ジルコンをめぐる最近の話題

I. U-Pb法によるジルコン年代学

抄録

In this paper (Part I), recent progress in U-Pb dating of zircon is reviewed. Rare-earth element (REE) partitioning of accessory minerals and REE geochemistry of zircon will be reviewed in separate papers, Part II and Part III, respectively.
Zircon (ZrSiO₄) is one of the most important minerals in geochronology because the zircon crystal is very resistant to alteration, metamorphic and magmatic events. Zircons therefore preserve their primary chemical properties, especially very high U and Th and extremely low Pb contents, in many cases, resulting in most Pb in zircon to be time-integrated radiogenic origin. For the above reasons, zircon has been extensively used in geochronology for over 30 years.
Conventional U-Pb dating of zircon has been performed using a thermal ionization mass spectrometer with isotope dilution (IDTIMS). With improvements in the techniques for the elimination of discordant parts and the chemical decomposition of zircon, it is now possible to measure U-Pb age with IDTIMS not only of a sircon grain but also of fragments of a single zircon grain. Recently, the single zircon evaporation technique has been developed to analyze for U/Pb only the closed-system domains in a single grain without chemical treatment.
The most outstanding advance in U-Pb dating of zircon has been achieved by the development of secondary ion mass spectrometry (SIMS). Using the SHRIMP (Sensitive High Resolution Ion Micro Probe) at Australian National University, a very small area (<40, μm) can be analyzed, making it possible to reveal with precise age determinations the multiple growth history in a single grain of zircon. However, one of the major problems in the SIMS analysis resides in preparation of a homogeneous zircon standard. The U and Pb concentrations and the Pb isotope ratios of the standard must be known but they can only be obtained by the conventional IDTIMS method. The conventional IDTIMS is, therefore, a prerequisite for SIMS U-Pb dating.
The oldest zircons, recorded >4.OGa, have been found with SHRIMP. The existence of such old zircons was confirmed by results obtained by IDTIMS and the single zircon evaporation technique with TIMS. Characterization of the latent primitive crust, based on the chemical compositions of the detrital zircons and their measured ages, has been attempted.