Shuhei Sakata

LA-ICP-MS Pb–U dating of young zircons from the Kos–Nisyros volcanic centre, SE Aegean arc

Zircon Pb–U dating has become a key technique for answering many important questions in geosciences. This paper describes a new LA-ICP-MS approach. We show, using previously dated samples of a large quaternary rhyolitic eruption in the Kos–Nisyros volcanic centre (the 161 ka Kos Plateau Tuff), that the precision of our LA-ICP-MS method is as good as via SHRIMP, while ID-TIMS measurements confirm the accuracy. Gradational age distribution over >140 ka of the Kos zircons and the near-absence of inherited cores indicate near-continuous crystallisation in a growing magma reservoir with little input from wall rocks. Previously undated silicic eruptions from Nisyros volcano (Lower Pumice, Nikia Flow, Upper Pumice), which are stratigraphically constrained to have happened after the Kos Plateau Tuff, are dated to be younger than respectively 124 ± 35 ka, 111 ± 42 ka and 70 ± 24 ka. Samples younger than 1 Ma were corrected for initial thorium disequilibrium using a new formula that also accounts for disequilibrium in 230Th decay.

U–Pb dating of CA/non-CA treated zircons obtained by LA-ICP-MS and CA-TIMS techniques: impact for their geological interpretation

Chemical Abrasion Isotope-Dilution Thermal Ionization Mass Spectrometry (CA-ID-TIMS) is known as a high precision technique for resolving lead loss and improving the interpretation of U–Pb zircon age data. Here, we argue that combining CA with the widely applied Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) improves the precision and accuracy of zircon dates, while removing the substantial parts with lead loss, reducing data scatter, and providing meaningful geological interpretations. The samples are magmatic rocks chosen from different geological time periods (one Paleozoic, one Mesozoic and three Cenozoic). All zircon separates are analysed by LA-ICP-MS before and after CA, and age data are compared with CA-ID-TIMS 206Pb/238U dates that are considered as the most accurately obtainable age. All CA-treated zircon crystals show up to 50% less data scatter compared to the non-CA treated zircon grains and thus reduction of the calculated uncertainties is apparent. The obtained wt average LA-ICP-MS 206Pb/238U ages of the CA-treated zircon grains are up to 4–6% higher than those of the non-CA treated crystals, exceeding the analytical uncertainties of the LA-ICP-MS dating technique of 1–2%. The damaged crystal parts, caused by U-decay, with lead loss are removed, so that we can exclude younging from the possible geological scenarios. CA-LA-ICP-MS age data are in good agreement with the CA-ID-TIMS dates and suggest advantages of using CA-LA-ICP-MS in order to define accurate ages. The use of the CA technique for very young zircons (∼0.2 Ma, Kos rhyolitic tuff, Greece) seems optional; as the obtained mean 206Pb/238U ages of non-CA and CA treated zircons coincide within the uncertainty. The negligible time to produce the lattice damage (based on alpha decay or spontaneous fission) makes lead loss less important for age dating and data interpretation of very young zircons (<1 Ma).

The eastern extension of Paleozoic South China in NE Japan evidenced by detrital zircon

The South Kitakami belt is unique in exposing a thick, well-preserved Paleozoic shelf sequence in Japan in which Phanerozoic accretionary complexes dominate. Its origin with respect to continental blocks has been debated in regard of two options, i.e., as belonging to the margin of North China or South China. Present work on U–Pb detrital zircon dating has identified Neoproterozoic mineral grains from the Silurian and Carboniferous sandstones in the S. Kitakami belt, and proved the link between Paleozoic Japan and South China with dominant Proterozoic basements. South China likely extended further to the east from the mainland China.

Detrital zircon ages of Cambrian and Devonian sandstones from Estonia, central Baltica: a possible link to Avalonia during the Late Neoproterozoic

Detrital zircon U–Pb ages of Lower Cambrian and Middle Devonian have been determined for sandstone in Estonia through LA-ICP-MS (Nu instruments, Wrexham, UK). Both sandstones have a similar zircon age spectrum with distinct age clusters that reflect the basement geology of Baltica, i.e. 2800–2700 Ma (Kola–Karelia), 1900–1700 Ma (Svecofennian), 1600–1500 Ma (Rapakivi) and 1200–1000 Ma (Sveconorwegian). Noteworthy is a cluster at 750–550 Ma, because rocks of such age are absent within the core of Baltica. The present results suggest a possible link between Baltica and Avalonia/Cadomia during the Late Neoproterozoic.

