Takeyoshi Yoshida

Arc Basalt Simulator version 2, a simulation for slab dehydration and fluid‐fluxed mantle melting for arc basalts: Modeling scheme and application

[1]xa0Convergent margin magmas typically have geochemical signatures that include elevated concentrations of large-ion lithophile elements; depleted heavy rare earth elements and high field strength elements; and variously radiogenic Sr, Pb, and Nd isotopic compositions. These have been attributed to the melting of depleted mantle peridotite by the fluxing of fluids or melts derived from subducting oceanic crust. High Mg # basalts and high Mg # andesites are inferred to make up the bulk of subduction-related primary magmas and may be generated by fluid or melt fluxing of mantle peridotite. The difference in contributions from the subducted slab found among various arcs appears to be mostly controlled by thermal structure. Cold slabs yield fluids, and hot slabs yield melts. Recent experimental studies and thermodynamic models better constrain the phase petrology of the slab components during prograde metamorphism and melting, mantle wedge melting, and mantle slab melt reaction. Experimental results also constrain the behavior of many elements in these processes. In addition, geodynamic models allow increasingly realistic, quantitative modeling of the temperature and pressure in the subducted slab and mantle wedge. These developments together enable generation of forward models to explain arc magma geochemistry. The Arc Basalt Simulator (ABS) version 2 (ABS2) uses an Excel® spreadsheet-based calculator to predict the partitioning of incompatible element and Sr-Nd-Pb isotopic composition in a slab-derived fluid and in arc basalt magma generated by an open system fluid-fluxed melting of mantle wedge peridotite. The ABS2 model is intended to simulate high Mg # basalt geochemistry in relatively cold subduction zones. The modeling scheme of ABS2 is presented and is applied to primitive arc magmas.

Simultaneous high P‐T measurements of ultrasonic compressional and shear wave velocities in Ichino‐megata mafic xenoliths: Their bearings on seismic velocity perturbations in lower crust of northeast Japan arc

[1]xa0Simultaneous measurements of ultrasonic compressional (Vp) and shear wave velocities (Vs) were conducted under pressures and temperatures representative of the lower crust on five mafic xenoliths derived from the northeast Japan arc crust. For comparison with the observed seismic heterogeneity of the lower crust of the northeast Japan arc, the velocity deviations (dVp and dVs) of the xenoliths were calculated using the average reference Vp (6.61 ± 0.1 km/s) and Vs (3.76 ± 0.1 km/s). The dVp and dVs values of the xenoliths at 700°C and 0.8 GPa are 0.8% and −1.2% for hornblende gabbro, 4.8% and 4.0% for hornblende-pyroxene gabbro, 0.9% and −1.5% for amphibolite, 2.5% and 1.7% for pyroxene amphibolite, and 0.9% and −5.6% for hornblendite, respectively. We compared the velocity perturbations in the lower crust determined by seismic tomography with dVp and dVs of the xenoliths using “Vp−Vs−Vp/Vs deviation diagrams.” On the basis of our measurements, we suggest that (1) the seismically high-Vp and Vs regions beneath the Tobishima Basin consist of hornblende-pyroxene gabbro, (2) hornblende gabbro is a predominant rock type beneath the Dewa Hills and Ou Backbone Range, (3) the low-velocity anomalies beneath the active volcano areas may be caused by the existence of partial melts of hornblende gabbro, and (4) the low-Vp and high-Vs regions beneath the Kitakami Mountains consist of quartz-plagioclase-bearing rocks. Our data demonstrate that the seismic heterogeneity in the lower crust of the northeast Japan arc reflects variations in rock composition and temperature that are related to the regional geological history.

Tectonic evolution and deep to shallow geometry of Nagamachi-Rifu Active Fault System, NE Japan

The Nagamachi-Rifu fault is an active reverse fault which trends NE-SW across the central part of Sendai City for over 21 km distance. The fault does not emerge at the surface and, accompanied with the Dainenji-yama fault, shows wedge thrusting in the Tertiary sediments. The amount of net slip of the master part of the Nagamachi-Rifu fault is estimated to be one mm/year. Seismic reflection profiles across the fault plus a gravity anomaly reveal the thicker Neogene sediments on the hanging wall rather than on the footwall. The Neogene sedimentary basin was formed by normal faulting in early Miocene under an extensional stress regime associated with the formation of the northern Honshu rift system. Due to shortening deformation since the Pliocene, this Miocene normal fault reactivated as a reverse fault. Judging from the CMP deep seismic reflection profile and location of the 1998 M5.0 Sendai earthquake, the deep geometry of the Nagamachi-Rifu fault is listric.

