Ion microprobe identification of 4,100–4,200 Myr-old terrestrial zircons

Abstract

We report here the existence of detrital zircons from Western Australia which are far older than any known terrestrial rocks. They are from quartzites at Mt Narryer (Fig. 1), a locality which has created interest because of the nearby occurrence of 3,630±40 Myr orthogneisses1. The older zircons were discovered during reconnaissance U–Pb age determinations of zircon concentrates from Archaean metasediments, using the ion microprobe SHRIMP2,3 at the Australian National University. These determinations are being made specifically to search for zircons having ages in the interval 3,800–4,500 Myr, a period unrepresented so far by reliable terrestrial age determinations. Grain-by-grain measurements of one particular concentrate revealed four zircons (Fig. 2) having near-concordant U–Pb ages between 4,100 and 4,200 Myr, in striking contrast to most grains whose ages are ∼3,750 and ∼3,500 Myr. These results show that pre-3,800 Myr silica-saturated rocks were indeed present on the Earth's crust.