U–Pb age determination for zircons using laser ablation-ICP-mass spectrometry equipped with six multiple-ion counting detectors

Precise zircon U–Pb age determinations have been made on Plesovice zircon using laser ablation-multiple ion counting-inductively coupled plasma-mass spectrometry (LA-MIC-ICP-MS). To achieve high precision and high spatial resolution age determination, multiple ion counting using six electron multipliers was employed. The intensities of Hg–Pb–U isotope (202Hg, 204(Hg + Pb), 206Pb, 207Pb, 208Pb, and 238U) signals were monitored simultaneously without mass scanning. In static acquisition mode, the resultant 238U–206Pb concordia age for Plesovice was 336.3 ± 1.9 Ma, demonstrating improved precision over that achieved using a magnetic sector-based single-collector-ICP-MS, which was 340.3 ± 3.5 Ma for Plesovice. A high duty cycle can be achieved, along with a short integration time or a small sample volume for analysis, allowing high spatial resolution. More importantly, downhole fractionation can be reduced with a shallow ablation pit. To take full advantage of the setup, a one-second LA analysis (8 laser shots with an 8 Hz repetition rate) was adopted for U–Pb age determination. The resultant concordia age for Plesovice was 339.5 ± 6.7 Ma, demonstrating that the repeatability and laboratory bias precision of the resultant age data were comparable to conventional ablation with a single-collector-ICP-MS. The depths and crater diameters of the ablation pits were, respectively, about >1 μm and 25 μm. The data presented herein demonstrate clearly that multiple ion counting-ICP-MS can become a fast and user-friendly tool for use in U–Pb zircon geochronology.

New U–Pb zircon ages of the Sandbian (Upper Ordovician) “Big K-bentonite” in Baltoscandia (Estonia and Sweden) by LA-ICPMS

Oscillatory-zoned euhedral single zircons from the upper Sandbian (Upper Ordovician) Kinnekulle K-bentonite exposed in a hillock at Pääsküla in Estonia and at the type locality on Mt Kinnekulle in Sweden were dated in a grain-by-grain manner by laser ablation-inductively coupled plasma mass spectrometer. The U–Pb (weighed mean) ages of the 25 grains from Mt Kinnekulle and 24 grains from Pääsküla are 453.4 ± 6.6 and 454.9 ± 4.9 Ma, respectively. This study provides the first ca. 454 Ma (late Sandbian) age for the Ordovician K-bentonite in northern Estonia and confirmed its correlation with the type Kinnekulle bed across the Baltic Sea.

Formation and Geological Sequestration of Uranium Nanoparticles in Deep Granitic Aquifer

The stimulation of bacterial activities that convert hexavalent uranium, U(VI), to tetravalent uranium, U(IV), appears to be feasible for cost-effective remediation of contaminated aquifers. However, U(VI) reduction typically results in the precipitation of U(IV) particles less than 5 nanometers in diameter, except for environmental conditions enriched with iron. Because these tiny particles are mobile and susceptible to oxidative dissolution after the termination of nutrient injection, in situ bioremediation remains to be impractical. Here we show that U(IV) nanoparticles of coffinite (U(SiO4)1−x(OH)4x) formed in fracture-filling calcium carbonate in a granitic aquifer. In situ U-Pb isotope dating demonstrates that U(IV) nanoparticles have been sequestered in the calcium carbonate for at least 1 million years. As the microbiologically induced precipitation of calcium carbonate in aquifer systems worldwide is extremely common, we anticipate simultaneous stimulation of microbial activities for precipitation reactions of calcium carbonate and U(IV) nanoparticles, which leads to long-term sequestration of uranium and other radionuclides in contaminated aquifers and deep geological repositories.

