A RECONNAISSANCE ION-PROBE STUDY OF HAFNIUM ISOTOPES IN ZIRCONS

A SIMS technique for the isotopic analysis of hafnium in zircons using the SHRIMP ion microprobe has been developed, and a precision of typically 0.5 parts per thousand (2-sigma) achieved in the mean reduced Hf-176/Hf-177 ratio measured at several spots on a single grain. Unfractionated (chondritic) initial Hf isotopic compositions have been measured on a number of Archaean zircon populations. These include the oldest-known terrestrial minerals, the 4.2 Ga-old Mount Narryer detrital zircons, thereby confirming their antiquity. In contrast, positive initial epsilon-Hf (relative to the chondritic model composition) has been found in several post-Archaean zircon populations, reflecting the increasing involvement of isotopically evolved depleted mantle sources in the formation of younger crust. The 570 Ma-old Sri Lankan zircon standard SL7 yielded an exceptionally low initial epsilon-Hf of -23, implying a metamorphic origin as a reworked product of ancient crust. SHRIMP U-Pb analyses of zircons from Archaean tonalitic gneiss at Watersmeet, Michigan, yield a precise crystallization age of 3636 +/- 6 Ma (2-sigma), and show that a previously reported correlation between Hf-176/Hf-177 and U-Pb isotopic discordance in bulk zircon samples (Patchett, 1983) was caused by the addition of radiogenic Hf in discrete overgrowths of new zircon ca. 2.7 Ga ago. The original 3.64 Ga grains show no evidence of disturbance to their original (chondritic) Hf isotopic composition. There is presently no evidence for significant isotopic exchange of Hf between zircon and other minerals in crustal rocks.