Ken-Ichi Hirauchi

Recent thermodynamic modeling indicates that C–O–H fluids released from subducting metasediments beneath the forearc can react with the overlying mantle wedge, causing carbonation in regions where episodic tremor and slip (ETS) events occur. However, the influence of serpentinization and carbonation on the rheological behavior of ETS‐generating shear zones remains poorly understood. To address this issue, we investigated the shallow mantle wedge‐derived Higuchi serpentinite body in the late Cretaceous Sanbagawa belt, central Japan. Our results show that infiltration of CO ₂ ‐rich fluids into massive antigorite serpentinite caused distributed extensional and extensional–shear fracturing under supra‐lithostatic pore fluid pressures ( P _f ). Such failure events, potentially analogous to bursts of tectonic tremor, led to transient drops in P _f and subsequent sealing by newly precipitated chrysotile, talc, and carbonate minerals, predominantly magnesite. An anastomosing network of chrysotile and talc governed the viscous behavior of the shear zone through dissolution–precipitation creep, potentially influencing slip rates during short‐term slow slip events. Subsequently, infiltration of externally derived, high‐flux CO ₂ ‐ and CaO‐rich fluids triggered large‐scale, dolomite‐rich extensional and shear veining. Mutual crosscutting relationships among the veins, together with paleostress analysis, indicate fluctuating principal stress orientations, with σ ₁ and σ ₃ axes occasionally switching positions under sustained near‐lithostatic P _f during the vein‐forming cycle. These findings suggest that mantle wedge carbonation is largely controlled by episodic, fracture‐mediated fluid flow, and its extent depends on the thickness of highly fluid‐overpressurized, ETS‐generating serpentinite shear zones.

The Nomo metamorphic rocks in western Kyushu, Japan, are distributed in the southwesternmost part of the Sanbagawa metamorphic belt; yet quantitative pressure-temperature (P-T) constraints have remained limited. Here, we apply Raman spectroscopy of carbonaceous material (RSCM) thermometry and quartz-in-garnet Raman barometry to pelitic schists from the biotite zone of the Sanwa Formation (metamorphic ages of 90-70 Ma) to constrain peak P-T conditions. RSCM thermometry of three samples yields peak metamorphic temperatures of 463-471 °C. Raman spectra of 92 quartz inclusions in garnet from three samples provide residual pressures of 0.46-0.57 GPa, which correspond to quartz entrapment pressures of 1.03-1.28 GPa within the garnet growth temperature range of ∼ 400-470 °C. These results suggest that the Sanwa Formation experienced peak P-T conditions of ∼ 460-470 °C at ∼ 1.0-1.3 GPa. The estimated temperatures overlap those reported for the garnet zone of the Sanbagawa belt in central Shikoku, whereas the inclusion-based pressures are higher than thermobarometric estimates for that zone. This discrepancy suggests that quartz inclusions in garnet may record conditions during prograde subduction along a Sanbagawa-type subduction-zone geotherm prior to exhumation.

Deep tectonic tremor occurs repeatedly at the base of a forearc mantle wedge corner, where a highly fluid-pressurized serpentinite shear zone is thought to develop. However, the deformation mechanisms that accommodate these tremors within the shear zone remain unclear. Here, we present observations of deformation experiments on water-saturated serpentinite conducted at pressure–temperature conditions relevant to the tremor zone. We find that increasing pore fluid pressure gradually decreases sample strength and leads to a transition in the deformation mechanism from frictional sliding on several fault surfaces to distributed extensional and extensional–shear fracturing. Combined with field observations of a shallow mantle-wedge-derived serpentinite shear zone, our experimental results suggest that numerous brittle failures developing simultaneously throughout the shear zone generate bursts of tectonic tremor. Furthermore, the recurrence interval of the tremors is likely controlled by the time required for the fractures to be hydrothermally sealed through serpentine precipitation.

The Mado Megamullion is an oceanic core complex (OCC) in the Shikoku back‐arc basin within the Philippine Sea Plate. Mantle peridotites (serpentinized) recovered by six dredge and submersible cruises exhibit signatures of extensive deformation. Amorphous pseudomorphs after plagioclase in many of the samples, as well as plagioclase‐spinel intergrowths, are clear evidence of melt stagnation and mantle reaction. Spinels show a wide range of compositions in terms of their Cr#, Mg#, and TiO₂ content. The presence of apparently magmatic high‐temperature pargasitic amphibole in veins and as replacement of clinopyroxene suggests that it may be a primary or near‐primary mineral crystallized from a hydrous melt which is unusual for abyssal peridotites. Two trace‐element populations of clinopyroxenes are in equilibrium with depleted and enriched basaltic melts, respectively. Rare‐earth element (REE) in the most depleted clinopyroxenes are modeled by 10% fractional melting except for a ubiquitous La‐Ce “kick.” Multiple models of open system melting combined with subsequent mixing of an enriched melt can explain the REE data. Broadly it appears that the peridotites underwent variable degrees of partial melting with moderate influx of enriched melts, which agrees with the other textural and chemical evidence of melt‐rock reaction and re‐fertilization. The compositions of the accumulated melts simulated by the open system models reproduce the enrichments in fluid mobile elements (Ba, U, and Pb) observed in basalts dredged from the Shikoku basin. Back‐arc basin peridotites at Mado Megamullion appear to have a unique petrographic and geochemical character that is distinct from those of peridotites exposed at the seafloor after formation from mid‐ocean ridges.

