3.7 GA OVERGROWTHS ON A 4.33 GA ZIRCON FROM JACK HILLS: EVIDENCE OF EARLY CRUSTAL RECYCLING

The results of an integrated study on the field relations, major element chemistry, mineralogy, d¹⁸O and geochronology, from the Jack Hills metasedimentary belt, Western Australia include several new findings that provide further insight into Earth's earliest crust. Whole-rock analyses from two measured sections through the metasedimentary sequence (one section includes Eranondoo Hill, the location of sample W74 which yielded a 4.404 Ga zircon and the other ca. 2 km east) reveal the sediments to be mature, (90-98 wt. % SiO₂) with zircon (41-498 ppm zirconium) and chromite (0-5300 ppm chromium) being the dominant detrital heavy minerals. Quartz d¹⁸O profiles through each section range from 10.97-11.65 ‰over 60 meters and 12.14-12.69 ‰ over 20 meters reflecting variable post-depositional oxygen isotope exchange during regional greenschist metamorphism.

Recent analyses on the Beijing Shrimp II of detrital zircons from samples collected along the two sections have confirmed the existence of >4.3 Ga zircons in the Jack Hills (Wilde et al., 2001, Nature), and identified the third oldest zircon known. The total range of ages at these localities is 4.4 to 1.6 Ga, representing half of Earth's history. One euhedral 300 mm-long grain contains a 4.33 Ga, low U core, and 3.7 Ga overgrowths at both terminations. This 4.33 Ga grain is the oldest known grain that preserves multiple growth generations, and therefore records one of Earth's earliest crustal events. The overgrowth age of ~3.7 Ga correlates to the oldest components of the adjacent Meeberrie gneiss of the Narryer Gneiss terrane, the oldest rock known in Australia (Myers, 1988, Precam. Res.). Possible scenarios for the formation of the overgrowth include either melting or metamorphism; either process requires the existence of > 4.3 Ga crust to be recycled during the ca. 3.7 Ga orogenic event.