Mitsuhiro Toriumi

First SHRIMP U-Pb zircon dating of granulites from the Kontum massif (Vietnam) and tectonothermal implications

Abstract The Kontum massif in Central Vietnam represents the largest continuous exposure of crystalline basement of the Indochina craton. The central Kontum massif is chiefly made of orthopyroxene granulites (enderbite, charnockite) and associated rocks of the Kannack complex. Mineral assemblages and geothermobarometric studies have shown that the Kannack complex has severely metamorphosed under granulite facies corresponding to P–T conditions of 800–850°C and 8±1 kbars. Twenty-three SHRIMP II U–Pb analyses of eighteen zircon grains separated from a granulite sample of the Kannack complex yield ca 254 Ma, and one analysis gives ca 1400 Ma concordant age for a zoned zircon core. This result shows that granulites of the Kannack complex in the Kontum massif have formed from a high-grade granulite facies tectonothermal event of Indosinian age (Triassic). The cooling history and subsequent exhumation of the Kannack complex during Indosinian times ranged from ∼850°C at ca 254 Ma to ∼300°C at 242 Ma, with an average cooling rate of ∼45°C/Ma.

P-T-T PATHS AND POST-METAMORPHIC EXHUMATION OF THE DAY NUI CON VOI SHEAR ZONE IN VIETNAM

Abstract The Day Nui Con Voi belt in Vietnam is the southeasternmost part of the Red River shear zone in Asia. It is a narrow high-grade metamorphic core complex consisting of garnet–sillimanite–biotite gneisses, mylonite bands, amphibolite layers and migmatites. Geothermobarometric study of the complex revealed that the peak metamorphism took place under amphibolite-facies conditions of 690 − 60 +30 °C and 0.65±0.15 GPa and the subsequent mylonitization occurred under greenschist-facies conditions of ∼480°C and under 0.3 GPa. Fifteen synkinematic hornblende and biotite separates from gneisses, amphibolites and mylonites were dated with the K/Ar method. Hornblende separates from the Day Nui Con Voi give K–Ar ages of 26.4–28.5 Ma, and the biotite separates do give 24.5–24.7 Ma. Combination of thermobarometric and geochronological data yields the cooling history of 500°C at 28 Ma and 300°C at 24 Ma with a cooling rate of 70–110°C Ma −1 , and 23 km post-metamorphic exhumation of the core complex. The first 16 km exhumation from the peak of metamorphism (at probably 31 Ma) to 28 Ma was triggered by the left-lateral strike-slip displacement of the Red River shear zone.

Silicon self-diffusion in MgSiO3 perovskite at 25 GPa

Abstract Silicon self-diffusion coefficients in MgSiO3 perovskite were measured under lower mantle conditions. The MgSiO3 perovskite was synthesized and diffusion annealing experiments were conducted at pressure of 25 GPa and temperature of 1673–2073 K using a MA8 type high-pressure apparatus. The diffusion profiles were obtained by secondary ion mass spectrometry. The lattice and grain boundary diffusion coefficients (D1 and Dgb) were determined to be D1 [m2/s]=2.74×10−10 exp(−336 [kJ/mol]/RT) and δDgb [m3/s]=7.12×10−17 exp(−311 [kJ/mol]/RT), respectively, where δ is the width of grain boundary, R is the gas constant and T is the absolute temperature. These diffusion coefficients play a key role for understanding the rheology of the lower mantle.

Grain Growth Rates of MgSiO3 Perovskite and Periclase Under Lower Mantle Conditions

The grain growth rates of MgSiO3 perovskite and periclase in aggregates have been determined at 25 gigapascals and 1573 to 2173 kelvin. The average grain size (G) was fitted to the rate equation, and the grain growth rates of perovskite and periclase were G10.6 = 1 × 10−57.4 t exp(−320.8/RT) and G10.8 = 1 × 10−62.3 t exp(−247.0/RT), respectively, where t is the time, R is the gas constant, and T is the absolute temperature. These growth rates provide insight into the mechanism for grain growth in minerals relevant to the Earths lower mantle that will ultimately help define the rheology of the lower mantle.

Experimental studies on the recovery process of deformed olivines and the mechanical state of the upper mantle

Abstract Naturally deformed olivines in peridotites from Eifel, Horoman, Red Hill and San Carlos were annealed at 1300–1500°C under controlled oxygen fugacity for the purpose of clarifying the dislocation annihilation process. The decreasing rate of dislocation density was well approximated by the following equation: dρ dt = −10 2 exp {−110,000 (cal/mo)/RT }ρ 2 (cm −2 sec −1 ) where ρ is the dislocation density. Thus the half-life time of dislocation density in annealing process depends both on temperature and on initial dislocation density. The activation energy obtained is very similar to the value given by Kohlstedt and Goetze (1974) for the steady state creep of olivine. Two types of dislocation structures were observed. One is an equi-axed cellular network structure (cellular type) which probably formed in steady-state creep, and the other is a tangled structure (tangled type) probably formed in transient creep. Thus, if the annealing time is short enough and the dislocation structure is characteristic of steadystate creep (i.e., cellular type), then the differential stress acting within the upper mantle can be inferred from the dislocation density of the olivine in peridotite nodules. The annealing time is estimated for the olivines in peridotite nodules from Ichinomegata, northern Japan and the differential stress is estimated as 100 to 300 bars. The successive partial melting and crystallization process is inferred from coexistence of two types of olivines in peridotite.