In the 1950s, the aim of the original mantle drilling projects was to obtain oceanic mantle samples in order to address the unanswered question of what constitutes the Earth's mantle. However, in the 21st century, it is widely accepted that the uppermost mantle is mainly composed of peridotite. Now, the challenge of mantle drilling is to understand crucial unsolved issues of earth science. Today's Earth is different from other planets due to the existence of life and plate tectonics. It is emphasized that mantle drilling is the only way to obtain the oceanic crust from top to bottom and an active mantle sample from an oceanic plate. The crucial issues that can only be addressed by mantle drilling are: (1) limits of life in an oceanic plate and its controlling factors, and (2) formation process of an oceanic plate and its modification. Modification of an oceanic plate, especially the weakening of plate strength, is required for plate tectonics. These two issues are interrelated. Long seismic profiles of oceanic plates reveal the diversity of Moho seismic reflection regions: clear, unclear, diffuse and non-Moho regions. Faults and/or fracturing in oceanic plates and subsequent seawater flow can modify oceanic plates locally, probably causing the diversity of oceanic Moho, as well as the rheological behavior of oceanic plates. Fluid flows along faults/fractures also extend the biosphere of oceanic plates. The first drilling sample should be a reference to the oceanic crust and the uppermost mantle, and define the nature of the Moho at the site, as well as constrain reasons for the diversity of the Moho in other areas. Deep sampling, such as mantle drilling in an old oceanic plate, can penetrate the biosphere/non-biosphere boundary, which tells us about the controlling factors of the limit of life. This information may help us find extraterrestrial life. After mantle drilling is completed, the borehole is the only window from the ocean floor to the mantle. An in-situ mantle observatory in the mantle hole to monitor plate movement and fluid flow with biological activity within an oceanic plate is also suggested. Detecting geoneutrinos at the mantle site allows the amounts and distributions of radioactive elements from the Earth's mantle to be measured. These provide basic information on the Earth's heat sources and the evolutionary history of the mantle.

We determined the mineralogical and petrological characteristics of ultramafic rocks dredged from two oceanic core complexes: the Mado Megamullion and 23°30′N non‐transform offset massif, which are located within the Shikoku back‐arc basin in the Philippine Sea. The ultramafic rocks are strongly serpentinized, but can be classified as harzburgite/lherzolite or dunite, based on relict primary minerals and their pseudomorphs. Strongly elongated pyroxene porphyroclasts with undulatory extinction indicate high‐temperature (≥700 °C) strain localization on a detachment fault within the upper mantle at depths below the brittle–viscous transition. During exhumation, the peridotites underwent impregnation by magmatic or hydrothermal fluids, lizardite/chrysotile serpentinization at ≤300 °C, antigorite crystallization, and silica metasomatism that formed talc. These features indicate that the detachment fault zones formed a fluid pathway and facilitated a range of fluid–peridotite interactions.

Slow earthquakes, including low‐frequency earthquakes and tremor, occur in the brittle‐ductile (plastic) transition zone on plate boundary faults. To understand how transitions in deformation mechanisms can influence seismic behavior, we conducted stick‐slip experiments on halite gouge at normal stresses (σ_n) of 8 to 113 MPa. In the brittle regime (σ_n ≤ 18 MPa), the halite gouge showed fast and regular stick‐slip associated with velocity‐weakening behavior. In contrast, increasing the normal stress within the semibrittle regime (σ_n ≥ 27 MPa) led to a significant decrease in slip velocity and stress drop, which was associated with a transition from velocity‐weakening to velocity‐strengthening behavior. Local stress measurements along the simulated faults, made using strain gauges, revealed that the critical nucleation length (L_c) increased with increasing normal stress. Macrostructural and microstructural observations showed that the transition from fast dynamic slip to slow quasi‐static slip could be attributed to an increasing contribution from semibrittle/plastic deformation occurring within domains that are heterogeneously distributed within the gouge layer. Our findings suggest that brittle‐plastic fault heterogeneity, which may be generated by spatial variations in pore fluid pressure and/or lithology, contributes to the emergence of slow earthquakes.

To understand the seismogenic potential of shallow plate‐boundary thrust faults (décollements) in relatively warm subduction zones, water‐saturated Na‐montmorillonite gouges were sheared at a pore fluid pressure of 10 MPa, effective normal stresses (σ_n^eff) of 10–70 MPa, temperatures (T) of 25–150°C, and axial displacement rates of 0.03–3 μm/s. The Na‐montmorillonite gouges were frictionally very weak at all conditions tested (steady state friction coefficient μ_ss = 0.05–0.09). At T ≤ 60°C, Na‐montmorillonite showed a transition from velocity‐strengthening to velocity‐weakening behavior with increasing σ_n^eff, whereas at T ≥ 90°C it was largely velocity neutral or velocity strengthening, irrespective of σ_n^eff. The rates of frictional healing (β) showed extremely low values (mostly <0.001) at all temperatures. Our results suggest that the existence of Na‐montmorillonite in the décollement zone at Costa Rica and Nankai promotes aseismic slip, particularly at shallow depths, forming weakly coupled regions.

Earth is the only terrestrial planet in our solar system where an oceanic plate subducts beneath an overriding plate. Although the initiation of plate subduction requires extremely weak boundaries between strong plates, the way in which oceanic mantle rheologically weakens remains unknown. Here we show that shear-enhanced hydration reactions contribute to the generation and maintenance of weak mantle shear zones at mid-lithospheric depths. High-pressure friction experiments on peridotite gouge reveal that in the presence of hydrothermal water, increasing strain and reactions lead to an order-of-magnitude reduction in strength. The rate of deformation is controlled by pressure-solution-accommodated frictional sliding on weak hydrous phyllosilicate (talc), providing a mechanism for the ‘cutoff’ of the high peak strength at the brittle-plastic transition. Our findings suggest that infiltration of seawater into transform faults with long lengths and low slip rates is an important controlling factor on the initiation of plate tectonics on terrestrial planets.