Other
Scientific paper
Sep 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986e%26psl..79..337k&link_type=abstract
Earth and Planetary Science Letters, Volume 79, Issue 3-4, p. 337-347.
Other
38
Scientific paper
Zircons from a sample of the tonalitic Amîtsoq gneisses near Godthhb, West Greenland, exhibit several stages of crystal growth. Measurements of the U-Th-Pb isotopic composition of 30 μm areas within individual grains using the ion microprobe SHRIMP reveal that components of magmatic zircon from the original igneous protolith are at least 3822 +/- 5 Ma (lσ) old, slightly but significantly older than the age of zircons from a fclsic volcanic unit in the Isua supracrustal belt as measured at high precision by the same technique, and older than all ages previously determined by mineral and whole-rock isotopic techniques for Amîtsoq gneisses at Godthåb and at Isukasia. Overgrowths of younger zircon formed at ca. 3630 Ma, coinciding with an episode of major Pb loss from the older grains. These results indicate that previous whole-rock isochron ages refer to later isotopic redistribution and/or that other phases of the Amîtsoq gneisses intruded during this later period and dominate the isotopic systematics.
Comparison of the ion-probe results with previous conventional U-Th-Pb data for the same zircon sample demonstrates the strong influence of discordant high-uranium parts of zircon on the mean isotopic composition of multi-grain samples.
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