Japan Arc Research Articles

宮城県北部，石越安山岩の地質・岩石学的特徴とマグマプロセス

Ishikoshi Andesite is the Middle Miocene, amphibole-bearing, two pyroxene andesite lava flow located in the forearc area of the Northeast Japan arc, which covers the area of 8 km EW and 7 km NS and has the thickness of >180 m. In this study it is divided into peripheral lava (PL) and central lava (CL), which are distributed in the peripheral and central parts of the volcanic edifice, respectively. PL bears groundmass pigeonite or shows higher FeO*/MgO trend with fractionation and CL does not bear pigeonite and lower FeO*/MgO trend, therefore PL is tholeiitic and CL is calc-alkaline. PL shows petrological features that are compatible with crystallization differentiation. CL has several features that suggest magma mixing such as non-equilibrium plagioclase phenocrysts, but evidences for magma mixing are not robust. In comparison to similar lava flows, PL and CL seem to form a composite lava flow because clear geological boundary between PL and CL cannot be observed and FeO*/MgO ratio of CL tend to be higher in the marginal portion. We also find that two-pyroxene equilibrium temperature of CL (760-850 ℃)is lower than that of PL (970-990 ℃). CL should have been too viscous at this temperature (>1010 Pa·s in anhydrous state) to form a dike, thus it should have effused through the vent which PL formed to be composite lava flow with PL. Ishikoshi Andesite has columnar joints rather than platy joints. In comparison to other lava flows which have platy joints in Japan, it is possible that less anisotropic crystals, low eruption temperature (high viscosity) and/or large phenocryst size have prevented Ishikoshi Andesite from formation of foliation along which platy joints develop in the lava.

Spatial U–Pb age distribution of plutonic rocks in the central Abukuma Plateau, northeastern Japan Arc

Spatial U–Pb age distribution of plutonic rocks in the central Abukuma Plateau, northeastern Japan Arc

東北日本弧に分布する白亜紀～古第三紀の花崗岩類のSr・Nd・Hf 同位体組成の空間分布

東北日本弧に分布する白亜紀～古第三紀の花崗岩類のSr・Nd・Hf 同位体組成の空間分布

Three‐dimensional electromagnetic imaging of upwelling fluids in the Kyushu subduction zone, Japan

AbstractA three‐dimensional (3‐D) lithospheric‐scale electrical resistivity model, developed using network‐magnetotelluric (network‐MT) data, contains structures associated with arc magmatism beneath Kyushu Island in the Southwest Japan arc. Kyushu Island, where the Philippine Sea Plate (PSP) subducts beneath the Eurasian plate, can be divided into northern and southern volcanic regions separated by a nonvolcanic region. Many active Quaternary volcanoes occur along the volcanic front (VF) associated with the PSP in the two volcanic regions. Our 3‐D electrical resistivity model shows three different shapes of upwelling fluid‐like conductive anomalies, indicative of either slab‐derived aqueous fluid and/or partial melt beneath the volcanic and nonvolcanic regions. A conductive anomaly in the northern volcanic region, located at some distance from the subducting PSP, extends from the surface to depths of <100 km, whereas another conductive anomaly in the southern volcanic region, located along the subducting PSP at >70 km depth, extends from the surface to depths of >100 km. In the nonvolcanic region, the upper region of a relatively conductive anomaly extends upward to a depth of ~50 km along the subducting plate. The degrees of magmatism and the relative contribution of slab‐derived fluids to the magmatism vary spatially in the one nonvolcanic and two volcanic regions.