Newtonian Dislocation Creep in Quartzites: Implications for the Rheology of the Lower Crust

Mechanical and microstructural evidence indicates that a natural and a synthetic quartzite deformed by Newtonian dislocation (Harper-Dorn) creep at temperatures higher than 1073 K and stresses lower than 300 megapascals. The observation of this creep in these materials suggests that the lower crust may flow like a Newtonian viscous fluid by a dislocation mechanism at stresses much smaller than those previously postulated.

Relation between dislocation density and subgrain size of naturally deformed olivine in peridotites

The subgrain size and the dislocation density of subgrain interiors were measured by the oxidation-decoration method under the optical microscope on naturally deformed olivine from peridotite xenoliths and alpine-type peridotites. Relation of the subgrain size, d and the dislocation density, ϱ, within subgrains is represented by the equation, d=15/√ϱ. Combining with relations of the differential stress and the dislocation density proposed by Kohlstedt and Goetze (1974), relation between the stress (σ) and the subgrain size becomes d=45 Gb/σ, where G and b are the rigidity and the magnitude of the Burgers vector of olivine. This relation is in good agreement with those in a simple oxide (MgO), and alkali halides (NaCl, LiF) given by Hüther and Reppich (1973), Poirier (1972), and Streb and Reppich (1973), respectively.

Preferred Orientation Development of Dynamically Recrystallized Olivine during High Temperature Creep

Single crystals of olivine change to polycrystalline aggregates through dynamic recrystallization during creep at temperatures of 1550-1650°C and stresses of 40-120 MPa. The deformation of the dynamically recrystallized specimens displays nearly plane strain, and the maximum elongation of samples is perpendicular to original c- (for 〈110〉c) or b-axis (for 〈101〉c). The preferred orientation development associated with dynamic recrystallization begins with the rotation of subgrains as a result of accumulation of dislocations due to crystalline slip. With increase of strain, new grains are formed at high angle subboundaries and at grain boundaries. The b-axis fabrics at this stage show two maxima: one near the original direction, and the other near parallel to the maximum compression (shortening) axis. In the final stage, where recrystallization is complete, recrystallized grains also undergo significant plastic strain, and strong a- and b-axis concentrations parallel to, respectively, the maximum elongation and maximum shortening (compression) axes were found. Thus two processes of preferred orientation development in syntectonically (dynamically) recrystallized olivine occur under our experimental conditions: lattice rotation due to dislocation glide, and nucleation and growth of new grains. Strong correlations of final preferred orientation with strain indicates that the former process dominates. Applying these results to deformation in the earth, it is suggested that the preferred orientation is kinematically controlled by crystalline slip even when dynamic recrystallization occurs, indicating that the seismic anisotropy in the deep upper mantle corresponds directly to flow pattern rather than to stress orientation.

Grain size distribution of the matrix in the Allende chondrite

Abstract HRTEM, ATEM and SEM studies of the Allende chondrite reveal that the matrix is composed of very fine-grained iron-rich olivine, Ca-poor and iron-rich clinopyroxene, iron-rich spinel, and Ni-bearing troilite. There are two types of very fine-grained aggregates: one is not sintered and the other is slightly sintered. Most (1–10 μm) olivine aggregates show the sintered microstructure but ultrafine-grained aggregates (1–10 nm) display little evidence of sintering. This means that coarse-grained olivine aggregates experienced a heating event but ultrafine-grained aggregates underwent no heating event. The size distribution of matrix grains were measured by SEM and TEM at magnifications of 570, 000 to 2000 corresponding to grain sizes ranging from 1 nm to 10 μm. The frequency distribution shows a log-normal pattern having a peak at 5 nm in the range from 1 to 10 nm, and it has a long-term tail with power law. This distribution means that the fine-grained matrix seems to be formed at conditions far from equilibrium in the protosolar cloud.

Markov random field modeling for mapping geofluid distributions from seismic velocity structures

We applied the Markov random field model, which is a kind of a Bayesian probabilistic method, to the spatial inversion of the porosity and pore shape in rocks from an observed seismic structure. Gaussian Markov chains were used to incorporate the spatial continuity of the porosity and the aspect ratio of the pore shape. Synthetic inversion tests were able to show the effectiveness and validity of the proposed model by appropriately reducing the statistical noise from the observations. The proposed model was also applied to natural data sets of the seismic velocity structures in the mantle wedge beneath northeastern Japan, under the assumptions that the fluid was melted and the temperature and petrologic structures were uniformly distributed. The result shows a significant difference between the volcanic front and the forearc regions, at a depth of 40 km. Although the parameters and material properties will need to be determined more precisely, the Markov random field model presented here can serve as a basic inversion framework for mapping geofluids.