2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 104-2
Presentation Time: 8:20 AM


GALSTER, Federico1, OVTCHAROVA, Maria2, SCHALTEGGER, Urs2 and MÜNTENER, Othmar3, (1)Institute of Earth Sciences, University of Lausanne, Lausanne, 1015, Switzerland; Department of Geological Sciences, University of Texas, Austin, TX 78712-1722, (2)Section of Earth and Environmental Sciences, University of Geneva, rue des Maraîchers 13, Geneva, 1205, Switzerland, (3)Institute of Earth Sciences, University of Lausanne, Lausanne, 1015, Switzerland, federico.galster@unil.ch

Since the pioneering work of Mezger et al. (1989), rutile U-Pb ages have been interpreted as cooling ages and Pb diffusion in rutile as governed by volume diffusion. However there is no consensus concerning the “closure temperature” of the system and “diffusion profiles” of Pb in rutile have not been rigorously verified by in-situ analysis.

Systematic analysis of rutile from partially molten, granulite-facies, meta-sediments of the Ivrea zone reveal a very complex behavior: articulated textures and chemical heterogeneities that developed during cooling and retrogression influence the spatial distribution of “immobile” uranium and the diffusion mechanism of the “mobile” Pb during the period of cooling from ~950 to ~400°C.

Rutile single grains appear systematically divided in subdomains characterized by different textures and in some cases even by different chemical compositions. In particular the presence/absence of oriented zircon exsolutions and the occurrence of irregular zircon lamellae within the grain highlight the different behaviors of rutile during cooling, re-equilibration and re-crystallization. This is evident in the Ivrea zone rutile thanks to the particular textures developed with the expulsion of large amounts of zirconium during cooling from UHT. However the same phenomenon is likely to occur even in rutile formed at lower temperatures, where the existence of subgrains cannot be detected solely by textural observations.

The occurrences of grain and subgrain boundaries within rutile provide fast-diffusivity paths for Pb and strongly reduce the effective diffusion radius with evident consequences for U-Pb ages. Coupled ID-TIMS and LA-ICPMS dating confirm this interpretation.

It seems however that the average size of individual subgrains is proportional to the grain size. This fact results in a general dependence of the ages on the grain sizes and was probably the main reason for which previous studies proposed that the Pb-diffusion length in rutile corresponds to the grain axis.

Our observations have important consequences for the Pb closure temperature in rutile and for the estimation of cooling histories based on rutile thermochronology.

Mezger et al., 1989. High-Precision UPb Ages of Metamorphic Rutile: Application to the Cooling History of High-Grade Terranes. EPSL, v 96, p 106-118.