北部フォッサマグナに産する後期新生代火山岩のSr-Nd 同位体組成からみた 2 つのソースマントルの分布

We analyzed the Sr-Nd isotopic and geochemical compositions of 112 late Cenozoic volcanic rocks from the northern Fossa Magna, central Japan. The volcanic rocks are divided, on the basis of Nd isotopic compositions, into a high-Nd group (HND) and a low-Nd group (LND). The HND has higher Zr/Nb and Hf/Nb ratios than the LND volcanic rocks. The isotopic ratios and high field strength element (HFSE) ratios indicate that the HND volcanic rocks were derived from a relatively depleted mantle compared with the LND rocks. The relationship between the HND and LND rocks, in terms of their isotopic and Zr/Nb, Hf/Nb ratios, is similar to that observed between Quaternary rear-arc and frontal-arc volcanic rocks of the NE Japan arc. The rear-arc rocks from NE Japan have higher Nd isotopic and HFSE ratios than the frontal-arc rocks, indicating the injection of depleted asthenospheric mantle beneath the rear-arc, associated with Miocene opening of the Sea of Japan. The HND rocks occur in the central part of the Fossa Magna region in the form of a NW-trending wedge. This wedge shape of the HND area is consistent with the wedge-shaped rifting that occurred perpendicular to the strike of the arc and that opened the Fossa Magna contemporaneously with the opening of the Sea of Japan. The Sr and Nd isotopic compositions of the HND and LND rocks fall within the mantle array and extend into the high-Sr isotopic field. Some of the LND rocks contain lower-crustal mafic xenoliths with Sr-Nd isotopic compositions that overlap those of the LND rocks. In the Sr-Nd isotope diagram, the trend of the HND rocks can be extrapolated to the most radiogenic Sr isotopic compositions obtained for the xenoliths. This result suggests that the lower-crustal materials, which have the most radiogenic Sr isotopic compositions, contributed to the Sr isotopic enrichment of the LND and HND magmas.

Back-arc rifting in the Korea Plateau in the East Sea (Japan Sea) and the separation of the southwestern Japan Arc from the Korean margin

The South Korea Plateau is a remnant of continental crust in the middle of the eastern Korean margin that was deformed by extension in association with back-arc rifting and separation of the southwestern Japan Arc in the Cenozoic. Multichannel seismic profiles show that the South Korea Plateau preserves structure of rift basins flanked by uplifted footwall blocks. Rift basins in the plateau are filled with distinct syn- and post-rift sequences separated by a prominent breakup unconformity resulting from uplift. The crustal and upper mantle structures computed by ambient noise tomography indicate that the South Korea Plateau defines the seaward limit of rifted continental crust; in addition, rifting occurred with vigorous asthenospheric upwelling induced along the Korean margin and depth-dependent stretching. Therefore, the South Korea Plateau underwent the entire sequence of tectonic events typical of a passive continental margin encompassing rifting with subsidence, uplift, erosion, and breakup prior to the separation of the southwestern Japan Arc. Breakup at the Korean margin may substantiate successive episodes of back-arc spreading which migrated toward the arc in response to trench retreat. We suggest that the southwestern Japan Arc moved to its present location from the southern part of the Korean margin with a significant amount of clockwise rotation during back-arc spreading.

PLATINUM-GROUP ELEMENT AND MINERAL CHARACTERISTICS OF SUB-ARC CHROMITITE XENOLITHS FROM THE TAKASHIMA ALKALI BASALT, SOUTHWEST JAPAN ARC

Chromitites in ophiolites are frequently used as a marker of sub-arc mantle magmatism. This has, however, rarely been verified by modern sub-arc mantle materials. Chromitite xenoliths from Takashima, southwest Japan, derived from the current sub-arc mantle, are here described with comparison to ophiolitic chromitites. The chromitite xenoliths from Takashima from the southwest Japan arc show various textures, including nodular, anti-nodular, and layered varieties. Laurite is commonly found as minute (<5 μm diameter) solitary grains of euhedral shape in Cr-rich spinel. Very fine (around 1 μm) grains of Co-bearing pentlandite are associated with silicate inclusions (mainly pyroxenes) in Cr-rich spinel. The chondrite-normalized PGE pattern of Takashima chromitites shows a slightly negative slope from Ru to Pt. The Takashima chromitite is very similar in its PGE chemistry, spinel chemistry, and textural variations to some discordant chromitites from the Oman ophiolite, a typical podiform chromitite interpreted to have formed in an arc setting, indicating a possible sub-arc origin for the latter. The characteristics of the Takashima chromitite xenoliths indicate that the Oman discordant podiform chromitites were formed in a supra-subduction mantle setting before ophiolite obduction.

