Cells harvest energy from ionic gradients by selective ion transport across membranes, and the same principle is recently being used for osmotic power generation from salinity gradients at ocean-river interfaces. Common to these ionic gradient conversions is that they require intricate nanoscale structures. Here, we show that natural submarine serpentinite-hosted hydrothermal vent (HV) precipitates are capable of converting ionic gradients into electrochemical energy by selective transport of Na⁺, K⁺, H⁺, and Cl^-. Layered hydroxide nanocrystals are aligned radially outwards from the HV fluid channels, constituting confined nanopores that span millimeters in the HV wall. The nanopores change the surface charge depending on adsorbed ions, allowing the mineral to function as a cation- and anion-selective ion transport membrane. Our findings indicate that chemical disequilibria originating from flow and concentration gradients in geologic environments generate confined nanospaces which enable the spontaneous establishment of osmotic energy conversion.

A Robust tectonic reconstruction of the Philippine Sea Plate around ∼52 Ma is a prerequisite in understanding the process of subduction initiation and establishment of the Izu‐Bonin‐Mariana arc. This study investigates origins and timing of basin formation in the still poorly understood oldest part of the northern Philippine Sea plate. We have established that andesitic magmatism in the form of the Northern Philippine Sea volcanics is widely distributed across this area. It is founded on both a Mesozoic arc terrane (Daito Ridge Group) and an intervening basin (Kita‐Daito Basin). Their narrow Eocene age range (45‐41 Ma) and lack of systematic spatial variation in geochemistry implies that this magmatism was not associated with on‐going subduction, but related to the rifting/spreading event forming the Kita‐Daito Basin. The arc‐like geochemistry of the volcanics seems to indicate melting of lithospheric mantle which had been previously metasomatized by Mesozoic subduction of a plate with Pacific‐MORB isotopic characteristics. Late Eocene basaltic magmatism also found in the Kita‐Daito Basin does not have arc‐like characteristics, and could have formed from low‐degree melts of asthenospheric mantle associated with the final stage of Kita‐Daito Basin spreading. As onset of activity of the Northern Philippine Sea volcanics is essentially synchronous with the magmatism associated with the Oki‐Daito mantle plume, it is possible that both magmatism and rifting of the Kita‐Daito Basin were triggered by the arrival of the Oki‐Daito mantle plume in this region. These results demonstrate that the Kita‐Daito Basin postdates subduction initiation of the Pacific Plate along the Izu‐Bonin‐Mariana arc.

Convergent margin igneous activity is generally limited to 100–200 km from the trench except where spreading ridges are subducted or in association with Subduction‐Transform Edge Propagators (STEP faults). The southernmost Mariana forearc, facing the Challenger Deep, subducts Mesozoic seafloor and is not in a STEP fault setting but includes at least one site where tholeiitic basalts recently erupted close to the trench, the SE Mariana Forearc Rift (SEMFR). We present evidence of young basaltic volcanism from ca. 100 km west of SEMFR. Shinkai 6500 diving during YK13‐08 (Dive 1363) recovered volcaniclastics from 5.5 to 6 km deep in the inner wall of the Mariana Trench, 50 km NE of the Challenger Deep. Glassy fragments are tholeiitic basalts similar to MORB except for much higher contents of magmatic water (approx. 2% H₂O vs. <0.2% H₂O in MORB) and enrichments in trace elements Rb ‐ Cs ‐ Ba , K , Pb , and Sr . Dive 1363 glasses are similar to basalts from SEMFR erupted near the trench and to Mariana Trough backarc basin basalts. Basalt fragments and palagonitized matrix dominate the studied samples, but small xenocrysts and xenoliths derived from mantle peridotite and Neogene volcanics are also present, probably torn from the vent walls. Dive 1363 hyaloclastites erupted at 3–6 km water depth accompanied by vigorous degassing of volatiles, most likely CO₂ . These results provide further evidence that the forearc adjacent to the Challenger Deep has been invaded by asthenospheric mantle and derivative hydrous melts. Extension, hydration, and melt invasion combine to further weaken Challenger Deep forearc lithosphere. Combined effects of: (i) absence of strong, cold lithosphere of the overriding plate; (ii) rapid rollback of a narrow, short subducted slab; and (iii) weak coupling between the subducting Pacific plate and the overriding Mariana plate may be responsible for the great depth of the Challenger Deep.