Paper No. 2
Presentation Time: 8:35 AM
JACK HILLS METACONGLOMERATE: EVIDENCE OF EARLY AND LATE ARCHEAN HYDROSPHERE
The Jack Hills belt includes BIF, chert, quartzite, and highly mature metaconglomerate and interlayered meta-sandstone. Cross-bedding and fining upward sequences suggest deposition of clastics in a fan delta environment (Baxter et al. 1986 GS Australia). Trace minerals include detrital zircon, chromite, and pyrite, and metamorphic muscovite (fuchsite) and tourmaline. SHRIMP ages from 4.4 to 1.6 Ga at Eranondoo Hill, indicate multiple sources and possibly multiple depositional ages (Cavosie et al. 2002 GSA). Taken together, these rocks bracket the evolution of the Archean and Early Proterozoic atmosphere and hydrosphere. The only solid evidence of the earliest Archean conditions is from detrital zircons. U-Pb ages older than 4 Ga are reported from 4 locations in the Yilgarn craton. Single zircons from Eranondoo Hill, including the oldest zircon, have been analyzed for age, oxygen isotope ratio, and REEs by ion probe (Wilde et al. 2001 Nature, Peck et al. 2001 GCA). Zircons are HREE enriched to 10,000x chondrite with positive Ce and negative Eu anomalies. CL imaging shows concentric growth zoning. Some zircons have correlated REE and oxygen isotope variation. The 4.4 Ga zircon measures 100 x 220 microns, and contains 1-20 micron inclusions of quartz and numerous 0.4-1.0 micron inclusions that appear to be K-feldspar ±qtz, albite, magnetite, and pyrite. Thus, the parent magma was differentiated, quartz saturated and probably granitic, consistent with continental crust. Oxygen isotope ratios range from 5.0 to 7.4%o VSMOW, similar to younger zircons throughout the Archean. Four crystals have higher 18/16 than in mantle melts, suggesting crystallization in magmas formed by burial and melting of protoliths elevated in 18/16 by interaction with low temperature fluids near the surface of the Earth. The best interpretation of these results is that liquid water existed on the surface of the Earth at least intermittently from 4.4-4.0 Ga. If the hydrosphere was similar to today, temperatures would be below 200 C. Taken together, this suggests rapid cooling of the Earth following accretion, and formation of the Moon and core. The flux of meteorite bombardment may have decreased more rapidly than previously thought permitting a cool early Earth from ca. 4.4 to 4.0 Ga.