Evolution of carbon dioxide-bearing saline fluids in the mantle wedge beneath the Northeast Japan arc

Lherzolite xenoliths containing fluid inclusions from the Ichinomegata volcano, located on the rear-arc side of the Northeast Japan arc, may be considered as samples of the uppermost mantle above the melting region in the mantle wedge. Thus, these fluid inclusions provide valuable information on the nature of fluids present in the sub-arc mantle. The inclusions in the Ichinomegata amphibole-bearing spinel–plagioclase lherzolite xenoliths were found to be composed mainly of CO2–H2O–Cl–S fluids. At equilibrium temperature of 920 °C, the fluid inclusions preserve pressures of 0.66–0.78 GPa, which correspond to depths of 23–28 km. The molar fraction of H2O and the salinity of fluid inclusions are 0.18–0.35 and 3.71 ± 0.78 wt% NaCl equivalent, respectively. These fluid inclusions are not believed to be fluids derived directly from the subducting slab, but rather fluids exsolved from sub-arc basaltic magmas that are formed through partial melting of mantle wedge triggered by slab-derived fluids.

Regional variations of the shear-wave polarization anisotropy in the crust and mantle wedge beneath the Tohoku district

We investigated the regional variations of the shear-wave polarization anisotropy in the upper crust, lower crust, and mantle wedge beneath the Tohoku district in the northeast Japan arc. The shear-wave splitting parameters (fast polarization direction (FPD) and split time) for the upper and lower crusts are estimated by shear-wave splitting analysis of the Ps phases, which are generated by the conversion of a P wave into an S wave at the Conrad and Moho discontinuity interfaces. The splitting parameters for the mantle wedge are determined by splitting analysis of the slab Ps phase converted at the oceanic crust on the subducting Pacific slab and the direct S waves from deep focus earthquakes located just below seismic stations. The Ps phases are identified on the P-wave receiver functions constructed from the teleseismic records. To accurately determine the splitting parameters for the lower crust from the Moho Ps phase, the Ps phase should be corrected for the shear-wave splitting effect in the upper crust. Similarly, to estimate the mantle wedge anisotropy, the direct S wave and slab Ps phase must be corrected for the splitting effect in the crust. The anisotropies of the crust and mantle wedge are estimated from the corrected Ps phases and direct S waves. The anisotropy of the upper crust exhibits a regional variation where FPD of the split shear wave is predominant in the N–S direction in the Pacific coast area and in the E–W direction in the rest of the Tohoku district. The split time is less than 0.2 s. The upper crustal anisotropy is attributed to the alignment of vertical cracks induced in the upper crust by tectonic stress. In the lower crust, FPD is predominant in the E–W direction with a split time similar to that in the upper crust, and the anisotropy of the lower crust is due to the lattice preferred orientation of rock-forming minerals. In the mantle wedge, FPD is predominant in the N–S direction (trench-parallel) in the fore-arc side of the volcanic front, but in the E–W direction (trench-perpendicular) in the back-arc side. The split time in the mantle wedge is similar to that of the upper crust. The most likely cause for the trench-perpendicular anisotropy in the back-arc mantle wedge is the lattice preferred orientation of A-type olivine along flows of the secondary mantle convection. For trench-parallel anisotropy in the fore-arc mantle wedge, we considered two possible causes: the preferred orientation of B-type olivine along the secondary mantle convection and that of dry olivine along the trench-parallel flow produced by the along-strike dip variation of the Pacific slab. However, we could not determine which one is the origin.

