High-reliability zircon separation for hunting the oldest material on Earth： An automatic zircon separator with image-processing/microtweezers-manipulating system and double-step dating  

作　　者：Yukio Isozaki Shinji Yamamoto Shuhei Sakata Hideyuki Obayashi Takafumi Hirata Ken-ichi Obori Toshinori Maebayashi Satoshi Takeshima Toshikazu Ebisuzaki Shigenori Maruyama 

机构地区：[1]Department of Earth Science and Astronomy, The University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan [2]Department of Human Environmental Studies, Yokohama National University, Hodogaya, Yokohama 240-8501, Japan [3]Department of Chemistry, Gakushuin University, Mejiro, Toshima, Tokyo 171-8588, Japan [4]Department of Geology and Mineralogy, Kyoto University, Kitashirakawa Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan [5]Geochemical Research Center, The University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0033, Japan [6]Horiba Ltd., Kanda, Chiyoda, Tokyo 101-0063, Japan [7]MicroSupport Co. Ltd., Hiratsuka, Kanagawa 254-0035, Japan [8]Mitani Corporation, Takada, Toshima, Tokyo 171-0033, Japan [9]Riken, Wako, Saitama 351-0198, Japan [10]ELSI, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan

出　　处：《Geoscience Frontiers》2018年第4期1073-1083,共11页地学前缘（英文版）

基　　金：supported by the Grant-in-Aid from Japan Society of the Promotion for Science (JSPS KAKAENHI; New Academic Research No. 26106005)

摘　　要：Despite the recent development in radiometric dating of numerous zircons by LA-ICPMS, mineral separation still remains a major obstacle, particularly in the search for the oldest material on Earth. To improve the efficiency in zircon separation by an order of magnitude, we have designed/developed a new machine-an automatic zircon separator（AZS）. This is designed particularly for automatic pick-up of100 μm-sized zircon grains out of a heavy mineral fraction after conventional separation procedures. The AZS operates in three modes：（1） image processing to choose targeted individual zircon grains out of all heavy minerals spread on a tray,（2） automatic capturing of the individual zircon grains with microtweezers, and（3） placing them one-by-one in a coordinated alignment on a receiving tray. The automatic capturing was designed/created for continuous mineral selecting without human presence for many hours. This software also enables the registration of each separated zircon grain for dating, by recording digital photo-image, optical（color） indices, and coordinates on a receiving tray. We developed two new approaches for the dating; i.e.（1） direct dating of zircons selected by LA-ICPMS without conventional resin-mounting/polishing,（2） high speed U-Pb dating, combined with conventional sample preparation procedures using the new equipment with multiple-ion counting detectors（LA-MIC-ICPMS）.With the first approach, Pb-Pb ages obtained from the surface of a mineral were crosschecked with the interior of the same grain after resin-mounting/polishing. With the second approach, the amount of time required for dating one zircon grain is ca. 20 s, and a sample throughput of 〉150 grains per hour can be achieved with sufficient precision（ca. 0.5%）.We tested the practical efficiency of the AZS, by analyzing an Archean Jack Hills conglomerate in Western Australia with the known oldest（〉4.3 Ga） zircon on Earth. Preliminary results are positive; we were able to obtain more than 194 zircoDespite the recent development in radiometric dating of numerous zircons by LA-ICPMS, mineral separation still remains a major obstacle, particularly in the search for the oldest material on Earth. To improve the efficiency in zircon separation by an order of magnitude, we have designed/developed a new machine-an automatic zircon separator（AZS）. This is designed particularly for automatic pick-up of100 μm-sized zircon grains out of a heavy mineral fraction after conventional separation procedures. The AZS operates in three modes：（1） image processing to choose targeted individual zircon grains out of all heavy minerals spread on a tray,（2） automatic capturing of the individual zircon grains with microtweezers, and（3） placing them one-by-one in a coordinated alignment on a receiving tray. The automatic capturing was designed/created for continuous mineral selecting without human presence for many hours. This software also enables the registration of each separated zircon grain for dating, by recording digital photo-image, optical（color） indices, and coordinates on a receiving tray. We developed two new approaches for the dating; i.e.（1） direct dating of zircons selected by LA-ICPMS without conventional resin-mounting/polishing,（2） high speed U-Pb dating, combined with conventional sample preparation procedures using the new equipment with multiple-ion counting detectors（LA-MIC-ICPMS）.With the first approach, Pb-Pb ages obtained from the surface of a mineral were crosschecked with the interior of the same grain after resin-mounting/polishing. With the second approach, the amount of time required for dating one zircon grain is ca. 20 s, and a sample throughput of 〉150 grains per hour can be achieved with sufficient precision（ca. 0.5%）.We tested the practical efficiency of the AZS, by analyzing an Archean Jack Hills conglomerate in Western Australia with the known oldest（〉4.3 Ga） zircon on Earth. Preliminary results are positive; we were able to obtain more than 194 zirco

关 键 词：HADEAN ZIRCON U-Pb dating LA-ICPMS Automatic mineral separator Primordial crust 

分 类 号：P597.3[天文地球—地球化学]