Stephen J. Mojzsis | Search USGS

USGS2014

Component geochronology in the polyphase ca. 3920 Ma Acasta Gneiss

The oldest compiled U–Pb zircon ages for the Acasta Gneiss Complex in the Northwest Territories of Canada span about 4050–3850 Ma; yet older ca. 4200 Ma xenocrystic U–Pb zircon ages have also been reported for this terrane. The AGC expresses at least 25 km 2 of outcrop exposure, but only a small subset of this has been documented in the detail required to investigate a complex history and resolve disputes over emplacement ages. To better understand this history, we combined new ion microprobe 235,238 U– 207,206 Pb zircon geochronology with whole-rock and zircon rare earth element compositions ([REE] zirc ), Ti-in-zircon thermometry (Ti xln ) and 147 Sm– 143 Nd geochronology for an individual subdivided &sim;60 cm 2 slab of Acasta banded gneiss comprising five separate lithologic components. Results were compared to other variably deformed granitoid-gneisses and plagioclase-hornblende rocks from elsewhere in the AGC. We show that different gneissic components carry distinct [Th/U] zirc vs. Ti xln and [REE] zirc signatures correlative with different zircon U–Pb age populations and WR compositions, but not with 147 Sm– 143 Nd isotope systematics. Modeled [REE] from lattice-strain theory reconciles only the ca. 3920 Ma zircons with the oldest component that also preserves strong positive Eu &lowast; anomalies. Magmas which gave rise to the somewhat older (inherited) ca. 4020 Ma AGC zircon age population formed at &sim;IW (iron–wüstite) to <FMQ (fayalite–magnetite–quartz) oxygen fugacities. A ca. 3920 Ma emplacement age for the AGC is contemporaneous with bombardment of the inner solar system. Analytical bombardment simulations show that crustal re-working from the impact epoch potentially affected the precursors to the Acasta gneisses.

Geochimica et Cosmochimica Acta↗

USGS2013

The impact environment of the Hadean Earth

Impact bombardment in the first billion years of solar system history determined in large part the initial physical and chemical states of the inner planets and their potential to host biospheres. The range of physical states and thermal consequences of the impact epoch, however, are not well quantified. Here, we assess these effects on the young Earth's crust as well as the likelihood that a record of such effects could be preserved in the oldest terrestrial minerals and rocks. We place special emphasis on modeling the thermal effects of the late heavy bombardment (LHB) – a putative spike in the number of impacts at about 3.9 Gyr ago – using several different numerical modeling and analytical techniques. A comprehensive array of impact-produced heat sources was evaluated which includes shock heating, impact melt generation, uplift, and ejecta heating. Results indicate that &sim;1.5–2.5 vol.% of the upper 20 km of Earth's crust was melted in the LHB, with only &sim;0.3–1.5 vol.% in a molten state at any given time. The model predicts that approximately 5–10% of the planet's surface area was covered by >1 km deep impact melt sheets. A global average of &sim;600–800 m of ejecta and &sim;800–1000 m of condensed rock vapor is predicted to have been deposited in the LHB, with most of the condensed rock vapor produced by the largest (>100-km) projectiles. To explore for a record of such catastrophic events, we created two- and three-dimensional models of post-impact cooling of ejecta and craters, coupled to diffusion models of radiogenic Pb*-loss in zircons. We used this to estimate what the cumulative effects of putative LHB-induced age resetting would be of Hadean zircons on a global scale. Zircons entrained in ejecta are projected to have the following average global distribution after the end of the LHB: &sim;59% with no impact-induced Pb*-loss, &sim;26% with partial Pb*-loss and &sim;15% with complete Pb*-loss or destruction of the grain. In addition to the relatively high erodibility of ejecta, our results show that if discordant ca. 3.9 Gyr old zones in the Jack Hills zircons are a signature of the LHB, they were most likely sourced from impact ejecta.

Chemie der Erde↗