Repeated magmatism at 34Ma and 23-20Ma producing high magnesian adakitic andesites and transitional basalts on southern Okushiri Island, NE Japan arc

Abstract The southern part of Okushiri Island in the present-day back-arc margin of the NE Japan arc is one of the rare convergent plate boundaries where similar magma types (high-magnesian adakitic andesite (HMAA) and high-TiO 2 basalt (HTB)) have been erupted concurrently at more than one time. Oligocene HMAA can be divided into two types: HMAA-I is characterized by high Sr/Y and low Y, and HMAA-II by relatively low Sr/Y and high Y. HMAA-I is primitive in terms of MgO (8.5 wt.%), Mg# (67), Ni (232 ppm) and Cr (613 ppm) contents, and the most Mg-rich olivine phenocrysts plot within the mantle olivine array in terms of Fo and NiO. The similar Cr versus Ni relations of types I and II HMAA indicate some interaction of slab-derived adakitic melts with mantle peridotite, whereas Ni contents are higher than those of most boninites derived by partial melting of mantle peridotite at a given Cr content. Types I and II HMAA have more enriched Sr and Nd isotopic compositions than N-MORB. The petrography and geochemistry of these rocks, combined with published results on the genesis of high-magnesian andesite (HMA) indicate that types I and II HMAA could be produced by interaction of slab (N-MORB and sediment)-derived adakitic melts with mantle peridotite. The comagmatism of HMAA and HTB is ascribed to the following model. A cool, less hydrous, adakite magma (spherical diapir) would rise from the subducting slab (Pacific Plate) and become more hydrous as a result of its interaction with overlying hydrous peridotite. This hydrated adakitic diapir further ascends and is heated on entering the overlying mantle wedge. Subsequently, the temperature and H 2 O gradients in the ascending adakitic diapir and surrounding mantle peridotite would have been established. The HTB magma segregated from the surrounding mantle peridotite region (high temperature and low H 2 O content) at a depth of 60 km or more, whereas the adakitic diapir (low temperature and high H 2 O content) continued to rise, with its chemical composition modified due to interaction with the surrounding mantle peridotite. Type I HMAA then segregated at about 50 km. The most attractive tectono-magmatic model to account for production of adakitic magma at two different periods in the same cool subduction zone region involves upwelling of depleted hot asthenosphere into the subcontinental lithosphere beneath the back-arc margin of the NE Japan arc, coincident with back-arc rifting which took place at the initiation of the Japan Sea opening. The unusually high temperature conditions established in the mantle wedge due to upwelling of depleted hot asthenosphere caused partial melting of a limited part of the cool oceanic crust subducting beneath the NE Japan arc, resulting in the generation of adakitic magma.

A 3-D electrical resistivity model beneath the focal zone of the 2008 Iwate-Miyagi Nairiku earthquake (M 7.2)

The 2008 Iwate-Miyagi Nairiku earthquake (M 7.2) was a shallow inland earthquake that occurred in the volcanic front of the northeastern Japan arc. To understand why the earthquake occurred beneath an active volcanic area, in which ductile crust generally impedes fault rupture, we conducted magnetotelluric surveys at 14 stations around the epicentral area 2 months after the earthquake. Based on 56 sets of magnetotelluric impedances measured by the present and previous surveys, we estimated the three-dimensional (3-D) electrical resistivity distribution. The inverted 3-D resistivity model showed a shallow conductive zone beneath the Kitakami Lowland and a few conductive patches beneath active volcanic areas. The shallow conductive zone is interpreted as Tertiary sedimentary rocks. The deeper conductive patches probably relate to volcanic activities and possibly indicate high-temperature anomalies. Aftershocks were distributed mainly in the resistive zone, interpreted as a brittle zone, and not in these conductive areas, interpreted as ductile zones. The size of the brittle zone seems large enough for a fault rupture area capable of generating an M 7-class earthquake, despite the areas distributed among the ductile zones. This interpretation implies that 3-D elastic heterogeneity, due to regional geology and volcanic activities, controls the size of the fault rupture zone. Additionally, the elastic heterogeneities could result in local stress concentration around the earthquake area and cause faulting.