Geology, petrology and tectonic setting of the Late Jurassic ophiolite in Hokkaido, Japan

Abstract The Gokurakudaira Formation, which has a N–S zonal distribution within a latest Jurassic greenstone belt in Hokkaido Island, Japan, constitutes the uppermost ultramafic–mafic unit of the Horokanai Ophiolite. The following three hypotheses for the origin of the ophiolite have been proposed: (1) a mid-oceanic ridge; (2) an oceanic plateau; and (3) an island arc. The Gokurakudaira Formation can be subdivided into four zones extending NNW to SSE, from east (Zone I) to west (Zone IV), based on lithofacies and areal distribution. Zones I and III consist of aphyric tholeiite resembling back-arc basin basalt (BABB), while Zone II is characterized by the coexistence of BABB-like tholeiite along with high-Mg andesite. Zone IV has a different lithology from the other zones, and is composed mainly of picrite and thick sedimentary sequences of island arc tholeiite (IAT) type andesitic subaqueous pyroclastic deposits and terrigenous sediments. These stratigraphic and petrological characteristics of the Gokurakudaira Formation cannot be explained by the oceanic plateau or mid-oceanic ridge models, but they could correspond to the marginal sea model, as in the Lau Basin. Therefore, we conclude that the Horokanai Ophiolite was formed in the Late Jurassic in a marginal basin above a supra-subduction zone on the margin of the Asian continent.

Structural control on Late Miocene to Quaternary volcanism in the NE Honshu arc, Japan

[1]xa0Volcanological and structural field data are used to define the tectonic control on the N–S volcanic arc of NE Honshu (Japan) since late Miocene. During late Miocene-Pliocene, bimodal products were mainly erupted from along-arc and NE–SW-aligned and elongated calderas. The deformation pattern mostly consisted of N–S dextral faults and subordinate NE–SW extensional structures produced by NE–SW compression. This pattern, because of the indentation of the Kuril sliver, is similar to that of oblique convergence settings. Magma rose and extruded along NE–SW areas of localized extension created by the dextral faults. These extensional areas were uncoupled with regard to those, ∼E–W trending, inferred to have focused the rise of melts from the subducting slab in the mantle. During Quaternary, a larger amount of andesite was mainly erupted from along-arc and ∼E–W-aligned and elongated stratovolcanoes. The deformation pattern mostly consisted of N–S thrust faults and subordinate ∼E–W extensional structures, produced by ∼E–W compression, resulting from orthogonal convergence due to the variation in the absolute motion of the Pacific Plate. The ∼E–W extensional structures are the shallowest expression of ∼E–W-trending hot mantle fingers, suggesting mantle-crust coupling for the rise of magma. Such a coupling ensures (1) higher extrusion and (2) mixing between a deeper mafic and a shallower felsic magma, generating the andesites. The significantly larger volumes (Ma−1 200 km−1 of length of the arc) of magma erupted during Quaternary show that pure convergence conditions do not necessarily hinder the rise and extrusion of magma.

Water content of primitive low-K tholeiitic basalt magma from Iwate Volcano, NE Japan arc: implications for differentiation mechanism of frontal-arc basalt magmas

The water content of low-K tholeiitic basalt magma from Iwate volcano, which is located on the volcanic front of the NE Japan arc, was estimated using multi-component thermodynamic models. The Iwate lavas are moderately porphyritic, consisting of ~8xa0vol.% olivine and ~20xa0vol.% plagioclase phenocrysts. The olivine and plagioclase phenocrysts show significant compositional variations, and the Mg# of olivine phenocrysts (Mg#78–85) correlates positively with the An content of coexisting plagioclase phenocrysts (An85–92). The olivine phenocrysts with Mg# > ~82 do not form crystal aggregates with plagioclase phenocrysts. It is inferred from these observations that the phenocrysts with variable compositions were primarily derived from mushy boundary layers along the walls of a magma chamber. By using thermodynamic calculations with the observed petrological features of the lavas, the water content of the Iwate magma was estimated to be 4–5xa0wt.%. The high water content of the magma supports the recent consensus that frontal-arc magmas are remarkably hydrous. Using the estimated water content of the Iwate magma, the water content and temperature of the source mantle were estimated. Given that the Iwate magma was derived from a primary magma solely by olivine fractionation, the water content and temperature were estimated to be ~0.7xa0wt.% and ~1,310xa0°C, respectively. Differentiation mechanisms of low-K frontal-arc basalt magmas were also examined by application of a thermodynamics-based mass balance model to the Iwate magma. It is suggested that magmatic differentiation proceeds primarily through fractionation of crystals from the main molten part of a magma chamber when it is located at <~200xa0MPa, whereas magma evolves through a convective melt exchange between the main magma and mushy boundary layers when the magma body is located at >~200xa0MPa.