SHOCKED ZIRCON SEEMS TO BE FOREVER, SO WHERE IS THE HADEAN RECORD…?

Investigation of Earth’s oldest intact remnants, the Jack Hills detrital zircon suite, initiated with U-Pb studies by Australian groups in the 1980s, and later became the subject of intense study by international groups. John W. Valley pioneered measuring oxygen isotope ratio in zircon in the 1990s [1], which led to benchmark studies showing δ¹⁸O in Hadean zircon is primary [2-4]. JV later conducted the first atom-scale study confirming concordant SIMS U-Pb dates up to 4.4 Ga are also primary igneous features [5], elegantly demonstrating that zircon is indeed ‘forever’. The seminal influence of JV spawned investigations into how early meteorite bombardment influenced establishment of habitable conditions. Although no craters or other impact evidence from early Earth have been identified, zircon has potential to record this process because of its ability to preserve microstructural [7] and geochronological evidence of impact over billions of years [8]. Detrital shocked zircon has been documented in sediment eroded from the Vredefort [9], Sudbury [10], and Santa Fe [11], impact structures, including grains transported ~2000 km by fluvial processes [12], and also in lunar samples [13]. Current models of early bombardment lead us to predict that zircon shocked by early impacts should be present in the Jack Hills and other Hadean suites. However, SEM surveys of significant numbers of Jack Hills zircon grains (>20,000 grains) have not detected microstructural evidence of impact-related deformation [14-15]. The absence of shocked zircon in the Jack Hills indicates that Hadean shocked grains were either not created, not preserved, or not detected. Destruction of shocked zircon by sedimentary or magmatic processes is inconsistent with studies of younger sites. The preservation of lunar shocked zircon, 4.4 Ga terrestrial zircon, and the absence of shocked Hadean grains together imply that our understanding of early impact processes and habitability may be incomplete.

[1] Valley et al. 1994 [2] Peck et al. 2001 [3] Wilde et al. 2001 [4] Cavosie et al. 2005 [5] Valley et al. 2014 [6] Cameron et al. 2017, this volume [7] Timms et al. 2017 [8] Cavosie et al. 2015 [9] Erickson et al. 2013 [10] Thomson et al. 2014 [11] Montalvo P. et al. in revision [12] Montalvo S. et al. 2017 [13] Timms et al. 2012 [14] Montalvo P. et al. 2014 [15] Cox et al. 2017