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Natural springs containing volcanic and magmatic components occur in association with these activities. However, features of deeply originated fluids and solutes were less documented from fields, where active volcanic and magmatic activities are not distributed. To characterize the presence of deep components and identify their major pathways 28 groundwater samples (~ 1230 m deep) were collected from hot spring sites located at western coast of Kumamoto, southwestern Japan, where the typical subduction related magmatisms are absent. The samples were measured for dissolved ion concentrations and stable isotope ratios (δ2HH2O, δ18OH2O, δ13CDIC and δ34SSO4) that were compared with data of 33 water samples from vicinity surface systems. The groundwaters were classified into three types based on major hydrochemistry: high Cl− fluid, low concentration fluid, and high HCO3− fluid. Our data set suggests that the high Cl− fluid was formed by saline water mixing with aquifer waters of meteoric origin and subsequently evolved by reverse cation exchange. The low concentration fluid is identical to regional aquifer water of meteoric origin that was subjected to cation exchange. The high HCO3− fluid showed the highest HCO3− concentrations (~ 3,888 mg/l) with the highest δ13CDIC (up to − 1.9‰). Based on our carbon mixing model and observed δ2HH2O and δ18OH2O shifts, it is suggested that dissolved carbon of mantle origin and small fraction of fluids generated in deep crust were transported towards surface through structural weakness under open tectonic setting. These deeply derived components were then mixed with waters in the surface systems and diluted. Their impacts on surface hydrological systems were limited in space except few locations, where deeply connected pathways are anticipated along active structural deformations. [ABSTRACT FROM AUTHOR]

In the subducting oceanic lithosphere, a significant part of the seismicity is triggered in the mantle, especially along the upper and lower Wadati-Benioff planes. Several studies have investigated the potential involvement of dehydration reactions in the triggering mechanism of mantle earthquakes. Recent experimental results reveal that, under subduction conditions, mechanical instabilities nucleate in strong stable mineral aggregates during the destabilization of minor amounts of antigorite, i.e. the high-temperature serpentine, through a stress transfer, without any fluid overpressure. Here I confront these laboratory results to seismological observations. On one hand, most of the natural hydrous magnesium silicates seem to be known, with experimentally deduced stability limits up to 7 GPa, at least, available as relatively accurate estimates. On the other hand, recent achievements in thermal structure of subduction zones combined with precise hypocentre relocation give access to pressure and temperature conditions at earthquakes hypocentres. A series of P-T diagrams summarizes the stability limit of minerals that may be part of natural peridotites or serpentinites with variable compositions at pressures from 0.5 to 6 GPa and temperatures from 200 to 950 °C, and compares it with seismicity. Both hydrous and anhydrous phases are considered. A myriad of minor metamorphic reactions could participate in a transformation-driven stress transfer, even if the stability limits of serpentine minerals seem to correlate with most of the observed seismicity. • Minor mineral phases play a major role in the triggering mechanism of mantle earthquakes. • A myriad of metamorphic reactions may participate in transformation-driven stress transfers within the subducting mantle. • Mantle seismicity within slabs is far from fitting the ideal dehydration curve of antigorite in a P-T diagram. • Considering variable paragenesis, chemistry and H2O saturation, serpentine stability limits correlate with most seismicity. • The lower Wadati-Benioff plane is due to both the dehydration reactions and the hydration limit of the subducting plate. • The upper plane of cold subducting mantle could be due to H2O transfer from low-grade serpentine minerals to antigorite. • The sharp seismicity increase at 90 km, 3 GPa, beneath Japan and Chile, would indicate H2O transfer to HP hydrous phases. [ABSTRACT FROM AUTHOR]

