The Cool Early Earth: Oxygen Isotope Evidence for Continental Crust and Oceans on Earth at 4.4 Ga

Details The Cool Early Earth: Oxygen Isotope Evidence for Continental Crust and Oceans on Earth at 4.4 Ga The Cool Early Earth: Oxygen Isotope Evidence for Continental Crust and Oceans on Earth at 4.4 Ga

May 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm...v51a01v&link_type=abstract

American Geophysical Union, Spring Meeting 2001, abstract #V51A-01 INVITED

Other

3672 Planetary Mineralogy And Petrology (5410), 8125 Evolution Of The Earth, 8494 Instruments And Techniques, 9330 Australia, 9619 Precambrian

Scientific paper

Zircons preserve the best record of U-Pb crystallization age and oxygen isotope ratios of igneous rocks. The d18-O of non-metamict zircon is unaffected even by hydrothermal alteration and high-grade metamorphism. Ion microprobe analysis of detrital zircons from the \sim3 Ga Jack Hills metaconglomerate (Narryer Gneiss Terrane, Yilgarn Craton, Western Australia) yield U-Pb ages from 3.1 to 4.4 Ga (SHRIMP II, Wilde et al. 2001 Nature) and d18-O from 5 to 8 permil (Cameca 4f, Peck et al. 2001 GCA). The d18-O of these zircons averages 6.3, and is 1 permil higher than that in equilibrium with the mantle and that of normal Archean granitic zircons (5.3+-0.3, 5.5+-0.4, respectively; King et al. 1998 Pre-C Res, Peck et al. 2000 Geology). The distribution of mantle-like vs. mildly elevated d18-O values for magmas is constant from 2.7 to 4.4 Ga, and on 4 continents. The age of 4.404+-0.008 Ga from one 200 micron zircon is >99% concordant and represents the oldest recognized terrestrial material. This crystal is zoned in d18-O (5.0+-0.7 vs. 7.4+-0.7) and REEs (La=0.3 to 13.6 ppm), and contains inclusions of SiO2. REE patterns are HREE enriched with positive Ce and negative Eu anomalies; calculated melts are LREE enriched. Taken together, these results suggest crystallization from a quartz-saturated granitic magma and thus the existence of continental crust, possibly in a setting like Iceland. The high d18-O portion of the crystal would be in equilibrium with a magma at d18-O(WR)= 8.5-9.5. There is no known mantle reservoir with such high values. d18-O(WR) values above 8.5 are typical of "S-type" granites that have melted or assimilated material that was altered by low temperature interaction with water at the surface of the Earth (i.e., weathering, diagenesis, low T hydrothermal alteration). Thus the high d18-O value of the 4.4 Ga zircon suggests that surface temperatures were cool enough for liquid water suggesting that the early steam-rich atmosphere condensed to form oceans at that time. The evidence for liquid water and possibly oceans at 4.4 Ga suggests a Cool Early Earth. This contrasts with the Hot Early Earth and global magma oceans envisioned at 4.5-4.3 Ga based on: an impact origin of the Moon (4.45-4.50 Ga), core formation, higher Hadean radioactive heat production, and intense early meteorite bombardment. Magma on the surface of the Earth cools quickly by radiation to form a crust, but a magma ocean caused by these processes might persist beneath the initially thin crust for up to 400 m.y. and might erupt as massive flood basalts in response to major meteorite impacts, boiling surface waters. The thermal contrasts presented by these lines of evidence are minimized if the Moon and core formed earlier (\sim4.5 Ga), if the Moon formed by a process not involving a Mars-size impactor, or if the early meteorite bombardment was less intense or irregular in timing. It is possible that periods of Cool vs. Hot Early Earth alternated, with boiling of early oceans after major impact events followed by periods of cooler surface conditions. If life evolved in these seas, multiple extinctions before 3.9 Ga are suggested.

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