therefore, at such densities, a density decrease of more than 10% was observed for some fluid inclusions, even at typical laser powers for inclusion analysis. Finally, to accurately estimate the density even in the presence of laser heating, we show that it is effective to estimate the intercept Δ from the correlation between Δ and laser power and substitute it into Δ–ρ relations.

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.