Lithium in Jack Hills zircons: Evidence for extensive weathering of Earth's earliest crust
Lithium in Jack Hills zircons: Evidence for extensive weathering of Earth's earliest crust
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DOI：
10.1016/j.epsl.2008.05.032
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发表时间：
2008-08
影响因子：
5.3
通讯作者：
T. Ushikubo;N. Kita;A. Cavosie;S. Wilde;R. Rudnick;J. Valley
中科院分区：
文献类型：
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作者：
T. Ushikubo;N. Kita;A. Cavosie;S. Wilde;R. Rudnick;J. Valley
通过 SIMS 对西澳大利亚杰克山 4348 至 3362 Ma 碎屑锆石进行的原位锂分析表明，锂丰度（通常为 10 至 60 ppm）通常比幔源岩浆结晶的锆石和幔源锆石巨晶（通常 <2 ppb）高 10,000 倍以上。在来自三种大陆母岩的火成锆石中也发现了锆石中高锂浓度（10 至 250 ppm）：花岗岩、富锂伟晶岩和泥质变沉积物中的混合岩。锆石中三价阳离子（REE 和 Y）的取代与 Li+1 和 P+5 相关，表明 Li 的间隙位点以及磷钇矿取代 P 为 REE 提供了电荷平衡。因此，Li 通过耦合取代固定在锆石结构中，并且 [Li] 成分的扩散变化受到 REE 缓慢扩散的速率限制。杰克山锆石的分馏锂同位素比（δ7Li=−19 至 +13‰）比原始海底玄武岩中记录的锂同位素比（2 至 8‰）变化约五倍，但与大陆地壳及其风化产物相似。在早在 4300 Ma 形成的锆石中发现了低于 -10‰ 的 δ7Li 值。高锂成分表明原始岩浆不是杰克山锆石的来源，δ7Li 的分馏值表明这些早期太古代岩浆采样了高度风化的风化层。这些新的锂数据提供了证据，表明来自杰克山的古代锆石的母岩浆包含了来自地球表面的物质，这些物质在低温下与液态水相互作用。这些数据支持这样的假设：大陆型地壳和海洋在 4300 Ma 之前就已存在，即地球形成后的 2.5 亿年内，而 δ7Li 的低值表明太古代早期风化作用广泛。
In situ Li analyses of 4348 to 3362 Ma detrital zircons from the Jack Hills, Western Australia by SIMS reveal that the Li abundances (typically 10 to 60 ppm) are commonly over 10,000 times higher than in zircons crystallized from mantle-derived magmas and in mantle-derived zircon megacrysts (typically <2 ppb). High Li concentrations in zircons (10 to 250 ppm) have also been found in igneous zircons from three continental parent rocks: granites, Li-rich pegmatites, and migmatites in pelitic metasediment. The substitution of trivalent cations (REEs and Y) in zircon correlates with Li+1and P+5, suggesting that an interstitial site for Li, as well as the xenotime substitution for P, provides charge balance for REEs. Li is thus fixed in the zircon structure by coupled substitutions, and diffusive changes in [Li] composition are rate-limited by slow diffusion of REEs. The Jack Hills zircons also have fractionated lithium isotope ratios (δ7Li=−19 to+13‰) about five times more variable than those recorded in primitive ocean floor basalts (2 to 8‰), but similar to continental crust and its weathering products. Values of δ7Li below −10‰ are found in zircons that formed as early as 4300 Ma. The high Li compositions indicate that primitive magmas were not the source of Jack Hills zircons and the fractionated values of δ7Li suggest that highly weathered regolith was sampled by these early Archean magmas. These new Li data provide evidence that the parent magmas of ancient zircons from Jack Hills incorporated materials from the surface of the Earth that interacted at low temperature with liquid water. These data support the hypothesis that continental-type crust and oceans existed by 4300 Ma, within 250 million years of the formation of Earth and the low values of δ7Li suggest that weathering was extensive in the early Archean.