Unusually high thermal stability of the Uranium-Xenon isotopic system in nonmetamict zircons

Abstract

Modern isotopic geology requires new approaches to geothermochronological problems for deciphering temperature evolution of magmatic and sedimentary rocks and related geological processes. One of the future research avenues in this field is the simultaneous application of genetically interrelated isotopic geochronological systems of noble gases: not only U‐Th‐He [1], but also U–Xe s , U–Xe n , U–Kr s , and U–Kr n [2, 3] in combination with the U‐Th‐Pb isotopic system (Xe and Kr are the products of spontaneous and neutroninduced fission [4]). This will maximally increase the geochronological reliability of the obtained ages and give opportunities unique for geothermogeochronology owing to a common mechanism of migration of different radiogenic isotopes having different kinetic parameters of migration. In order to make this approach applicable to practice, it is necessary to carry out extensive studies of migration characteristics of He, Xe, and Kr and the stability of the Xe/U, He/U, and Kr/U isotopic systems in mineral geochronometers, primarily, zircons. In this work, we experimentally determined the migration parameters of radiogenic xenon and energetic characteristics of the U‐Xe isotopic system in the nonmetamict zircons with perfect crystalline structure; demonstrated for the first time the unique stability of the uranium‐xenon isotopic system in the nonmetamict zircon samples; studied and compared the state of genetically linked isotopic systems: U‐Xe and U‐Pb (using local methods of ion microprobe). The study objects were thoroughly selected zircons with perfect structure, without signs of metamictization: a large fragment of a gem-quality zircon crystal from Sri Lanka (Ceylon Island); a zircon from Nigeria that had a nonmetamict undisturbed structure of grains; two samples from plagiogranites (enderbites), MacMaxon Island, Antarctica: sample 11v32-b represented by opaque lilac‐yellow grains with fused crystal faces; and sample 11g-1/32 from plagiogranite (enderbite) from MacMaxon Island, Antarctica, consisting of almost transparent long-prismatic crystals with very rare inclusions.