Takaomi D. Yokoyama

Tectonic erosion in a Pacific-type orogen: Detrital zircon response to Cretaceous tectonics in Japan

U-Pb dating of detrital zircons from the Lower Cretaceous Sanbagawa and the recently recognized Upper Cretaceous Shimanto high-pressure (HP) metamorphic rocks in southwestern Japan has revealed the presence of abundant Proterozoic (ca. 1500–2000 Ma) detrital grains. In contrast, coeval non- to weakly metamorphosed accretionary complex (AC) and forearc basin sediments in southwestern Japan lack these older signatures. The only possible source of the Proterozoic detrital grains is the Jurassic AC in southwestern Japan, which structurally overlies the Cretaceous HP units. The Proterozoic grains were incorporated into the protoliths of HP-ACs, without polluting coeval forearc basin to trench sediments, likely by tectonic erosion in the forearc domain. Along the Cretaceous Wadati-Benioff plane, the tectonic erosion peeled off the sole part of the pre-existing forearc crust and mixed it with the subducting trench sediments prior to the peak HP metamorphism. In the Cretaceous subduction-related margin around Japan, the tectonic erosion likely occurred twice.

Determinations of rare earth element abundance and U-Pb age of zircons using multispot laser ablation-inductively coupled plasma mass spectrometry.

We have developed a new calibration technique for multielement determination and U-Pb dating of zircon samples using laser ablation-inductively coupled plasma mass spectrometry (ICPMS) coupled with galvanometric optics. With the galvanometric optics, laser ablation of two or more sample materials could be achieved in very short time intervals (~10 ms). The resulting sample aerosols released from different ablation pits or different solid samples were mixed and homogenized within the sample cell and then transported into the ICP ion source. Multiple spot laser ablation enables spiking of analytes or internal standard elements directly into the solid samples, and therefore the standard addition calibration method can be applied for the determination of trace elements in solid samples. In this study, we have measured the rare earth element (REE) abundances of two zircon samples (Nancy 91500 and Prešovice) based on the standard addition technique, using a direct spiking of analytes through a multispot laser ablation of the glass standard material (NIST SRM612). The resulting REE abundance data show good agreement with previously reported values within analytical uncertainties achieved in this study (10% for most elements). Our experiments demonstrated that nonspectroscopic interferences on 14 REEs could be significantly reduced by the standard addition technique employed here. Another advantage of galvanometric devices is the accumulation of sample aerosol released from multiple spots. In this study we have measured the U-Pb age of a zircon sample (LMR) using an accumulation of sample aerosols released from 10 separate ablation pits of low diameters (~8 μm). The resulting (238)U-(206)Pb age data for the LMR zircons was 369 ± 64 Ma, which is in good agreement with previously reported age data (367.6 ± 1.5 Ma). (1) The data obtained here clearly demonstrate that the multiple spot laser ablation-ICPMS technique can become a powerful approach for elemental and isotopic ratio measurements in solid materials.

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.