Kentaro Terada

Ion micro-probe U–Pb zircon geochronology of peak and retrograde stages of ultrahigh-pressure metamorphic rocks from the Kokchetav massif, northern Kazakhstan

Sensitive high resolution ion micro-probe (SHRIMP) U–Pb analyses of zoned zircons were performed to constrain the age and geodynamic significance of the Kokchetav ultrahigh-pressure (UHP) metamorphic rocks, which recrystallized at P>60 kbar and T>1000°C. Cathodoluminescence images were correlated with micro-assemblages of mineral inclusions in zircons from diamond-bearing gneisses. These revealed UHP mineral-bearing cores and low-pressure mineral-bearing rims, some zircons also retain inherited cores. Our SHRIMP data identified four discrete stages: Middle Proterozoic protolith age, 537±9 Ma for UHP metamorphic conditions, 507±8 Ma for the late-stage amphibolite facies overprint, and 456–461 Ma for post-orogenic thermal events. Thus, we conclude that Middle Proterozoic supracrustal protoliths of the Kokchetav UHP–HP rocks were subducted to mantle depths in the Middle Cambrian, and exhumed to mid-crustal levels in the Late Cambrian. The exhumation rate calculated from the SHRIMP data and relative P–T conditions is approximately 5 km/Ma. The rapid exhumation rate for the massif is in the same range as other UHP metamorphic terranes, and suggests that slab decoupling may be a driving force for exhumation of UHP metamorphic rocks from mantle depths.

High mass resolution ion microprobe analysis of rare earth elements in silicate glass, apatite and zircon: lack of matrix dependency

Abstract The concentrations of all rare earth elements (REE: La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) in silicate glass, apatite and zircon were measured by an ion microprobe method with a high mass resolving power of 9300 at 1% peak height. Observed contents of REE in apatite extracted from a dacite agree well with those measured by the isotope dilution-thermal ionization mass spectrometry (ID-TIMS) method within 30% error at 2σ. The contents of heavy REE in several zircons are consistent with those measured by ID-TIMS and instrumental neutron activation analysis (INAA), while the light REE abundances are significantly lower by ion microprobe than by the other methods. The discrepancy is partly due to the presence of 1–5-μm inclusions of apatites within zircon crystals. Secondary ion yields of REE observed in a silicate glass show a very similar trend to those in apatite and zircon. Since the chemical composition of the silicate glass is appreciably different from the apatite and zircon, relative secondary ion yields of REE are probably independent from matrix effect, which is the most important finding of this work. Calculated partition coefficients of REE between melt and apatite agree well with reported values while the coefficients between melt and zircon are discrepant from those in literature. The REE coefficients in zircon increase with increase of the ionic radius logarithmically, suggesting power law dependence of the coefficients.

Canonical time variations of X-rays from black hole candidates in the low-intensity state

X-rays from black hole candidates, Cyg X-1, GX 339-4, and GS 2023+338 in their low-intensity state, consist of various shots or burst events, and shapes of the shots are similar among these sources. In their low-intensity states, the normalized power spectrum density functions of the X-rays except for the Fourier frequencies below about 0.2 Hz are the same not only in their shape but also in their absolute values, even if the X-ray energy range is different.

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.

North-south extension in the Tibetan crust triggered by granite emplacement

We combine zircon sensitive high-resolution ion microprobe U-Pb spot dating and mica 4 0 Ar- 3 9 Ar plateau ages with field-geological and geochemical constraints from the Malashan area of Southern Tibet to show that the deformed granite core of the North Himalayan metamorphic domes in this area is not Indian basement, but was intruded and deformed during the Himalayan orogeny. Microstructural observations reveal that a transition from top-to-the-south thrust-related to top-to-the-north extension-related deformation occurred during granite intrusion and related metamorphism. This suggests that intrusion triggered the onset of extensional tectonics in the Tibetan middle to upper crust. Expected positive feedback mechanisms between decompression melting leading to more intrusion and more extensional deformation suggest that this mechanism may have been important on a regional scale.

Cenozoic and Mesozoic metamorphism in the Longmenshan orogen: Implications for geodynamic models of eastern Tibet

New zircon U-Pb and mica 40Ar/39Ar dating combined with structural studies in the Longmenshan orogen confirm that most of the upper crustal deformation in the eastern margin of Tibet is Mesozoic. However, at lower structural levels, apatite U-Pb and monazite electron microprobe dating reveals a previously unknown domain of Cenozoic (ca. 65 Ma) Barrovian-type metamorphism and deformation. This discovery shows that the crust in the eastern margin of Tibet was already a substantial thickness around the time of the India-Asia collision. Associated deformation has a N-S-oriented stretching lineation, implying that deformation was not driven by topographic gradients in the Tibetan Plateau. The observed moderate amounts of distributed postmetamorphic E-W shortening can probably explain the present thickness of the continental crust in the area. These results do not support models of crustal thickening caused by solid-state lateral flow of midcrustal metamorphic rocks.