Three-dimensional electromagnetic imaging of fluids and melts beneath the NE Japan arc revisited by using geomagnetic transfer function data

The three-dimensional distribution of fluids and melts under the NE Japan arc was imaged using its resistivity structure, modeled with geomagnetic transfer functions. The data were collected at 37 stations located on a 20-km grid, at periods ranging from 16 to 256 s. In spite of the narrow period band nature, these periods turn out to be sensitive to conductors in the deep crust and upper mantle. The geomagnetic transfer functions represent lateral resistivity variations, which yield inherently nonunique model results when using the geomagnetic transfer functions alone. However, by fixing the resistivity structure of the surrounding seawater distribution, the intrinsic nonuniqueness is alleviated. In this study, we show an inversion result using a 100-Ωm uniform Earth with fixed resistivity of surrounding oceans. As a result, it was found that the features of the short period transfer function require shallow conductors in the upper crust, which is suggested to represent the northern Tohoku conducting belt of a previous study. The final model is characterized by a highly conductive zone along the quaternary volcanic arc in the depth range of the lower crust to the upper mantle. The conductor, which is obtained mainly from the features of longer-period data, is particularly clear beneath the Sengan geothermal area. The deep crustal conductor implies the existence of partial melt and/or high-salinity fluids.

Numerical forecasting of the time interval between successive M8 earthquakes along the Nankai Trough, southwest Japan, using ocean bottom cable network data

One possible approach to estimating the time interval between large-scale Tōnankai (Tōkai) and Nankai earthquakes on the Japan arc is sequential assimilation of crustal deformation data. We conducted numerical modeling of sequential assimilation using surface deformation calculated from earthquake generation cycle simulations along the Nankai Trough. To account for observation noise, we used measured ocean bottom pressure gauge data, excluding tidal modulation, from a station on the ocean bottom cable network Dense Oceanfloor Network System for Earthquakes and Tsunamis in the Kumano basin. We used sequential importance sampling as our data assimilation method. We found that as the amount of data increased, the estimated time interval between the Tōnankai and Nankai earthquakes approached the “true” observed interval. In addition, the noise in the pressure gauge data was sufficiently small that simulated crustal deformation patterns could be distinguished for different time intervals.

Role of silica for the progress of serpentinization reactions: Constraints from successive changes in mineralogical textures of serpentinites from Iwanaidake ultramafic body, Japan

Serpentinization of peridotite in subduction zones and mid-ocean ridges is a key process that controls not only the geodynamic behavior of the mantle but also the activity of modern (and probably primordial) microbial systems on the seafloor. However, there is still controversy about what factors promote the mineralogical reactions of serpentinization in natural ultramafic rocks. Here we report textures, chemistry, and magnetic susceptibility of variably serpentinized harzburgite and dunite samples from Iwanaidake ultramafic body, Japan, which originated from the forearc mantle of the Northeast Japan arc. Successive changes in textures, mineral chemistry, and magnetic susceptibility during serpentinization of harzburgite involved two stages: replacement of olivine by serpentine and brucite, and subsequent formation of magnetite along with more-magnesian serpentine and brucite. The later reactions occurred concurrently with serpentinization of orthopyroxene, which supplied the silica component. In serpentinized dunite, which does not contain orthopyroxene, serpentinization involved replacement of olivine by serpentine and brucite, and the fraction of magnetite did not increase with the progress of serpentinization. These observations suggest that the silica supply from serpentinization of orthopyroxene is an essential factor for the formation of magnetite during serpentinization. Magnetite formation facilitated by addition of silica has often been reported for many serpentinite systems, suggesting that the magnetite formation triggered by silica addition is one of the key reactions for the progress of serpentinization in natural ultramafic rocks.

