Mathematics – Logic
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p51a0906t&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #P51A-0906
Mathematics
Logic
6200 Planetary Sciences: Solar System Objects
Scientific paper
A geological record of the Hadean Earth remained inaccessible to geologists due to the apparent absence of a rock record until the discovery of pre-4.0 Ga detrital zircons in the Narryer Gneiss Complex (Western Australia) sediments (Froude et al., 1983; Compston and Pidgeon, 1986). Since then, follow-up studies have found evidence for large (present-day) amounts of continental crust with plate tectonic-style recycling (Harrison et al., 2005), liquid water at or near Earth's surface (Mojzsis et al., 2001), presence of short-lived nuclides like 244Pu (Turner et al., 2004) and zircon crystallization temperatures similar to present day water-saturated granitoids (Watson and Harrison, 2005), all within the first 200 Myr of Earth history. We report results of a continuous U-Th-Pb depth profile survey of six individual zircons ranging in age from 4317-4055 Ma by ion microprobe which provides sub-μm scale resolution bearing on zircon responses to discrete thermal events during their residence time in the crust. The earliest recognized thermal events appear coincident with mid to late Archean granitoid and gneissic assembly and emplacement in the Narryer Gneiss Complex. Two of the six zircons contain large (~25 μm) mid-Archean overgrowths mantling igneous cores. Four other zircons profiled to their core regions record separate and geologically uncorrelatable events: common ~3.9 Ga overgrowths 3-6 μm thick are present on a spatial scale otherwise inaccessible to a conventional "spot" mode (~30 μm). Post main-accretion impact fluxes of comets and asteroids would potentially have had direct influence on the terrestrial rock record; lunar meteorite melts and melt-sheet ages for the Moon predict a statistical increase in impactor flux between 4.0-3.92 Ga (Tera et al., 1974; Cohen et al., 2000; Ryder, 2002). Dynamical models predict scaled impact events on the Earth during the "lunar cataclysm" (e.g. Sleep and Zahnle, 1998). Our depth profile results from Hadean zircons record contemporaneous 3.97-3.94 Ga metamorphic overgrowths. Whether these rims are of impact origin (i.e. metamorphism within or under impact ejecta melt) or metamorphic processes in the crust awaits further investigation.
Harrison Matthew T.
Mojzsis Stephen J.
Trail Dustin
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