Mixon Emily E, Bauer Ann M, Blum Tyler B, Valley John W, Rizo Hanika, O'Neil Jonathan, Kitajima Kouki
Department of Geoscience, University of Wisconsin-Madison, WI.
Department of Earth Sciences, Carleton University, Ottawa, ON, Canada.
Proc Natl Acad Sci U S A. 2024 Sep 24;121(39):e2405378121. doi: 10.1073/pnas.2405378121. Epub 2024 Sep 16.
Determining the mechanisms by which the earliest continental crust was generated and reworked is important for constraining the evolution of Earth's geodynamic, surface, and atmospheric conditions. However, the details of early plate tectonic settings often remain obscured by the intervening ~4 Ga of crustal recycling. Covariations of U, Nb, Sc, and Yb in zircon have been shown to faithfully reflect Phanerozoic whole-rock-based plate-tectonic discriminators and are therefore useful in distinguishing zircons crystallized in ridge, plume, and arc-like environments, both in the present and in deep time. However, application of these proxies to deciphering tectonic settings on the early Earth has thus far been limited to select portions of the detrital zircon record. Here, we present in situ trace-element and oxygen isotope compositions for magmatic zircons from crystalline crustal rocks of the Acasta Gneiss Complex and the Saglek-Hebron Complex, Canada. Integrated with information from whole-rock geochemistry and zircon U-Pb, Hf, and O isotopes, our zircon U-Nb-Sc-Yb results reveal that melting of hydrated basalt was not restricted to a single tectonomagmatic process during the Archean but was operative during the reworking of Hadean protocrust and the generation of juvenile crust within two cratons, as early as 3.9 Ga. We observe zircon trace-element compositions indicative of hydrous melting in settings that otherwise host seemingly differing whole-rock geochemistry, zircon Hf, and zircon O isotopes, suggesting contemporaneous operation of stagnant-lid (oceanic plateau) and mobile-lid (arc-like) regimes in the early Archean.
确定最早的大陆地壳是如何形成和改造的机制，对于限制地球地球动力学、地表和大气条件的演化至关重要。然而，早期板块构造环境的细节往往被约40亿年的地壳再循环所掩盖。锆石中U、Nb、Sc和Yb的协变已被证明能忠实地反映显生宙基于全岩的板块构造判别标志，因此有助于区分在现代和地质历史时期在洋中脊、地幔柱和弧状环境中结晶的锆石。然而，迄今为止，将这些指标应用于解读早期地球的构造环境仅限于碎屑锆石记录的某些部分。在这里，我们展示了来自加拿大阿卡斯塔片麻岩杂岩和萨格莱克-希伯伦杂岩结晶地壳岩石中岩浆锆石的原位微量元素和氧同位素组成。结合全岩地球化学以及锆石U-Pb、Hf和O同位素信息，我们的锆石U-Nb-Sc-Yb结果表明，含水玄武岩的熔融在太古宙期间并不局限于单一的构造岩浆过程，而是在冥古宙原生地壳的改造以及早在39亿年前两个克拉通内新生地壳的形成过程中都起作用。我们观察到，在其他方面具有看似不同的全岩地球化学、锆石Hf和锆石O同位素的环境中，锆石微量元素组成指示了含水熔融，这表明太古宙早期停滞盖层（洋中脊高原）和活动盖层（弧状）机制是同时运作的。
