Double dating technique of detrital zircons using combined fission track and LA-ICPMS U/Pb analyses

Tamás Mikes, István Dunkl, Hilmar von Eynatten (GZG Sedimentologie/Umweltgeologie)
Teresa E. Jeffries (Mineralogy Dept., Natural History Museum, London)

Tuesday, 3rd June 2008, 16:15, MN16

Zircon usefully carries a diverse range of petrogenetical, age and thermal information about its metamorphic, igneous and sedimentary host rocks. Many in-situ techniques are used to examine zircon genetical aspects (e.g. structure, chemistry, O isotope composition, isotopic age, "cooling" age), yet attempts to combine them on a single-grain level have been limited so far. In sedimentary provenance analysis, a fundamental question is the age of a crystalline source rock and the timing of subsequent thermotectonic events that affected crystalline terrains prior to exhumation and erosion. We present an approach which assesses this question by combining fission track and U/Pb isotopic measurements from the same grain.
The results of this study are significant for at least two reasons:
(1) The analytical protocol we developed provides an unprecedented trade-off between analysis quality and high (cost-effective) sample throughput. Figures of merit include achieved sensitivities of >3200 cps/ppm for 238U and >2000 cps/ppm for 207Pb in the case of a 30 µm pit, U/Pb elemental fractionation <1 % for a 70 s ablation run (rendering any subsequent linear correction unnecessary). Age bias can be kept minimal owing to CL-control and good spatial resolution, as judged from the notably high proportion (>90%) of analyses within the ± 5% band of concordia. Our protocol allows the dating of 70-100 sample grains per day.
(2) The double dating approach yields valuable insights into the thermal history of source terrains of Alpine synorogenic sediments of the Dinarides. We can separate several age clusters: (i) pre-Mesozoic magmatic age populations that were not affected by a later thermal event; (ii) Upper Cretaceous grains with coeval cooling above the mid-crustal depth; Permian groups with (iii) Late Jurassic and with (iv) Late Cretaceous cooling; (v) Devonian group with Late Cretaceous cooling. The "age-pairs" clearly match typical Alpine tectonostratigraphic units and the results can be effectively used for building a more realistic model of hinterland erosion and sediment dispersal patterns than previously available.