Petit-spot volcanoes erupt on old oceanic plates far from mid-ocean ridges. Their magmas are characterised by exceptionally high initial CO2 concentrations of up to ∼10 wt%. Although petit-spot magmas have been inferred to originate from CO2-rich melts at the lithosphere–asthenosphere boundary, their mantle source and melting mechanism remain debated. Here, we determined the noble gas and CO2 compositions of vesicle gases in a glassy petit-spot basalt, 6K#1466R3–003 (1466R3), from the western Pacific plate using stepwise crushing and laser ablation. We then estimated the initial noble gas concentrations of the 1466R3 magma. The highest measured 20Ne/22Ne and 40Ar/36Ar ratios are 12.56 ± 0.11 and 24,100 ± 1100, respectively. The estimated mantle 21Ne/22Ne ratio of 0.0563 ± 0.019 is similar to those of mid-ocean ridge basalts (MORBs). The inferred noble gas elemental and isotopic ratios of the 1466R3 magma are similar to those of typical MORB magma. The δ13C values of CO2 (−4 to −7‰) are consistent with those of MORBs. The CO2/22Ne ratio in the 1466R3 magma is approximately twice that of average MORB magma, suggesting a heterogeneous carbon distribution in the upper mantle. The initial noble gas concentrations in the 1466R3 magma are >10-fold higher than those of average MORB magma. The extremely high noble gas concentrations suggest low-degree partial melting of the source mantle, with a melt fraction of ≤1%. These results provide evidence for the generation of CO2- and noble-gas-rich 1466R3 petit-spot magma by low-degree partial melting of the upper mantle.
With the continuous growth of data storage demand, precise and stable magnetic-head positioning in hard disk drives (HDDs) remains essential. This paper proposes a disturbance-rejection-oriented control design for triple-stage actuator (TSA) systems. The approach employs Bode-plot analysis to achieve robust performance against plant perturbations while explicitly incorporating the stroke limits of Lead Zirconate Titanate (PZT) micro-actuators into controller synthesis. Stroke constraints are represented as forbidden regions on the Bode diagram, providing an intuitive feasibility criterion for controller design. The physical stroke limitation is formulated as an inequality condition and embedded directly in the loop-shaping procedure. Performance is evaluated through hybrid simulations in which numerical plant models identified from measured actuator frequency responses are driven by disturbance signals generated from measured disturbance characteristics. The results demonstrate that the proposed controllers significantly reduce the risk of micro-actuator saturation while preserving high positioning accuracy.
We construct a class of regular black hole solutions of the Fan-Wang type within quasi-topological gravity (QTG) in arbitrary spacetime dimensions greater than four. In contrast to the original Fan-Wang solution, which was obtained in four-dimensional general relativity coupled to nonlinear electrodynamics, our higher-dimensional generalization does not require any matter fields. Instead, regularity is achieved purely through an infinite tower of higher-curvature corrections. We demonstrate that the Fan-Wang-type metric is a solution to the QTG field equations by explicitly determining the corresponding coupling constants for each curvature order. Within an appropriate parameter regime, the solution describes an asymptotically flat black hole spacetime with a regular center. Remarkably, even in the case of negative mass, the geometry can remain completely regular, in sharp contrast to Einstein gravity.
We present independent polarimetric observations of the interstellar object 3I/ATLAS, including the first near-infrared polarimetric measurements. Using imaging polarimeters, we measured the degree of linear polarization from the visible RC band (0.64 mu m) to the near-infrared Ks band (2.25 mu m), and investigated its dependence on solar phase angle (polarization phase curve, PPC) and wavelength (polarization color curve, PCC). We confirm that the PPC of 3I/ATLAS differs significantly from those of typical solar system comets, showing an unusually large polarization amplitude. This PPC shows no significant change in the RC band across perihelion passage, despite the perihelion lying within the water snow line. This indicates that the unusual polarimetric behavior of 3I/ATLAS is unlikely to be driven by transient volatile activity, but instead reflects intrinsic optical properties of refractory dust particles. The PCC increases with wavelength over 0.6-1.2 mu m and peaks at 1.5-2.0 mu m, suggesting that the dominant scattering units are dust aggregates composed of submicron-sized monomers, broadly consistent with interstellar dust and solar-system cometary aggregates. Taken together, our results indicate that 3I/ATLAS preserves polarimetric properties characteristic of a primitive cometary planetesimal formed in another planetary system, with a refractory dust composition that differs from that typically observed among solar system comets, despite sharing a similar size scale of the aggregate building blocks.
The Shikoku Basin, a Miocene back-arc basin in the Philippine Sea, hosts numerous oceanic core complexes (OCCs) that reflect melt-poor lithospheric extension. Based on bathymetric mapping, gravity and magnetic surveys, dredging, and submersible dives conducted between 2007 and 2023, we identified multiple OCCs in its southern part, including the Mado, Sui-Shin, Tosa, Sanuki, and Awa Megamullions. These OCCs expose mantle and lower crustal rocks and are associated with high mantle Bouguer anomalies, indicating thin crust. Geochemical and geochronological data from the Sui-Shin Megamullion suggest its formation immediately after rifting of the proto-Izu-Bonin arc ( 24 Ma). The Tosa Megamullion, dominated by plagioclase-peridotites with only minor gabbros, exemplifies melt-poor lithospheric conditions and may represent Hess-type oceanic crust. The spatial and temporal distribution of OCCs, including the triplet OCCs (Tosa, Sanuki, and Awa), implies multiple ridge jumps during basin evolution. These findings suggest that a significant portion of the Shikoku Basin formed largely through amagmatic or melt-poor processes, contrasting with typical mid-ocean ridge settings. The OCCs serve as tectonic windows into back-arc lithosphere evolution and provide a framework for future investigations into the role of melt supply and slab-derived fluids in shaping oceanic crustal architecture.
