Oxygen isotope ratios and rare earth elements in 3.3 to 4.4 Ga zircons; ion microprobe evidence for high δ18O continental crust and oceans in the early Archean

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doi: 10.1016/S0016-7037(01)00711-6
Authors:Peck, William H.; Valley, John W.; Wilde, Simon A.; Graham, Colin M.
Author Affiliations:Primary:
University of Wisconsin, Department of Geology and Geophysics, Madison, WI, United States
Curtin University of Technology, Bentley, West. Aust., Australia
University of Edinburgh, Edinburgh, United Kingdom
Volume Title:Geochimica et Cosmochimica Acta
Source:Geochimica et Cosmochimica Acta, 65(22), p.4215-4229. Publisher: Pergamon, Oxford, International. ISSN: 0016-7037
Publication Date:2001
Note:In English. 88 refs.; illus., incl. 2 tables
Summary:Ion microprobe analyses of oxygen isotope ratios in Early Archean (Hadean) zircons (4.0- to 4.4-Ga) reveal variable magmatic δ18O values, including some that are high relative to the mantle, suggesting interaction between magmas and already-formed continental crust during the first 500 million yr of Earth"s history. The high average δ18O value of these zircons is confirmed by conventional analysis. A metaconglomerate from the Jack Hills in the Yilgarn Craton (Western Australia) contains detrital zircons with ages >4.0 Ga and one crystal that is 4.40-Ga old . The newly discovered 4.40-Ga grain is the oldest recognized terrestrial mineral. The Jack Hills metaconglomerate also contains a large 3.3- to 3.6-Ga-old zircon population with an average δ18O value of 6.3±0.1 per mil (1 s.e.,; n = 32 spot analyses). Two 4.15-Ga zircons have an average δ18O of 5.7±0.2 per mil (n = 13). In addition, a 4.13-Ga zircon has an average δ18O of 7.2±0.3 per mil (n = 8) and another 4.01-Ga zircon has an average δ18O of 6.8±0.4 per mil (n = 10). The oldest grain (4.40 Ga) is zoned with respect trace element composition (especially LREE), and intensity of cathodoluminescence, all of which correlate with oxygen isotope ratios (7.4 per mil vs. 5.0 per mil). High LREE and high-δ18O values from the 4.01- to 4.40-Ga grains are consistent with growth in evolved granitic magmas (δ18O(WR) = 8.5 to 9.5 per mil) that had interacted with supracrustal materials. High δ18O values show that low-temperature surficial processes (i.e., diagenesis, weathering, or low-temperature alteration) occurred before 4.0 Ga, and even before 4.40 Ga, shortly following the hypothesized date of core differentiation and impact of a Mars-sized body to form the Moon at ∼4.45 Ga. This is the first evidence of continental crust as early as 4.40 Ga and suggests differentiation during the period of intense meteorite bombardment of the early Earth. The magnitude of water and rock interaction that would be necessary to cause the high δ18O values suggests the presence of liquid water and thus the possibility of an ocean at 4.40 Ga. Abstract Copyright (2001) Elsevier, B.V.
Sections:Age determination
Subjects:Absolute age; Archean; Continental crust; Crust; Dates; Detritus; Experimental studies; Geochemistry; Ion probe data; Isotope ratios; Isotopes; Mass spectra; Metaconglomerate; Metals; Metamorphic rocks; Metasedimentary rocks; Nesosilicates; O-18/O-16; Orthosilicates; Oxygen; Paleoarchean; Precambrian; Rare earths; Silicates; Spectra; Stable isotopes; Water-rock interaction; Zircon; Zircon group; Australasia; Australia; Jack Hills; Western Australia; Yilgarn Craton
Coordinates:S320000 S313000 E1170000 E1160000
Abstract Numbers:02M/1254
Record ID:2002003854
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands
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