Behaviour of subducted water and its role in magma genesis in the NE Japan arc: A combined geophysical and geochemical approach

Water at subduction zones is carried to mantle depths by the subducting oceanic plate and then released by dehydration. It then migrates upwards and contributes to melting of the mantle wedge to form primary arc magma. The magma thus captures and transfers water to the crust, or outgasses water to the atmosphere. Water, either in fluids or melts in both the slab and the mantle, promotes the dissolution and mobilization of elements and affects the physical properties of the sub-arc slab, mantle, and seismicity. In this paper, we present a coherent model to explain the geophysical and geochemical role of water beneath NE Japan. We first investigate the seismic structures of the downgoing slab and sub-arc mantle and examine the role of subducted water in forming these structures. We then use the Arc Basalt Simulator version 4, a petrological–geochemical model developed to describe the geochemical behaviours of water and elements in the slab, mantle, and arc basalt. Parameters governing these petrogenetic processes are also estimated by the model and compared to geophysical observations. The combined approach shows that (1) subducted sediment and igneous oceanic crust are almost fully hydrated, whereas only partial hydration occurs in the oceanic mantle; (2) this high slab water content leads to melting of the slab sediment and the uppermost basalt layer beneath the arc; (3) the released water via slab liquid promotes 3–25% melting of the mantle wedge at a depth of 50–30km at a mantle temperature of 1250–1350°C; (4) virtually 89% of slab water is released, 22% of the water returns to the forearc, and 38% enters the arc crust with the magma; and (5) 11% of the subducted water retained beyond a depth of 180km is held in the slab, and 29% in nominally anhydrous minerals in the wedge mantle.

Temporal and spatial change of Quaternary volcanism at the northern end of NE Japan arc: Re-investigation of eruptive age and magma chemistry

Temporal and spatial change of Quaternary volcanism at the northern end of NE Japan arc: Re-investigation of eruptive age and magma chemistry

Seismic attenuation tomography of the Northeast Japan arc: Insight into the 2011 Tohoku earthquake (Mw9.0) and subduction dynamics

Detailed three-dimensional (3-D) P and S wave attenuation (Qp and Qs) models of the crust and upper mantle under the entire Northeast Japan (Tohoku) arc from the Japan Trench to the Japan Sea coast are determined, for the first time, using a large number of high-quality t* data measured precisely from P and S wave spectra of local earthquakes. The suboceanic earthquakes used in this work are relocated precisely using sP depth phases. Our results reveal a prominent landward dipping high-Q zone representing the subducting Pacific slab, a landward dipping intermediate- to high-Q zone in the mantle wedge between the Pacific coast and the volcanic front, and significant low-Q anomalies in the crust and mantle wedge between the volcanic front and the Japan Sea coast. Prominent high-Q patches surrounded by low-Q anomalies are revealed in the interplate megathrust zone under the Tohoku fore arc where the great 2011 Tohoku-oki earthquake (Mw 9.0) occurred. The high-Q patches in the megathrust zone generally exhibit large coseismic slips of megathrust earthquakes and large slip deficit on the plate interface. We think that these high-Q patches represent asperities in the megathrust zone, whereas the low-Q anomalies reflect weakly coupled areas. We also find that the hypocenters of the 2011 Tohoku-oki interplate earthquakes (Mw > 7.0) are located in areas where Qp, Qs, and Qp/Qs change abruptly. These results suggest that structural heterogeneities in the megathrust zone control the interplate seismic coupling and the nucleation of megathrust earthquakes.

Generation of Cenozoic granitoids in Hokkaido (Japan): Constraints from zircon geochronology, Sr-Nd-Hf isotopic and geochemical analyses, and implications for crustal growth