Large-scale displacement along the Median Tectonic Line, Japan: evidence from SHRIMP zircon U–Pb dating of granites and gneisses from the South Kitakami and paleo-Ryoke belts

Abstract In this paper we present new U–Pb zircon ages determined with Sensitive High-Resolution Ion MicroProbe (SHRIMP) for nine plutonic rocks or orthogneisses and one paragneiss from the Higo and Maana belts (here referred to as the paleo-Ryoke belt) in southwest Japan, and the South Kitakami belt in northeast Japan. Both belts are Paleozoic–Mesozoic continental terranes discontinuously distributed along the Median Tectonic Line (MTL), and structurally lying on the Sambagawa belt in the Japanese Islands. Three groups of U–Pb zircon ages showing the timing of different magmatic events were determined for the plutonic rock or orthogneiss samples: ca. 500 Ma (Early Ordovician, two samples), a mean age of 292.0±12.4 Ma (Late Carboniferous–Early Permian, one sample) and ca. 110 Ma (Early Cretaceous, six samples). Furthermore, zircons with core–rim microstructures from a garnet–biotite–cordierite paragneiss in the Higo belt were also dated with SHRIMP. The cores yielded 2155–184 Ma inherited U–Pb ages (mostly varying between 330 and 184 Ma), while the rims yielded a mean age of 116.5±18.7 Ma (Early Cretaceous) U–Pb recrystallization ages comparable with the igneous ages, limiting the sedimentary age of the protolith to between Early–Middle Jurassic (ca. 180 Ma) and Early Cretaceous time. These SHRIMP U–Pb ages of plutonism, metamorphism and sedimentation, together with previously reported radiometric ages, have revealed a strong similarity in the tectonic histories of the paleo-Ryoke and South Kitakami and Abukuma belts, suggesting that these belts can be correlated. At present the paleo-Ryoke belt and the South Kitakami and Abukuma belts are separated by more than 1000 km. However, these belts may have originally comprised a continuous continental terrane that was later displaced and juxtaposed, together with the underlying Sambagawa belt, against the Ryoke belt by large-scale sinistral faulting along the MTL and associated major strike–slip faults during the Latest Cretaceous.

Zircon SHRIMP U-Pb dating on plagiogranite from Kuerti ophiolite in Altay, North Xinjiang

Field observation, petrological and geochemical characteristics of plagiogranite from Kuerti ophiolite indicate a similar origin to those in shearing zones. It is derived from partial melting of amphibolite that is developed from gabbro within the ocean layer 3 shear zone by the low-angle shearing deformation during the oceanic crust migrating process. Zircon SHRIMP age of 372±19 Ma for the plagioganite from Kuerti ophiolite indicates that this ophiolite formed in the Devonian period and it also represented the time of extension of the Kuerti backarc basin that is relevant to the northwards subduction of the Paleo-Asian oceanic crust. Therefore, the northwards subduction of the Paleo-Asian Ocean beneath the Siberian Plate began in the early stage of the Late Paleozoic era.

Cryptomare magmatism 4.35 Gyr ago recorded in lunar meteorite Kalahari 009

The origin and evolution of the Moon remain controversial, with one of the most important questions for lunar evolution being the timing and duration of basaltic (mare) magmatism. Here we report the result of ion microprobe U–Pb dating of phosphates in a lunar meteorite, Kalahari 009, which is classified as a very-low-Ti mare-basalt breccia. In situ analyses of five phosphate grains, associated with basaltic clasts, give an age of 4.35 ± 0.15 billion years. These ancient phosphate ages are thought to represent the crystallization ages of parental basalt magma, making Kalahari 009 one of the oldest known mare basalts. We suggest that mare basalt volcanism on the Moon started as early as 4.35 Gyr ago, relatively soon after its formation and differentiation, and preceding the bulk of lunar volcanism which ensued after the late heavy bombardment around 3.8-3.9 Gyr (refs 7 and 8). Considering the extremely low abundances of incompatible elements such as thorium and the rare earth elements in Kalahari 009 (ref. 9) and recent remote-sensing observations illustrating that the cryptomaria tend to be of very-low-Ti basalt type, we conclude that Kalahari 009 is our first sample of a very-low-Ti cryptomare from the Moon.

Dating of zircon from Ti-clinohumite–bearing garnet peridotite: Implication for timing of mantle metasomatism

Garnet peridotites from the Kokchetav ultrahigh-pressure (UHP) massif contain abundant volatile and high field strength element (HFSE) bearing minerals, such as Ti-clinohumite and zircon. These characteristics are interpreted to be related to fluid-infiltrated mantle metasomatism from the oceanic lithosphere that had been subducted below the area. The zircons from the peridotites were dated by using sensitive high-resolution ion microprobe (SHRIMP) and yielded apparent U-Pb ages of 554–494 Ma (weighted mean age, 528 Ma) that are mostly consistent with the timing of UHP metamorphism deduced from diamond-bearing country rocks in this massif. These zircons have an almost flat rare earth element (REE) pattern and very low REE concentrations; these characteristics are similar to those observed in kimberlitic zircons. Inherited zircon cores, although only rarely preserved, yielded apparently Proterozoic ages and have different trace element characteristics compared to the overgrowth rims. These features indicate that the mantle metasomatic events and the recrystallization of Ti-clinohumite and zircon were due to HFSE-enriched fluid infiltration during the UHP metamorphism at great depths. The metasomatized mantle may have been transported farther into the deep upper mantle and contributed to the source of intraplate magmas such as kimberlites and alkali basalts, because these rocks have characteristically high volatile and HFSE concentrations such as those of the Ti-clinohumite–bearing garnet peridotites.