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
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
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DOI：
10.1016/s0016-7037(01)00711-6
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发表时间：
2001-11
影响因子：
5
通讯作者：
W. Peck;J. Valley;S. Wilde;C. Graham
中科院分区：
文献类型：
--
作者：
W. Peck;J. Valley;S. Wilde;C. Graham
离子探针对早太古代（冥古宙）锆石（4.0- 4.4-Ga）氧同位素比值的分析揭示了岩浆δ 18 O值的变化，包括一些相对于地幔较高的值，表明在地球历史的前5亿年期间岩浆和已经形成的大陆地壳之间的相互作用。常规分析证实了这些锆石具有较高的δ ~（18）O值。来自Yilgarn盆地（西澳大利亚）Jack Hills的变砾岩含有年龄> 4.0 Ga的碎屑锆石（Compston和Pidgeon，1986年）和一个年龄为4.40 Ga的晶体（Wilde等人，2001年）。新发现的4.40 Ga颗粒是目前已知的最古老的陆源矿物，杰克山变质砾岩中还含有大量年龄为3.3- 3.6 Ga的锆石，其δ 18 O平均值为6.3 ± 0.1‰（1 s.e.，; n = 32点分析）。2个4.15Ga锆石的δ 18 O平均值为5.7 ± 0.2‰（n = 13）。此外，一个4.13 Ga锆石的δ 18 O平均值为7.2 ± 0.3‰（n = 8），另一个4.01 Ga锆石的δ 18 O平均值为6.8 ± 0.4‰（n = 10）。最古老的颗粒（4.40 Ga）的微量元素组成（特别是LREE）和阴极发光强度，所有这些都与氧同位素比值（7.4‰对5.0‰）相关。4.01- 4.40 Ga颗粒的高LREE和高δ 18 O值与演化的花岗质岩浆（δ 18 O（WR）= 8.5 - 9.5‰）的生长相一致，这些岩浆与表壳物质相互作用。高δ 18 O值表明，成岩作用、风化作用或低温蚀变作用）发生在4.0 Ga之前，甚至在4.40 Ga之前，紧接着假设的核心分化和火星大小的天体在1.45 Ga形成月球的日期。这是早在4.40 Ga的大陆地壳的第一个证据，表明早期地球在强烈的陨石撞击期间发生了分化。水和岩石相互作用的强度是导致高δ 18 O值所必需的，这表明存在液态水，因此可能在4.40 Ga存在海洋。
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 (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 δ18O value of 6.3 ± 0.1‰ (1 s.e.,; n = 32 spot analyses). Two 4.15-Ga zircons have an average δ18O of 5.7 ± 0.2‰ (n = 13). In addition, a 4.13-Ga zircon has an average δ18O of 7.2 ± 0.3‰ (n = 8) and another 4.01-Ga zircon has an average δ18O 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-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‰) 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.