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We present Early Cretaceous partial melting recorded by pelitic gneiss from the Nagasaki Metamorphic Complex, western Kyushu, Japan. The existence of the partially melted rock in the complex may imply initiation of long-lived high-T metamorphism in proto-southwest Japan at Cretaceous eastern margin of Eurasia. The gneisses are composed of three garnet-bearing layers and one felsic lens, and they show mylonitic foliation. Application of Zr-in-rutile geothermometer and quartz-in-garnet Raman geobarometer for inclusion rutile and quartz in garnet provided prograde condition of 670–692°C and 1.01–1.26 GPa and peak condition of 755–769°C and 1.17–1.38 GPa. These conditions correspond to those of lower crustal depths between volcanic arc and forearc where thermal advection by wedge mantle corner flow occurs. Presence of the felsic lens whose mineral assemblage can undergo partial melting under the peak condition and presence of a crystallized melt inclusion in garnet suggest that distinct partial melting occurred during the metamorphism. The concordant U–Pb ages of metamorphic zircon rims are ranging from 131.5 ± 4.3 to 120.1 ± 3.9 Ma. The mean square weighted deviation (MSWD) of the metamorphic age was as large as 6.4, implying a significant duration of high-T condition for growth of metamorphic zircon. The obtained ages are within the period of the magmatic hiatus that has previously proposed around southwest Japan and Korean Peninsula (ca. 160–120 Ma) that is due to stop of wedge mantle corner flow by flatting of subducting slab. Our results suggest that high-T metamorphism associated with partial melting at lower crustal depths along Early Cretaceous proto-southwest Japan has already been initiated at the late stage of the hiatus (ca. 130 Ma). This may be due to slab rollback and trench retreat at eastern margin of Eurasia. [ABSTRACT FROM AUTHOR]

Behavior of zircon at the schist/migmatite transition is investigated. Syn-metamorphic overgrowth is rare in zircon in schists, whereas zircon in migmatites has rims with low Th/U that give 90.3 ± 2.2 Ma U-Pb concordia age. Between inherited core and the metamorphic rim, a thin, dark-CL annulus containing melt inclusion is commonly developed, suggesting that it formed contemporaneous with the rim in the presence of melt. In diatexites, the annulus is further truncated by the brighter-CL overgrowth, suggesting the resorption and regrowth of the zircon after near-peak metamorphism. Part of the zircon rim crystallized during the solidification of the melt in migmatites. Preservation of angular-shaped inherited core of 5-10 µm in zircon included in garnet suggests that zircon of this size did not experience resorption but developed overgrowths during near-peak metamorphism. The Ostwald ripening process consuming zircon less than 5-10 µm is required to form new overgrowths. Curved crystal size distribution pattern for fine-grained zircons in a diatexite sample may indicate the contribution of this process. Zircon less than 20 µm is confirmed to be an important sink of Zr in metatexites, and ca. 35-µm zircon without detrital core are common in diatexites, supporting new nucleation of zircon in migmatites. In the Ryoke metamorphic belt at the Aoyama area, monazite from migmatites records the prograde growth age of 96.5 ± 1.9 Ma. Using the difference of growth timing of monazite and zircon, the duration of metamorphism higher than the amphibolite facies grade is estimated to be ca. 6 Myr. Keywords Zircon * Migmatite * Melt inclusion * Glass * Crystal size distribution * Duration of metamorphism, Introduction Behavior of zircon during the metamorphism is a matter of great interest because zircon could grow during many stages of metamorphism and the U-Pb spot ages of this mineral [...]

Ocean-plate stratigraphy (basalt, chert, clastics) of the Balcreuchan Group in the Ballantrae complex is repeated by layer-parallel thrusts to form duplex structures south of Bennane Head, ~5 km north of Ballantrae. Five tuff beds, all in chert, have similar U-Pb zircon ages of ca. 470 [+ or -] 10 Ma. The geometrical polarity of the duplexes and the zircon ages provide new constraints on the tectonic evolution of the accretionary wedge of the Ballantrae complex. Northwestward thrusting of the duplexes suggests that subduction was from the northwest to the southeast, and this polarity at the leading edge of the main ophiolite body is consistent with the northward younging of the volcanic arc rocks of the Ballantrae complex. doi: 10.1130/B26329.1