In situ 207Pb/206Pb isotope ratio measurements using two Daly detectors equipped on an ICP-mass spectrometer

The simultaneous detection of 206Pb and 207Pb ions has been made by multiple-ion counting ICP-mass spectrometry using two Daly detectors (MC-ICPMS). To evaluate the long-term gain stability of the detectors, 135Ba/138Ba and 136Ba/138Ba ratios have been measured by a combination of Daly and Faraday detectors (135Ba(D)/138Ba(F)) and an electron multiplier and Faraday detectors (136Ba(EM)/138Ba(F)). The measured 136Ba(EM)/138Ba(F) ratio changed 2% through the 10-hour analysis, whereas the 135Ba(D)/138Ba(F) showed smaller changes (<0.5%) over the 10-hour period, demonstrating that the Daly detector could provide better gain stability against conventional electron multipliers. After the correction for the counting loss due to dead time, the Daly detector is capable of accepting signal intensities as high as 107 cps. This indicates that the overlap of the analysis range, between the Daly detector (100 to 107 cps) and the Faraday detector (104 to 1010 cps), would be at least two orders of magnitude, suggestive of easier cross calibration of the collector gain between the detectors. With the present two Daly detectors, in situ207Pb/206Pb ratio measurements have been made on the Nancy 91500 zircon standard through the sample introduction technique using laser ablation. The overall analytical precision and the relative deviation from the literature values were 5.1% and 0.04%, respectively. The data obtained here clearly demonstrate that the LA-MC-ICPMS technique equipped with the Daly detectors would become a major analytical tool for in situ U–Pb geochronology.

Isotopic analysis of tungsten using multiple collector-inductively coupled plasma-mass spectrometer coupled with electrothermal vaporization technique.

We have developed a micro-electrothermal vaporization (μETV) device for the multiple collector-ICP-mass spectrometry (μETV-MC-ICPMS) to improve analytical precision in the (182)W/(183)W and (184)W/(183)W ratio measurements from nanogram quantities of W. The W solution was loaded onto the Re-filament, and the gradual evaporation of W was achieved by controlling the incident current onto the Re filament and D-Glucose. With the W evaporation under the Ar atmosphere, the measured W isotope ratios became erroneous mainly due to the contribution of signal spikes Ala-Arg-Gly-Phy-Tyr. In strike contrast, signal intensity profile became smooth when the He ambient/carrier gas was employed, and this resulted in better precision in the isotope ratio measurements. The measured UV-vis isotope ratio data obtained with present μETV technique were significantly deviated from the ratio data obtained with solution nebulization technique, mainly due to the contribution of the isotope fractionation effect through the evaporation process. Rigorous testing for the correction of the isotope fractionation processes pH-activity curve revealed that the Rayleigh fractionation law, rather than the conventional exponential law, provided the most reliable ratio data (1.851720±0.000018 for (182)W/(183)W and 2.141248±0.000028 for (184)W/(183)W ratios), which agreed well with the ratio data obtained through the conventional solution nebulization technique (1.851718±0.000039 for (182)W/(183)W and 2.141248±0.000022 for (184)W/(183)W). Moreover, mass dependency for the mass fractionation law suggested that W was evaporated as oxides (WO3), rather than the metallic form (W), from the Re filament, and therefore, information concerning the chemical form of the analytes could also be derived by the ETV technique developed in this study. The data presented here demonstrate clearly that the ETV sample introduction technique has a potential to become a sensitive tool for the precise isotope analysis for the MC-ICPMS technique.

iQuant2: Software for Rapid and Quantitative Imaging Using Laser Ablation-ICP Mass Spectrometry

We report on the development of a software program named iQuant2 which creates visual images from two-dimensional signal intensity data obtained by a laser ablation-ICP-mass spectrometry (LA-ICPMS) technique. Time-resolved signal intensity profiles can be converted to position resolved signal intensity data based on the rastering rate (μm s−1) of the laser ablation. Background signal intensities obtained without laser ablation (gas blank) are used as the background, and all of the blank-subtracted intensity data can be used for the imaging analysis. With this software, deformation of the created image can be corrected visually on a PC screen. The line profile analysis between the user-selected points can be observed using the iQuant2 software. To accomplish this, data points on the profile line were automatically calculated based on the interpolation between the analysis points. The resulting imaging data can be exported and stored as JPEG, BMP or PNG formats for further processing. Moreover, a semi-quantitative analysis can be made based on the coupling of the external correction of the RSF (relative sensitivity factor) using NIST SRM610 with normalization of the corrected signal intensity data being 100%. The calculated abundance data for major elements are in reasonable agreement with the values obtained by electron probe micro analyzer (EPMA). With the software developed in this study, both the rapid imaging and semi-quantitative determinations can be made.