Across-arc Li-B-Pb isotope systematics, together with trace element geochemistry and phase relationships in subducting slabs are used to understand the petrogenesis of lavas from the northeastern Japan arc. Our results suggest that accessory minerals and minor phases in the subducting slab play important roles in determining the chemical compositions of fluid released by dehydration. The strikingly different results for the northeastern Japan arc and the Izu arc in Li isotope across-arc variation show that not only chemical characteristics of the subducting slab but also the physical characteristics of the subduction zone could affect the Li isotope compositions of subducting slabs, producing heterogeneity in the Li isotope composition of the mantle.

View full abstract

[in Japanese]

View full abstract

[in Japanese]

View full abstract

[in Japanese]

View full abstract

Understanding the trace element and isotopic compositions of individual lithologies subducted to the subarc mantle is indispensable to constrain the geochemical models for subduction-zone material recycling. Recent petrographical studies focusing on trace element distributions between minerals in subduction-related metamorphic rocks occurring as a coherent regional metamorphic belt have pointed out that the trace elements were preferably partitioned into accessory minerals. However, these rocks commonly suffered varying extents of mineralogical and chemical overprinting during their exhumation. Here we report such trace element distributions by using eclogite xenoliths from the Colorado Plateau, because they represent fragments of the subducted oceanic crust, and they are mostly free from the effect of retrogressive overprinting due to the rapid entrainment by kimberlite-like magma.

View full abstract

[in Japanese]

View full abstract

Dike rocks preceding the Early Cretaceous plutonic activity distributed in southern part of the Kitakami Mountains, northeast Japan, are petrologically investigated.The high-Sr andesites and high-Mg andesites show similar petrochemical characteristics to those of adakite except higher Cr, Ni, and Mg contents.Petrochemical investigation indicates that the high-Sr andesite and high-Mg andesite magma can be resulted from interaction of slab derived adakitic melt with overlying wedge mantle.The shoshonites are characterized by the similar petrochemical characteristics to those of adakite except higher K and Rb contents.The chemical characteristics of shoshonites can be explained by the hypothesis of partial melting of mantle peridotite assimilated by adakitic slab melt.

View full abstract

Resent tomographic observations suggest that some subductig slabs stagnate at mantle transition zone. Irifune and Ringwood (1978) show that there is density crossover occurs at the depth 660km to 800km between MORB and peridotite, which causes buoyancy and stagnate slab or tear off upper oceanic crust at 660km. It is important to determine the depth of post-garnet transition occurs in MORB system to discuss the behavior of the slab at 660 km. In this study, we determined post-garnet phase boundary in MORB system using in-situ X-ray experiment at 20-25GPa and 1100-1200 ºC under hydrous conditions.

View full abstract

Diffusion experiments of mantle minerals are important for understanding the rheological properties of the Earth's mantle and subducting slab, because the atomic diffusion in the crystals controls plastic deformation in silicate minerals. Wadsleyite is a major constituent mineral in the mantle transition zone. Hence experimental examination of the diffusion rates is essential for debating the rheology of mantle transition zone. In this study, Si self-diffusion rates in wadsleyite were measured at 18 GPa, 1703-1903 K and the viscosity of wadsleyite deformed by diffusion creep was calculated. Our results imply some portions in cold subducting slabs, where the grain size reduces to less than 1 micron after the olivine-spinel transformation, become weaker than the surrounding mantle.

View full abstract

The Al-bearing phases with orthorhombic (phase CF) and hexagonal (phase NAL) structures were observed in the system of average Mid-Ocean Ridge basalt at pressures above 25 GPa. Structure and composition of these two phases are very similar. Moreover, they can coexist in the single experimental sample. Here we report data on chemical composition, structure, and stability fields of the Al-bearing phases NAL and CF and investigate their relationship in the lower mantle.

