The 238U-206Pb decay system in minerals is affected by initial excess or deficit in the intermediate daughter nuclide 230Th relative to 238U (Mattinson, 1973) due to chemical fractionation of U and Th immediately before or during crystallization. For high resolution U-Pb dating, in particular in the case of young Th-enriched samples, 238U-206Pb ages have to be corrected for initial radioactive disequilibrium. To make this correction, it is commonly assumed that the Th/U ratio of the reservoir from which the minerals grew is approximated by that of the host rock (Schärer, 1984). In addition, it is assumed that the reservoir was in a state of radioactive equilibrium at that time. Because fractional crystallization of Th- or U-enriched phases may produce changes in Th/Ureservoir and induce disequilibrium in 230Th/238Ureservoir, these assumptions do not necessarily hold for natural crystallization environments. In contrast, Th-Pb dating is not affected by this problem, because there are no short-lived intermediate daughter nuclides in the 232Th-208Pb decay system (Barth et al., 1993).
Viewed from a different perspective, systematic studies of the relationship between potentially disequilibrium-affected 238U-206Pb ages and "unbiased" 232Th-208Pb ages have considerable potential not only for the assessment of age validity, but also to provide detailed information on the nature of geological processes documented by those ages. Of particular importance is the fact that preservation of an
initial disequilibrium signature by the U-Th-Pb system of a mineral indicates that the age refers to a crystallization age rather than to reheating of the mineral beyond its blocking temperature by a metamorphic event (Copeland et al., 1988) or by exposure to prolonged post-magmatic cooling (von Blankenburg, 1992).
We have applied single-crystal U-Th-Pb dating
techniques to zircon, allanite and titanite of a tonalite sample collected from the feeder zone of the Tertiary Bergell pluton (S Switzerland/N Italy), The tonalitic-granodioritic pluton intrudes the Penninic nappe system just N of the Insubric line, a lineament separating the Alpine metamorphic units in the N from Hercynian basement gneisses to the S. Hornblende barometry (~0.8 GPa) suggests an intrusion depth in excess of 20 km at the sample locality located near the W end of the pluton. The country rocks in this region experienced regional high-grade metamorphic conditions at the time of emplacement as documented by conformable
textures in migmatites and in the tonalite. These observations and published geochronological data suggest an extended cooling history for this part of the pluton.
U-Pb isotopic data obtained for abraded single zircon grains suggest a crystallization interval from 32.9 Ma to 32.0 Ma. Similarly, Th-Pb ages of crystal fragments representing distinct density fractions of strongly zoned allanite document an extended crystallization interval from 32.0 Ma to ~28 Ma. The presence of large quantities of excess 230Th-derived 206Pb in all allanite analyses argues against a simple Pb loss mechanism leading to variable resetting of the
isotopic ages during magmatic and regional cooling. The data imply that closure temperatures for U-Th-Pb diffusion in zircon and allanite are in excess of the solidus temperature for tonalite (>700 C). A uniform increase of U contents in zircon (110 to 150 ppm) with decreasing age, and in particular, increase of U and Pbcommon in allanite (80 to 560 ppm and 6 to 17 ppm, respectively) and a drastic decrease of Th (13400 to 1140 ppm) from 32 Ma to 28 Ma mirrors incompatible element enrichment in an evolving magma environment and concomitant depletion of Th which is consumed by allanite. Comparison of Th/U(t)reservoir calculated from disequilibrium systematics with Th/U measured in the host rock indicates that substantial amounts of allanite have been removed by fractional crystallization or other crystal/melt separation processes during the earlier part of allanite crystallization. Absence of cumulate features at the sampling location therefore indicates that zircon and substantial amounts of allanite had crystallized prior to ascent of the magma to the present sampling level ¾ 31.5 Ma ago.
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