The distribution function of the ocean nadir radiance, defined by the upward radiance-to-irradiance ratio, is investigated as functions of the absorption coefficient and the volume scattering function to understand their relationship rather than to develop a numerical algorithm. It is shown for oceanic waters that the distribution function is directly proportional to the volume scattering function normalized by the backscattering coefficient. The relatively small spectral variation of the distribution function is explained by the small spectral variation of the normalized volume scattering function, as well as by a function that describes the contribution of the backscattering-to-absorption ratio to the distribution function. The interpretation described was verified against in situ data, highlighting factors controlling the distribution function of oceanic waters. OCIS codes: 010.4450, 010.5630, 010.5620, 010.4458, 010.1350, 010.0280.

A new gain calibration method (roundabout method) that entails the use of a 1013 Ω amplifier-equipped multiple-collector inductively coupled plasma mass spectrometer was developed to correctly determine amplification gains of Faraday detectors. The amplification gains were successfully calibrated by implementing various configurations of Faraday cups and amplifiers in 142Nd/144Nd, 143Nd/144Nd, 145Nd/144Nd, 148Nd/144Nd, and 150Nd/144Nd isotopic measurements. Here, Sr, Hf, W, and Pb isotopic analysis was applied for several reference materials (NIST SRM 987, JMC475, NIST SRM 3163, NIST SRM 981, and NIST SRM 982) to validate the new calibration method. Regarding the resulting isotopic data, the roundabout method yielded isotopic results that were dissimilar to those of an already existing protocol based on constant current calibration. The 208Pb/204Pb isotopic analysis of NIST SRM 981 and 982 resulted in the observed biases reaching >10ε. Alternatively, the isotopic data obtained by using a conventional 1011 Ω amplification system were largely consistent with the roundabout calibration protocol-derived results. This indicates that gains of 1013 Ω amplifiers can be accurately determined by using the roundabout technique. Compared to the previous gain calibration technique, the roundabout method shows approximately twice better analytical precision. A significant advantage of the new calibration protocol is that it does not require a reference material for each element. This advantage is critical to improving the analytical accuracy of gain measurements and extending the analytical applicability of MC-ICPMS isotopic measurements in the field of isotope geoscience. The resulting isotopic data clearly demonstrate that precise isotopic measurements can be performed by applying 1013 Ω resistors to Faraday detectors. The new calibration protocol has the potential to validate a gain calibration protocol, and its practical advantage is that it effectively downsizes the sample amount used for high-precision isotopic analysis. [ABSTRACT FROM AUTHOR]

We present a detailed petrological, structural and geochronological study of the mylonitic Ryoke metamorphic rocks within the granitic mylonite (Kashio mylonite) and Sanbagawa metamorphic rocks along the Median Tectonic Line (MTL), Japan. Located in the Oshika area of the Chubu District, the Kashio mylonite is one of the few geologic units that can be used to determine detailed pressure–temperature–time–deformation (P–T–t–D) paths during mylonitization because it occurs as many small tectonic blocks of mylonitic metasediment. Detailed petrological analysis coupled with conventional thermobarometry and P–T pseudosection modelling give estimated peak P–T conditions (M1a) of 650–790°C at 4.6–5.6 kbar for the Ryoke metamorphic rocks. The gneissose Ryoke granitoids were emplaced subhorizontally at around 685–710°C and 4.6–5.8 kbar, after peak metamorphism. The Kashio shear zone developed immediately after the last igneous activity at ca. 71 Ma, and two stages of mylonitization (stages D1 and D2) can be identified from microstructural observations. The retrograde P–T conditions (M1b) recorded in the Kashio mylonite exhibit a systematic change in temperature from 710°C to 450°C at 5.2–2.6 kbar with decreasing distance from the MTL. By contrast, highly deformed mylonites with zoned garnets demonstrate a striking increase in pressure from 4.0 to 8.3 kbar with decreasing temperature from 590°C to 450°C after low‐P/T‐type metamorphism. Such a temperature range indicating isothermal compression is consistent with deformation temperatures of stage D1 determined from quartz microstructures and quartz c‐axis fabric opening‐angle deformation thermometer. Moreover, the timing of the two mylonitization episodes during retrograde metamorphism are estimated to be 69–67 and 66–64 Ma, respectively, with a high cooling rate of ~34°C/Ma using the revised time–temperature relationship of the host Ryoke granitoids. The rapid change in tectonic setting with strain localization occurred during the brief period between 69 and 64 Ma. Our field and petrological observations imply that a thick D1 mylonite zone was formed by rapid subsidence (≥10 km) with cooling of the hangingwall rocks from the middle crust to the subduction interface. It is considered that the underplating of exhumed high‐P/T‐type metamorphic rocks led to further cooling between hangingwall and footwall rocks and the formation of a narrow D2 mylonite zone, which served as an old plate boundary. Thus, low and high‐P/T‐type metamorphic belts had already been amalgamated as paired metamorphic belts beneath the brittle–ductile transition of the subduction zone before exhumation. The rapid cooling of hangingwall rocks at the subduction interface is proposed to play an essential role in the thermal overprinting of exhumed high‐P/T‐type metamorphic rocks. [ABSTRACT FROM AUTHOR]

