ASSESSING THE RELIABILITY OF OXYGEN ISOTOPE MEASUREMENTS OF HADEAN ZIRCONS

The Jack Hills detrital (JHD) zircons are the primary source of >4.0-Ga terrestrial material. Values of δ¹⁸O in >4.0 Ga detrital zircons that are mildly elevated above mantle values are used to infer that protoliths of the zircons interacted at low temperature with liquid water before burial and melting; that Earth cooled quickly after core and Moon formation; and that habitable conditions for life existed 800 Ma before the oldest known microfossils.

However, values of δ¹⁸O in some radiation damaged zircons correlate with high OH/O ratios, suggesting cryptic alteration and that high δ¹⁸O values in JHD zircons are not magmatic (Pidgeon et al., 2017). Radiation damaged zircons (above the first percolation point) are often hydrous with fast pathways for exchange and alteration. In contrast, some studies have applied tests to avoid analysis of damaged domains in zircon. Tests previously applied include evaluation of CL zoning and U-Pb concordance. Here we look at OH/O ratios and Raman spectroscopy.

We present new analyses of 129 >4.0-Ga JHD zircons for δ¹⁸O, OH/O, CL, and FWHM of the ν₃(SiO₄) vibrational mode to test the hypothesis that the mildly elevated δ¹⁸O in JHD zircons results from alteration. We reanalyzed δ¹⁸O by SIMS in JHD zircons previously interpreted to be unaltered by Cavosie et al. (2004, 2005, 2006) and found excellent agreement with the earlier studies. Our new data include measurement of background-corrected ¹⁶OH/¹⁶O to monitor “water”, which can indicate alteration (Wang et al. 2014). FWHM data measured by laser Raman, are used to approximate the present-day effective damage state, D(eff). Each dated zircon was analyzed by SIMS with 2 or 3 10-μm spots (IMS 1280, UW-Madison). Most zircons have OH/O ratios indistinguishable from background. There is no correlation of OH/O to δ¹⁸O in our data. This demonstrates that mildly elevated δ¹⁸O is a primary geochemical signature. Raman data document significant annealing; D(eff) is uniformly less than the first percolation point (2×10¹⁵ α-events/mg), with most zircons having effective doses <1×10¹⁵. Furthermore, there is no correlation between D(eff) and δ¹⁸O, age, concordance, or OH/O. These data strengthen the conclusion that mildly elevated δ¹⁸O magmas existed during the Hadean, providing evidence for oceans and a Cool Early Earth before 4.3 Ga.