View full abstract

It has been widely recognized that magmas undergoing fractional crystallization can simultaneously assimilate the surrounding crust, as a consequence of heat transfer from hot magmas to the cool crust. Because of the importance and interest in the geochemical and dynamical phenomena associated with assimilation and fractional crystallization (AFC), the underlying physics and the geochemical consequences of the required mass and energy balance, have received considerable attention. However, there is still a shortage of good field examples, constrained by geological observations and detailed geochemisty, which can be used to test the various hypothesis for AFC. Geological approach has a potential to extract direct information on the AFC process involved in individual magmatic systems, through careful examinations of the couplings of the fundamental processes, such as crystallization, solid-melt separation, and transport of crustal melt. However, the previous geological studies have commonly just used geochemical data to evaluate the AFC process in terms of the well-known model of DePaolo (1981), and few studies have elucidated the mechanisms responsible for the AFC process. In this study, we investigate mechanisms of AFC process involved in the evolution of the Kutsugata and Tanetomi lavas, an alkali basalt-dacite suite erupted sequentially from Rishiri Volcano, northern Japan. The Kutsugata and Tanetomi lavas have been the subject of detailed petrologic studies, and the major element variations within the suite have been explained by boundary layer fractionation; i.e. mixing of a magma in the main part of the magma body with a fractionated interstitial melt transported from the mushy boundary layer at the floor. Systematic variations in SiO₂ correlate with variations in the Pb, Sr and Nd isotopic compositions of the lavas. It is suggested that crustal melt, transported from the floor crust, was mixed with the fractionated interstitial melt in the floor mush zone, and the mixed melt was further transported to the main magma, causing its geochemical evolution characteristic of AFC process. In order to estimate the ratio of assimilated mass to crystallized mass (r-value) in the magma chamber, a mass balance model including both AFC and boundary layer fractionation is newly developed. The geochemical variations of the lavas can be explained by a constant and relatively low value of the parameter r. Using the estimated r-value, as well as the estimated rate of the inward solidification of the floor boundary layer, the volume flux of the assimilant from the crust to the magma chamber is estimated. The volume flux is suggested to have decreased progressively with time (proportional to t^-1/2), and was about 3 x 10^-2 m/year at t = 10 years and 1 x 10^-2 m/year at t = 100 years.

View full abstract

We carried out subliquidus viscosity measurement on the 1707 basalt of Fuji volcano, utilizing an atmosphere-controlled furnace with viscometer, which allow temporal sampling during the run. Liquidus temperature of the basalt is 1227°C, and the viscosity of magma changed from 52 Pa sec at 1230°C to 1950 Pa sec at 1130°C. The crystallinity varied from 0 to 24 %. We analyzed the glass, plagioclase and olivine (only at 1130°C), and calculated the melt viscosity at each temperature. Melt viscosity does not vary due to counterbalancing of temperature decrease and decrease of plagioclase component in the melt. Relative viscosity (bulk viscosity/melt viscosity) increased to 14 at 1130°C, which is 3.9 times larger than the commonly used Einstein-Roscoe-Marsh equation.

View full abstract

The 1914-1915 eruption of Sakurajima volcano produced Plinian fall pumice and lava flows. Crystal size distributions (CSDs) and textures of the groundmass minerals of Plinian fall pumice and lava flow and vesicle size distributions (VSDs) and vesicle texture of Plinian fall pumice were analyzed. Plinian fall pumice samples show lower crystallinity and smaller crystal size compared with lava flow samples. Moreover, Plinian fall pumice samples are divided into three types according to their microlite and vesicle textures. Type-1 and type-2 Plinian fall pumice samples have high vesicularities (53-65vol%) and low (9.2*10¹² to 9.3*10¹³m^-3) to middle (1.6*10¹⁴ to 4.0*10¹⁴m^-3) number densities of plagioclase microlite. In these samples, vesiculation and expansion of magmas occurred in nearly closed system, deriving faster uprise velocity and inefficient crystallization of microlite. Type-3 Plinian fall pumice sample has low vesicularity (40-53vol%) and high number density (5.0*10¹⁴ to 1.5*10¹⁵m^-3) of plagioclase microlite, indicating that vesiculation and expansion occurred in a semi-open system. The CSDs analysis of the type-1 to type-3 samples indicate that increase of number density and crystallinity is due to the increase of microlite of small size. Opening of new conduit in relatively shallow level than that of Plinian conduit enhanced effective degassing of magmas and resulted in the change of eruption style from explosive to effusive. Higher crystallinity and larger microlite size of the lava flow samples compared with Plinian fall pumice samples are interpreted as due to their lower ascent and degassing rate of the magma.

