Geochronology of the Jack Hills detrital zircons by precise U–Pb isotope dilution analysis of crystal fragments

Abstract

U–Pb ages were determined for 12 whole grains and 45 fragments from further 14 grains of detrital zircon from the Jack Hills metaconglomerate, Western Australia, using isotope dilution thermal ionization mass-spectrometry in combination with air abrasion and HF leaching. These data indicate the presence of two groups of detrital zircons with ages of 3.31–3.52 Ga (`young group') and 3.82–4.11 Ga (`old group'), in agreement with earlier SHRIMP and direct Pb-evaporation data for zircons from the same sample. The zircons of the `old group' have complex internal age distributions, but still provide clear evidence for extensive zircon growth during two discrete periods 3.96–4.00 Ga and 4.02–4.03 Ga ago, in addition to older (>4.1 Ga) episodes. Precise crystallization ages of detrital zircons in the `young group', determined by multiple fragment analyses, are compared with published ages of Archean rocks exposed in the Narryer Gneiss Complex. This comparison suggests that the sources of Jack Hills conglomerates are distinct from the rocks presently exposed in the area. The results of this study demonstrate that the grain fragmentation method can be used to determine precise crystallization ages of detrital zircons, and to study complex internal zircon structures. Air-abrasion treatment of crystal fragments helps to remove internal zones influenced by metamictization and lead loss, and appears to be even more efficient if applied to fragments rather than to whole zircon grains.