The island of Hokkaido is a young accretionary terrane, basically built with a Jurassic accretionary complex and Cretaceous arc in the west (= NE Japan arc terrane), a Cretaceous-Paleogene forearc basin and accretionary complex with the Hidaka metamorphic belt in the center, and a Cenozoic island arc with Cretaceous basement in the east (= Chishima or Kuril arc terrane). Though volumetrically small, Paleogene and Neogene granitoids are widespread in central Hokkaido (Hidaka Belt). Granitoids are the most representative component of the continental crust, so in this work we aimed to study the mode of generation and source characteristics of these granitoids in order to assess the crustal composition of Hokkaido and examine the general problem of continental growth. New zircon geochronology on nine granitic and one gabbroic rocks from the Hidaka Belt reveals three distinct magmatic episodes, two in the Eocene at 45–46 Ma (3 granites), and 37.0 ± 0.5 Ma (1 granite), and one in the Miocene at 18 to 19 Ma (5 granites and 1 gabbro). The Miocene episode represents the most important granitic emplacement in Hokkaido. The early Eocene zircon ages of 45 to 46 Ma are identified for the first time for granitoids that occur in the northern part of the Hidaka Belt. The zircon age of 37 Ma for a granite from Shirataki is rather rare in Hokkaido, but similar ages had been reported for a tonalite and a granite from the Hidaka metamorphic belt. Geochemically, all granites are slightly peraluminous but not S-type, and they possess volcanic arc granitoid characteristics. Their REE distribution patterns are typically “granitic,” showing fractionated patterns with LREE enrichment and distinct negative Eu anomaly. The whole-rock isotopic signatures [ISr = 0.7044 to 0.7061; eNd(t) = +1.0 to +4.7; TDM-1 = 400-1000 Ma] reveal their largely juvenile characteristics. This is corroborated by the zircon Hf isotopic compositions [eHf(t) = +8 to +19]. The Eocene granites were most probably generated by melting of subducted accretionary complex in a prolonged period from 46 to 37 Ma in supra-subduction zone; whereas the Miocene granites were also generated by melting of accretionary complex in a back-arc rifting setting. In both cases, the involved accretionary complex was probably dominated by the mantle-derived lithological component with little Paleozoic or older crustal material. Hokkaido provides an excellent example of juvenile crust addition to the continental crust.

Primary melt from Sannome-gata volcano, NE Japan arc: constraints on generation conditions of rear-arc magmas

The conditions under which rear-arc magmas are generated were estimated using primary basalts from the Sannome-gata volcano, located in the rear of the NE Japan arc. Scoriae from the volcano occur with abundant crustal and mantle xenoliths, suggesting that the magma ascended rapidly from the upper mantle. The scoriae show significant variations in their whole-rock compositions (7.9–11.1 wt% MgO). High-MgO scoriae (MgO > ~9.5 wt%) have mostly homogeneous 87Sr/86Sr ratios (0.70318–0.70320), whereas low-MgO scoriae (MgO 0.70327); ratios tend to increase with decreasing MgO content. The high-MgO scoriae are aphyric, containing ~5 vol% olivine microphenocrysts with Mg# [100 × Mg/(Mg + Fe2+)] of up to 90. In contrast, the low-MgO scoriae have crustal xenocrysts of plagioclase, alkali feldspar, and quartz, and the mineralogic modes correlate negatively with whole-rock MgO content. On the basis of these observations, it is inferred that the high-MgO scoriae represent primary or near-primary melts, while the low-MgO scoriae underwent considerable interaction with the crust. Using thermodynamic analysis of the observed petrological features of the high-MgO scoriae, the eruption temperature of the magmas was constrained to 1,160–1,220 °C. Given that the source mantle was depleted MORB-source mantle, the primary magma was plausibly generated by ~7 % melting of a garnet-bearing spinel peridotite; taking this into consideration, and considering the constraints of multi-component thermodynamics, we estimated that the primary Sannome-gata magma was generated in the source mantle with 0.5–0.6 wt% H2O at 1,220–1,230 °C and at ~1.8 GPa, and that the H2O content of the primary magma was 6–7 wt%. The rear-arc Sannome-gata magma was generated by a lower degree of melting of the mantle at greater depths and lower temperatures than the frontal-arc magma from the Iwate volcano, which was also estimated to be generated by ~15 % melting of the source mantle with 0.6–0.7 wt% H2O at ~1,250 °C and at ~1.3 GPa.

Formation process of olivine–clinopyroxene cumulates inferred from Takashima xenoliths, Southwest Japan arc

Formation process of olivine–clinopyroxene cumulates inferred from Takashima xenoliths, Southwest Japan arc