View full abstract

Lithium (Li), boron (B), and lead (Pb) isotopic compositions of glass inclusions in olivine phenocrysts from Hawaiian lavas, (Kilauea Iki, Mauna Loa and Koolau volcano) were measured by high mass resolution ion probe (HR-SIMS) to search for possible signatures of recycled materials in the Hawaiian plume.

View full abstract

We present clinopyroxene REE variations in certain peridotite xenoliths recovered from Malaita, Solomon Islands, to address the role of garnet during melting in a mid-oceanic ridge setting. On a Sm/Yb ratio versus Yb content diagram, the measured clinopyroxenes yield a significant deviation from the field of global abyssal peridotites toward higher Yb at given Sm/Yb, that can be recognized as strong garnet signature. However, most likely explanation for the deviation is subsolidus REE redistributions caused by changes of modal abundance ratio and REE partition coefficients between orthopyroxene and clinopyroxene. Thus, caution should be excercised while using clinopyroxene REE variation to address the melting history of peridotite.

View full abstract

ODP Leg 209 was performed from May to July, 2003 to drill mantle peridotites and associated gabbroic rocks (at 8 Sites, 19 holes) along the Mid- Atlantic Ridge (MAR) from 14-16 N, both sides of 15 20'N FZ. In this area, a series of outcrops of residual mantle peridotite on both sides of the rift valley. Site 1268, 1270, 1271 ans 1272 yielded substantial proportions of gabbroic rocks intrusive into mantle peridotite (25% of gabbros and 75% of mantle peridotite). Abundant of gabbro and dunite were recovered from this area suggest that main volume of melt body were consumed in the upper mantle by melt-wallrock interaction.

View full abstract

The Onsen site in the Desmos caldera of the Manus back-arc basin is a typical acid-sulfate mineralisation on the seafloor which is denoted by three stages of advanced argillic alteration. High concentration of Mg and SO4 in the Onsen fluid, depletion of major, minor and REE is altered rock and presence of high and low temperature acid stable minerals suggest that mineralisation is related to fluctuating influx of magmatic fluid.

View full abstract

[in Japanese]

View full abstract

[in Japanese]

View full abstract

[in Japanese]

View full abstract

We report on analytical procedures for Li isotopic analysis of olivine by a High Resolution Secondary Ion Mass Spectrometry (HR-SIMS), Cameca ims-1270 with mono-collection system.In order to accomplish high spatial resolution and accurate Li isotope analysis, analytical conditions determined and reliable standards prepared.

View full abstract

[in Japanese]

View full abstract

[in Japanese]

View full abstract

The effect of non-equilibrium at an interface to chemical zoning of metamorphic garnet is theoretically studied. The formation conditions of normal and reverse zonings are derived.

View full abstract

Phenocrysts of black plagioclase from the Lac Saint-Jean Proterozoic massif type anorthosite in Quebec, Canada were studied to explain the simultaneous existence of ilmenite, magnetite, hematite, hemo-ilmenite and fine unidentified Fe-Ti dust, and the unusual presence of the ⟨Si₄O₈ and AlAl₃SiO₈ feldspar end-members in the plagioclase. Our result indicate that significant amounts of Fe2+ were originally present in the tetrahedral and cavity sites of the plagioclase and that Ti⁴⁺ was also present in the tetrahedral site. We conclude that plagioclase in the Lac Saint-Jean anorthosite crystallized under fairly reducing conditions and later underwent postmagmatic oxidation when the plutonic mass was uplifted and came into contact with crustally-derived

View full abstract

The TiO2 solubility in quartz depends on increasing temperature. We present the quartz geothermometry.

View full abstract

[in Japanese]

View full abstract

[in Japanese]

