Initial results from the mission to really early earth

Details Initial results from the mission to really early earth Initial results from the mission to really early earth

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....8060h&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #8060

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Scientific paper

Isotopic evidence suggests that life was present on Earth prior to 3.83 Ga, placing its emergence prior to the end of an intense bombardment of the inner solar system. This raises the possibility that life originated during the Hadean Eon (4.5-4.0 Ga), a period for which there is no known rock record. If so, how can we determine when the necessary ingredients for life, notably liquid water, first appeared? Detrital zircons older that 4 Ga from the Jack Hills, Western Australia, offer the prospect of unprecedented insights into surface environmental conditions during the earliest phase of Earth history. For example, zircons as old as 4.3 Ga were discovered to contain δ18OSMOW up to +8ppm. These and other data indicate that Jack Hills zircons formed from multiple processes that require a hydrosphere (subduction-related melting and anatexis of clay-rich protoliths) within 200 m.y. of planetary accretion and challenge the view that continental and hydrosphere formation were frustrated by meteorite bombardment and basaltic igneous activity until ˜4 Ga. These ancient zircons offer other opportunities to constrain the earliest evolution of the atmosphere, hydrosphere, and continents provided sufficient quantities can be obtained for analysis. \underline{Age distribution}: As >4.2 Ga zircons make up only ˜0.2% of the detrital population, we have refined a method to rapidly survey large numbers of 207Pb/206Pb ages. Using an ion microprobe in multi-collector mode, an age with ±1% precision can be obtained in ˜30 sec. We have thus far surveyed over 15,000 grains (typically ˜2 μg each) from which two age peaks are evident at ˜3.3 Ga and ˜4.0 Ga, tailing off at older ages. Zircons identified as being >4 Ga are then U-Pb dated using SHRIMP RG. We have thus far identified 105 zircons in the age range 4.1 to 4.2 Ga and 31 >4.2 Ga, including three that are ge4.35 Ga. The oldest zircon (˜8 μg) thus far dated is 4.37 Ga. Our inability to identify a zircon as old as 4.4 Ga raises the possibility that an earlier report of such an age reflects later U-Pb mobility (see Tera, 2002, EOS 83). \underline{Inclusion mineralogy}: Many ˜10-100-μm-sized inclusions have been found in >4 Ga zircons, including peraluminous mineral assemblages and sulfide crystals (targeted for Δ33S analysis). \underline{Geodynamo origin}: Ultra-sensitive analysis methods demonstrate that a Jack Hills zircon carries an intrinsic remenant magnetism raising the possibility of constraining the time of geodynamo activation. \underline{Extinct radioactivities}: RELAX Xe isotope analyses using 4.15 Ga zircons yield plutogenic 136Xe as high as 35% and indicate a terrestrial Pu/U ratio indistinguishable from chondritic. This result has implications for interpreting mantle-derived Xe isotopes in terms of mantle evolution and the age of the atmosphere.

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