For a better understanding of plate dynamics such as the opening of Japan Sea back-arc basins, it is critical to know why and how back-arc spreading/rifting occurs adjacent to convergent plate boundaries. Two models have been proposed for the formation of Japan Sea back-arc basins: the "slab rollback model" and the "plume model". We report here on 12 spinel peridotite xenoliths in the alkali basalt of the Cenozoic Kawashimo volcano in southwest Japan. These xenoliths are dominantly harzburgite with some lherzolite. The harzburgites and lherzolites show coarse-grained or porphyroclastic textures that record variable degrees of deformation. Olivine and spinel compositions indicate the samples are residual mantle peridotites formed under various degrees of partial melting. Olivine crystallographic preferred orientations in the xenoliths have orthorhombic patterns characterized by a strong [010] maximum, and this orthorhombic pattern implies that the olivine deformed by dislocation creep on the (010)[100] slip system. Using a sub-grain size piezometer, the maximum differential stress varies from 3 to 13 MPa in the samples. We also found that the peridotite xenoliths deformed under relatively high strain-rates, such as 10−13 to 10−10 s−1, using olivine flow laws and the obtained stress and grain size. The range of high strain-rates is comparable to that predicted using a thermomechanical model of back-arc spreading/rifting. Therefore, it is probable that the peridotite xenoliths preserve textures that formed during the mantle deformation that accompanied the Japan Sea back-arc spreading/rifting. We estimated an equilibrium temperature of 1238 ± 20 °C using the two-pyroxene geothermometer, and this temperature is higher than any temperature reported previously for peridotite xenoliths in southwest Japan. The obtained equilibrium P–T condition for the xenoliths implies that the hot mantle plume hypothesis best explains the mechanism of formation of Japan Sea back-arc basins. • Spinel peridotite xenoliths with coarse-grained or porphyroclastic textures were deformed at ∼1200 °C and ∼2 GPa. • Deformation involved dislocation creep and a relatively high strain rate (10−13 to 10−10 s−1). • The peridotites preserve deformation textures formed during the opening of Japan Sea back-arc basins. [ABSTRACT FROM AUTHOR]

It has long been thought that the low-K tholeiitic (TH) and medium-K calc-alkaline (CA) lavas in the NE Japan arc were produced by fractional crystallization from mantle-derived basalt magmas, and that the latter formed from mixing of mafic and felsic magmas, both derived from a common primary TH basalt through fractionation. An alternative view was recently proposed on the basis of Sr isotope microanalysis of plagioclase phenocrysts from the Zao volcano, suggesting that (1) the low-K TH basaltic andesites formed by melting of lower crustal amphibolite and that (2) the medium-K CA basalts to andesites formed by mixing of mantle-derived basalt and crustal TH melts. To investigate further the origin of the ‘primary’ low-K TH and medium-K CA basalts, we investigated basalts and andesites from Azuma volcano. Azuma is a Quaternary eruption center at the volcanic front in the NE Japan arc that has erupted two types of basalt: (1) radiogenic-Sr (€87Sr/€86Sr = 0·7058–0·7062) low-K TH basalt lavas without evidence of magma mixing and assimilation; (2) unradiogenic-Sr (€87Sr/€86Sr = 0·7039–0·7041) medium-K CA basalt lavas with subtle evidence for magma mixing. Associated intermediate lavas are voluminous and are all (3) mildly radiogenic-Sr (€87Sr/€86Sr = 0·7044–0·7055) medium-K andesites, all of which have CA affinities with evidence for rigorous magma mixing but no crustal assimilation. The low-K TH basalt has an isotopic composition similar to that of crustal granitoids beneath Azuma and has a composition indicating that it potentially formed from a high-degree lower crustal amphibolite melt. The medium-K CA basalt has a basaltic groundmass with Mg-rich olivine (Fo89) and calcic plagioclase phenocrysts (An90) and the most unradiogenic Sr (€87Sr/€86Sr = 0·7037–0·7038), suggesting that it originated from a primary mantle melt. Major and trace element microanalysis of the basaltic groundmass indicates that the primary magma composition is close to high-K. We conclude that the mantle-derived basalt at Azuma is the result of a high- to medium-K magma that was later mixed with a low-K TH basalt melt from the amphibolitic lower crust to form medium-K CA basalts and andesites. This supports the view of a lower crustal origin of the low-K TH basalts and simultaneously requires a reappraisal of the origin of the across-arc variation in K contents of the mantle-derived primary arc basalts, as the high- to medium-K CA basalt is geochemically fairly similar to the high-K rear-arc basalt in the NE Japan arc. [ABSTRACT FROM PUBLISHER]

Abstract: Identifying the amount and composition of slab-derived fluid and its spatial variation is key to quantifying fluid processes in subduction zones. Based on the isotopic systematics of arc lavas, we found regional variations over the Japan arcs in terms of the amount and composition of slab-derived fluid added to the melting source region. The average amounts of slab-derived fluid differ among the arcs: 2.6 wt.% for Central Japan, 0.69 wt.% for Ryukyu, 0.17 wt.% for NE Japan, and 0.12 wt.% for both Kurile and Izu–Bonin. These differences may be attributed to the arc setting (oceanic or continental) and the geometry of the slabs. Contribution of sediment involved in the slab-derived fluid is dominant in NE Japan compared to the Izu–Bonin and Central Japan arcs. This could be attributed to mechanical features such as fractures near the subducting plate surface, in addition to the arc setting and the slab geometry. Therefore, the amount and composition of slab-derived fluid are thought to be controlled not only by the thermal conditions, but also by the tectonic and mechanical features around the subduction zone. On top of the variability of slab-derived fluid, the mantle wedge shows the regional variation in terms of proportion of the Pacific-type and Indian-type MORB-source mantle components, which also contributes to the compositional variations of arc magmas. [Copyright &y& Elsevier]

