Jack Hills, evidence of more very old detrital zircons in Western Australia

Abstract

The age of the Earth's oldest crustal minerals sets a time-limit on the earliest preservation of buoyant solid crust. The oldest mineral ages reported so far are ∼4,180 Myr for detrital zircons from quartzites at Mount Narryer¹, in the Yilgarn Block, Western Australia. The oldest-known intact rocks, as distinct from individual minerals, are substantially younger; they formed ∼3,813-Myr ago² in the Isua supracrustal belt, West Greenland. We report here a further occurrence of old detrital zircons, again identified using the ion-microprobe SHRIMP³, in conglomerate from the Jack Hills Metasedimentary Belt^4,5 (26°11′ S, 116°58′ E), ∼60km north-east of Narryer. This discovery is important for several reasons. First, one zircon registers the exceptionally old age of 4,276±6 Myr, which is, moreover, a minimum estimate for its original age; 16 other grains may have the same age or may be slightly younger. Second, the new occurrence considerably extends the area over which the old Narryer suite is known, thus amplifying its geological significance. Third, the frequency of old zircons in the Jack Hills occurrence is 12%, about five times higher than at Narryer, which suggests that the intact ∼4,180 Myr-old rocks, if they still exist, might be closer, and facilitates future comparison of the SHRIMP data with conventional analyses of these grains. (Recent conventional U–Pb work⁶ on other zircons from the same Narryer concentrate confirmed the younger ages but failed to observe any at ∼4,180 Myr, probably because of their low abundance⁷). The likelihood that zircon would dissolve in magmas undersaturated in zirconium⁸ argues against its survival in long-term contact with such magmas during the disaggregation and subduction of crust. Consequently, the existence of zircon in the Jack Hills area from ∼4,300 Myr ago implies that this part of the crust has been preserved, since then, from recirculation in the mantle through plate-tectonics or any other mechanism.