It is widely recognized that the natural isotopic variation of Mo can provide crucial information about the geochemical circulation of Mo, and the ocean is an important reservoir of Mo. To obtain precise isotopic data on Mo in seawater samples using multiple collector-inductively coupled plasma mass spectrometry (MC-ICPMS), we have developed a preconcentration technique using 8-hydroxyquinoline bonded covalently to a vinyl polymer resin (TSK-8HQ). By optimizing the procedure, Mo in seawater could be effectively separated from matrix elements such as alkali, alkaline earth, and transition metals. With this technique, even with a 50-fold enrichment factor, the changes in the [sup.98]Mo/[sup.95]Mo ratio during preconcentration were smaller than twice the standard deviation (SD) in this study. Mass discrimination of Mo isotopes during the measurement was externally corrected for by normalizing [sup.86]Sr/[sup.88]Sr to 0.1194 using an exponential law. We evaluated [[delta].sup.98/95]Mo to a precision of [+ or -] 0.08%0 ([+ or -]2 SD); this value was found to be less than one-third of previous reported values. Moreover, we were able to determine an accurate ratio for every pair of stable Mo isotopes, which was impossible with previous methods owing to the isobaric interference from the external elements (Zr and Ru). In this study, [[delta].sup.92/98]Mo in seawater was first determined so that it had the smallest relative error. We applied the proposed method to four kinds of seawater samples. The Mo compositions were constant among them, with average [[delta].sup.98/95]Mo and [[delta].sup.92/98]Mo values of 2.45 [+ or -]0.11 and -4.94 [+ or -] 0.09%o ([+ or -]2 SD), respectively. Our data indicate that seawater is enriched in heavy Mo isotopes than previously reported.

This work introduces a newly developed on-line flow injection (FI) Sr/Rb separation method as an alternative to the common, manual Sr/matrix batch separation procedure, since total analysis time is often limited by sample preparation despite the fast rate of data acquisition possible by inductively coupled plasma-mass spectrometers (ICPMS). Separation columns containing ~100 [micro]L of Sr-specific resin were used for on-line FI Sr/matrix separation with subsequent determination of [sup.87]Sr/[sup.86]Sr isotope ratios by multiple collector ICPMS. The occurrence of memory effects exhibited by the St-specific resin, a major restriction to the repetitive use of this costly material, could successfully be overcome. The method was fully validated by means of certified reference materials. A set of two biological and six geological Sr- and Rb-bearing samples was successfully characterized for its [sup.87]Sr/[sup.86]Sr isotope ratios with precisions of 0.01-0.04% 2 RSD (n = 5-10). Based on our measurements we suggest [sup.87]Sr/[sup.86]Sr isotope ratios of 0.713 15 [+ or -] 0.000 16 (2 SD) and 0.709 31 [+ or -] 0.000 06 (2 SD) for the NIST SRM 1400 bone ash and the NIST SRM 1486 bone meal, respectively. Measured [sup.87]Sr/[sup.86]Sr isotope ratios for five basalt samples are in excellent agreement with published data with deviations from the published value ranging from 0 to 0.03%. A mica sample with a Rb/Sr ratio of ~1 was successfully characterized for its [sup.87]Sr/[sup.86]Sr isotope signature to be 0.718 24 [+ or -] 0.000 29 (2 SD) by the proposed method. Synthetic samples with Rb/Sr ratios of up to 10/1 could successfully be measured without significant interferences on mass 87, which would otherwise bias the accuracy and uncertainty of the obtained data.

