Oxygen isotope ratios and rare earth elements in 3.3 to 4.4 Ga zircons: Ion microprobe evidence for high δ~(18)O continental crust and oceans in the Early Archean

摘要

Ion microprobe analyses of oxygen isotope ratios in Early Archean (Hadean) zircons (4.0- to 4.4-Ga) reveal variable magmatic δ~(18)O 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 δ~(18)O value of these zircons is confirmed by conventional analysis. A metaconglomerate from the Jack Hills in the Yilgarn Cration (Western Australia) contains detrital zircons with ages > 4.0 Ga (Compston and Pidgeon, 1986) and one crystal that is 4.40-Ga old (Wilde et al., 2001). 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 δ~(18)O value of 6.3 ±0.1‰ (1 s.e., ; n = 32 spot analyses). Two 4.15-Ga zircons have an average δ~(18)O of 5.7 ± 0.2‰ (n = 13). In addition, a 4.13-Ga zircon has an average δ~(18)O of 7.2 ± 0.3‰ (n = 8) and another 4.01-Ga zircon has an average δ~(18)O of 6.8 ±0.4‰ (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‰ vs. 5.0‰). High LREE and high-δ~(18)O values from the 4.01- to 4.40-Ga grains are consistent with growth in evolved granitic magmas (δ~(18)O(WR) = 8.5 to 9.5‰) that had interacted with supracrustal materials. High δ~(18)O 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 δ~(18)O values suggests the presence of liquid water and thus the possibility of an ocean at 4.40 Ga.