View full abstract

Large zircon porphyroblasts (up to 3 mm) occur in jadeitite of the Osayama serpentinite melange (OSM), central Chugoku Mountains. Zircons coexist with jadeite, grossular and rutile, and they contain tiny inclusions of jadeite and rutile. Zircons analysed by SHRIMP-RG are characterized by low Th- and U-abundances (Th = 1-81 ppm; U = 6-149 ppm). Textural and geochemical features define three zones: normal growth zone with Th/U ratio of 0.2-0.5 (Type I), texturally inherited cores with Th/U ratio of 0.7-0.8 (Type II), and replacement zone (Type III). The ratio of common Pb to radiogenetic Pb is clearly high (10-26 %) in Type III zones, whereas other zones are less than 5.4 % (mostly < 0.5 %). The Type I and Type II zones yield a homogeneous age population with a weighted mean of ²⁰⁶Pb/²³⁸U spot ages of 476 ± 10 Ma (n = 34). Since zircon porphyroblasts contain high-P mineral inclusions, the U-Pb age represents the time of jadeitite formation, i.e., the timing of interaction between alkaline fluid and ultramafic rocks in a subduction-zone, and constraints the minimum age for serpentinization. The jadeitite formation is significantly older than phengite K-Ar ages of the OSM blueschists (273-327 Ma). This relationship suggests the jadeitite formation is not related to regional Late Paleozoic blueschist metamorphism. The jadeitite formation and serpentinization may instead be related to Early Paleozoic high-P epidote-amphibolites with hornblende K-Ar age of 403-443 Ma in a serpentinized peridotite body in the eastern Chugoku Mountains, about 180 km to the east of the OSM.

View full abstract

We have developed an improved method for examining the metamorphic history of subducting oceanic crust and its impact on chemical cycling at convergent margins. Our approach provides a much-needed metamorphic complement to records of subduction-zone cycling derived from studies of igneous rocks produced at volcanic arcs. By merging methods utilizing garnet zoning to establish prograde reaction histories with in situ high-resolution trace element geochemistry, and application to coesite eclogites representing subduction to depths beneath arcs, we are able to directly identify geochemical manifestations of reactions contributing to element mobility in the subducting slab that are only inferred in studies of volcanic arcs or theoretical metamorphic models.

View full abstract

[in Japanese]

View full abstract

[in Japanese]

View full abstract

[in Japanese]

View full abstract

[in Japanese]

View full abstract

[in Japanese]

View full abstract

Ryoke gneisses in Tomiyama were intruded by granitic veins several to 30cm in width. Folded and boudinaged veins were emplaced around 93-90 Ma, and non-deformed veins c.85 Ma. Ductile deformation continued beyond 93-90 Ma and ended before 85 Ma. Quartz in the non-deformed veins has chessboard-patterned SGBs, similar to those in deformed veins. The density of SGBs in the non-deformed veins is much lower than in the deformed veins, and is compatible with that in the late-stage two-mica granite, emplaced c.74 Ma. A period of thermal inactivity is thus envisaged before the reheating of the area by the two-mica granite. Uplift subsequent to the final plutonism may have produced minor deformation, expressed as chessboard SGBs in quartz grains in non-deformed veins and the two-mica granite.

View full abstract

[in Japanese]

View full abstract

[in Japanese]

View full abstract

The Pliocene Kurobegawa Granitic Pluton, exposed in the Northern Japan Alps, is vertically divided into the lower, the middle and the upper units. Elliptic mafic enclaves occur densely in the most part of the pluton and hardly mixed with the host magma. In chondrite normalized REE patterns for the host granite, the upper part is depleted in LREE and the lower part is enriched in HREE. Modal analyses of accessory minerals suggest that the granitic magma was probably saturated in allanite after the lower unit was formed during fractional crystallization. Assuming a starting melt composition equivalent to the average composition of the middle and the upper unit, the calculated REE patterns of the granite using modal compositions of accessories and their distribution coefficients show that the characteristics of the REE patterns for 40% fractional crystallization are similar to those of the upper granite.

View full abstract

[in Japanese]

View full abstract

[in Japanese]

View full abstract

[in Japanese]

View full abstract

[in Japanese]

View full abstract

[in Japanese]

View full abstract

Lower crustal materials such as granulite terranes and deep-seated xenoliths often suffer metasomatism by late-stage magmatism or fluids. To discuss formation process of the lower crust, determination of initial compositions of the lower crustal materials is very important. In this study, we present evidence of metasomatism under the lower crust beneath Oki-Dogo, southwest Japan using lower crustal xenoliths, and discuss the metasomatic agent.

View full abstract

[in Japanese]

View full abstract

[in Japanese]

View full abstract

[in Japanese]

View full abstract

Granulite-facies rocks in the Sefuri Mountains, North Kyushu, were equilibrated at c. 900oC and 5 kbar. Ultramafic rocks preserve microstructures indicative of retrograde reaction that consumes plagioclase.

View full abstract

Laser Raman analysis of micro-fluid inclusions in ultrahigh-temperature metamorphic rocks

View full abstract