Production of laser ablation-induced sample aerosols has been visualized using a high-speed camera device coupled with shadowgraphy technique. The time resolution of the method is 1 [micro]s, and production of the sample grains was successfully defined by the imaging system. An argon--fluoride excimer laser operated at 193-nm wavelength was used to ablate the solid samples. When the laser was shot onto the sample (Si wafer), a dome-shaped dark area appeared at the ablation pit. This dark area reflects changes in refractive index of ambient He probably due to emission of electrons or ions from the ablation pit. The dark area expanded hemispherically from the ablation pit with a velocity close to the speed of sound (~1000 m/s for He at 300 K). This was followed by the excitation or ionization of the vaporized sample, known as the plasma plume. Immediately after the formation of the plasma plume, sample aerosols were produced and released from the ablation pit along the propagation of the laser-induced shockwave. Production of the sample aerosols was significantly delayed (~4 [micro]s) from the onset of the laser shot. The typical speed of particles released from the ablation pit was 100-200 m/s, which was significantly slower than the reported velocity of the plasma plume expansion ([10.sup.4] m/s). Since the initial measured speed of the sample particles was rather close to the speed of sound, the sample aerosols could be rapidly decelerated to the terminal velocity by a gas drag force with ambient He. The release angle of the sample aerosols from the ablation pit was very shallow (

A semianalytical approach to the problem of determining inherent optical properties from satellite and in situ ocean color data is presented. The model uses empirically derived spectral slopes between neighboring wavebands in combination with radiative transfer modeling to determine the spectral absorption (a) and backscatter ([b.sub.b]); these values are then further decomposed into absorption due to phytoplankton, detrital, and colored dissolved organic matter components. When compared with over 400 in situ data points the model makes good retrievals of the total absorption and backscatter across the entire spectrum, with regression slopes close to unity, little or no bias, high percentage of variance explained, and low rms errors.

Evidence for the existence of continental crust older than 4.06 Ga has so far been obtained only from zircons in the Yilgarn Craton of Western Australia. In this paper we report the first occurrence of a very old zircon with a U-Pb age of 4.2 Ga in the Acasta Gneiss Complex of northwestern Canada, based on a laser ablation-inductively coupled plasma-mass spectrometry and sensitive high-resolution ion microprobe study. The U-Pb data reveal that the 4.2 Ga zircon occurs as a xenocryst in a 3.9 Ga granitic rock. Trace element compositions of the xenocryst suggest that it crystallized from a granitic magma. Our results, suggesting the existence of granitic rocks outside the Yilgarn Craton at 4.2 Ga, imply that granitic continental crust was more widespread than previously thought, and that it was reworked into Early Archean continental crust. Keywords: Acasta Gneiss Complex, Hadean, ancient zircon, crustal reworking, laser ablation-inductively coupled plasma-mass spectrometry, sensitive high-resolution ion microprobe, U-Pb dating.

Author(s): Zhong-Yuan Ren (corresponding author) [1]; Stephanie Ingle [1, 3]; Eiichi Takahashi [1, 2]; Naoto Hirano [1]; Takafumi Hirata [1] The Hawaiian-Emperor volcanic island and seamount chain is usually attributed [...]

We carried out in situ U-Pb and Lu-Hf isotope analyses of detrital zircons from the Mississippi River in order to understand crustal reworking and continental growth rates. The U-Pb analyses for 416 zircons reveal three major peaks of crust formation at 2.8-2.6 Ga, 1.8-0.9 Ga, and after 0.2 Ga. Initial Hf isotope ratios were obtained for 402 of the dated zircons, and only 8% of the zircons have [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] values less negative than -2.5. These data correspond to a crustal residence time of Keywords: zircon, U-Pb, Lu-Hf, continental growth, crustal reworking, laser ablation ICP-MS.