OXYGEN ISOTOPES IN EARLY ARCHEAN ZIRCONS: CORRELATING IN-SITU δ18O ION PROBE ANALYSES WITH CONCORDANT U/PB DOMAINS AND CL ZONING

The existence of Early Archean (>3.8 Ga) crust enriched in d¹⁸O is demonstrated by >3800 Ma zircons with d¹⁸O values too high to be in equilibrium with the mantle. Calculated d¹⁸O_{(WR magma)} values vary widely, however, from ~9‰ (Peck et al., 2001) up to 17.0‰ (Mojzsis et al., 2001). To better characterize these rocks, we analyzed a group of 43 detrital zircons > 3900 Ma from the Jack Hills, Western Australia for d¹⁸O by ion microprobe.

Analyses were made with a Cameca 1270 ion microprobe using dual Faraday detectors at the University of Edinburgh and standardized with zircon KIM5 (d¹⁸O=5.09 ‰; Valley, 2003). As zircons can have both crystalline and radiation damaged domains, oxygen analyses targeted the re-polished locations beneath concordant SHRIMP spots, as imaged by optics, SEM, BSE, and CL, thus avoiding damaged areas. Results are based only on in-situ analyses of domains in zircon (15-20 μm) that are inferred to be retentive of magmatic composition. Cracks, areas with disturbed CL zoning, and radiation damaged or discordant zones are susceptible to alteration and may not reliable indicators of magmatic d¹⁸O. All analyses on cracks, imaged by SEM, were rejected.

The d¹⁸O values in grain cores for 4400-3800 Ma zircons range mostly from 5.3-7.4 ‰, with only one outlier above 8 at 10.3 ‰. A statistical t-test shows that the Jack Hills mean of 6.22 ‰ (1sd=±0.70, 1se=±0.11, n=38, each datum=grain average) differs from Superior Province Late Archean volcanics (5.52‰, 1sd=±0.63, 1se=±0.10, n=43) and TTG’s (5.48‰, 1sd=±0.39, 1se=±0.06, n=39) (King et al., 1998), and also from mantle zircon KIM5 measured in this study (5.09‰, 1sd=±0.32, 1se=±0.02, n=269) at > 99.95 % confidence level (t-values=4.74, 5.75, and 17.63, respectively). However, zircons with higher d¹⁸O values (6.37‰, 1sd=±0.24, 1se=±0.06, n=15) are known from more evolved igneous rocks in the Superior Province (King et al., 1998).

Crustal recycling produced magmas with d¹⁸O_(WR) up to ~9‰ before 3900 Ma. Similar values are found in more evolved rocks later in the Archean, suggesting similar magmatic processes occurred throughout the Archean. While no >4000 Ma rocks are known, these results are consistent with hypotheses that surface waters and continental-type crust were extant in the so-called ‘Hadean’.