Abstract: We examine the upper mantle and transition zone beneath the western Pacific using multiple ScS reverberations. A low-velocity zone (LVZ) is found above the 410-km discontinuity oceanward of the subducting Honshu slab at an average depth of 356km, with a thickness that ranges from 50 to 75km assuming the LVZ continues to the 410-km discontinuity, which is locally elevated. The low-velocity region is evident in previous tomographic studies, and our results suggest that the anomaly is best explained by a layer of partial melt. The layer may be entrained from above by subduction or produced in situ by the combined effects of water and temperature. A self-consistent model that explains local P-wave velocities (Obayashi, M., Sugioka, H., Yoshimitsu, J., Fukao, Y., 2006. High temperature anomalies oceanward of subducting slabs at the 410-km discontinuity. Earth and Planetary Science Letters 243, 149–158) and our observations calls for a maximum temperature anomaly of ∼150°C and a resulting maximum olivine water content after melting of 0.100wt. %. [Copyright &y& Elsevier]

Two distinctive differentiation trends, tholeiitic and calc-alkalic, are recognized in Zao volcano, which is located immediately behind the volcanic front of the NE Japan arc. The genetic relationship between these two magma series is critical for a better understanding of andesite genesis, because they often coexist in close spatial and temporal proximity in arc volcanoes. Petrographic features indicative of ‘disequilibrium’, such as reversely zoned pyroxene phenocrysts, the wide and bimodal compositional distribution in Ca/(Ca + Na) of plagioclase phenocrysts, honeycomb textures and dusty zones that these plagioclase phenocrysts often exhibit, and the presence of olivine–pyroxene pairs with different Mg/Fe, are observed exclusively in calc-alkalic rocks. In tholeiitic rocks the Sr isotopic ratios of plagioclase phenocrysts, determined by both micromilling combined with thermal ionization mass spectrometry, and laser-ablation inductively coupled plasma mass spectrometry techniques, are constant at 0·7042–0·7044. On the other hand, those in calc-alkalic rocks (0·7033–0·7042) show more complex characteristics, which can be best understood if at least three end-member components, a calc-alkalic basaltic melt, a tholeiitic basaltic melt and a tholeiitic felsic melt, contribute to the production of mixed calc-alkalic magmas. The 87Sr/86Sr and trace element compositions of the least-differentiated basalt magmas, which are inferred from the composition of the calcic plagioclase [Ca/(Ca + Na) >0·9], suggest that two types of basaltic magma, calc-alkalic and tholeiitic, exist beneath the volcano. The tholeiitic basalt magma has a higher 87Sr/86Sr than the calc-alkalic magma (0·7042 vs 0·7038) and a characteristic trace element signature consistent with the presence of plagioclase and amphibole as melting residues. This suggests that the tholeiitic magmas are produced via anatexis of amphibolitic crust caused by underplating and/or intrusion of mantle-derived calc-alkalic basalt magmas into the sub-Zao crust. The mantle-derived calc-alkalic basalt magma mixes with crust-derived tholeiitic melts to form calc-alkalic andesite magmas. The hypothesis proposed here requires revision (or even abandonment) of the general consensus that calc-alkalic magmas have greater contributions of a crustal component than tholeiitic magmas. [ABSTRACT FROM PUBLISHER]

Abstract: In Japan, the crust and uppermost mantle seismic character is yet unimaged although many refraction surveys have been recorded. The longest seismic profiles are analyzed. A remarkable feature, a long-duration coda wave after the PmP wave (reflected wave at the Moho boundary), is observed on the record sections. Several possible models are considered to explain the long-duration coda wave. The model with many scatterers located in the uppermost mantle explains the observed data well while the undulating Moho and continuous layering models do not account for some aspects of the observed data. The scatterer distributed uppermost mantle is not consistent with that of continental region which is often characterized as transparent. We estimate the scattering coefficient of the uppermost mantle and crust using simulations. The scattering coefficients obtained for upper crust, lower crust, and uppermost mantle are 0.01, 0.02, and 0.025, respectively. The scattering coefficient of the uppermost mantle is slightly larger than that of lower crust, which is characterized as being reflective. The many scatterers in the uppermost mantle might be related to magmatism in Japan. This will be one of the important observations for understanding formation processes of the Moho boundary and uppermost mantle in the island-arc environment. [Copyright &y& Elsevier]