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Hafnium isotope evidence for a transition in the dynamics of continental growth 3.2 Gyr ago

机译：G同位素证据表明，在3.2年前，大陆增长动力发生了转变

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Earth's lithosphere probably experienced an evolution towards the modern plate tectonic regime, owing to secular changes in mantle temperature. Radiogenic isotope variations are interpreted as evidence for the declining rates of continental crustal growth over time with some estimates suggesting that over 70% of the present continental crustal reservoir was extracted by the end of the Archaean eon Patterns of crustal growth and reworking in rocks younger than three billion years (Gyr) are thought to reflect the assembly and break-up of supercontinents by Wilson cycle processes and mark an important change in lithosphere dynamics. In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8 Gyr agosuggesting that modern-day tectonic regimes operated during the formation of the earliest crustal rock record. Here we report in situ uranium-lead, hafnium and oxygen isotope data from zircons of basement rocks in southern West Greenland across the critical time period during which modern-like tectonic regimes could have initiated. Our data show pronounced differences in the hafnium isotope-time patterns across this interval, requiring changes in the characteristics of the mag-matic protolith. The observations suggest that 3.9-3.5-Gyr-old rocks differentiated from a >3.9-Gyr-old source reservoir with a chondritic to slightly depleted hafnium isotope composition. In contrast, rocks formed after 3.2 Gyr ago register the first additions of juvenile depleted material (that is, new mantle-derived crust) since 3.9 Gyr ago, and are characterized by striking shifts in hafnium isotope ratios similar to those shown by Phanerozoic subduction-related orogens These data suggest a transitional period 3.5-3.2 Gyr ago from an ancient (3.9-3.5 Gyr old) crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3.2 Gyr ago that involved juvenile crust generation by plate tectonic processes.
机译：由于地幔温度的长期变化，地球的岩石圈可能经历了向现代板块构造体制的演变。放射性同位素的变化被解释为大陆地壳生长速率随时间下降的证据，一些估计表明，目前的大陆地壳储层中有70％以上是在太古新世末期开采的。 30亿年（吉尔）被认为反映了威尔逊循环过程对超大陆的组装和破裂，标志着岩石圈动力学的重要变化。然而，在西格陵兰南部，许多研究都主张通过俯冲环境和弧壳的增生可以追溯到3.8 Gyr之前，这表明现代构造机制是在最早的地壳岩石记录形成过程中起作用的。在这里，我们报告了西格陵兰岛南部南部基底岩石锆石在关键时期内的原位铀铅，ha和氧同位素数据，在此期间可能启动了类似现代的构造机制。我们的数据表明，在此间隔内in同位素-时间模式存在明显差异，这需要改变岩浆原生岩的特性。观测结果表明，3.9-3.5-Gyr的岩石与> 3.9-Gyr的烃源岩相区分，具有软骨状至轻度ha同位素组成。相比之下，自3.2吉尔之前开始，在3.2吉尔之前形成的岩石记录了自3.9吉尔之前首次添加的少年贫化物质（即新的地幔衍生的地壳），其特征是ha同位素比发生了惊人的变化，与变质生代俯冲所显示的相似。相关造山带这些数据表明，从古代（3.9-3.5 Gyr）古老的地壳演化机制到现代Gyl过渡期在3.5-3.2 Gyr之前的过渡时期，到3.2 Gyr之后的涉及地壳构造过程生成幼硬壳的地球动力学背景。

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  • 来源
    《Nature》 |2012年第7400期|p.627-630|共4页
  • 作者

    T. Nasraa; A. Schersten; M. T. Rosing; A. I. S. Kemp; J. E. Hoffmann; T. F. Kokfelt; M. J. Whitehouse;

  • 作者单位

    Geological Survey of Denmark and Greenland (GEUS), Oster Voldgade 10,1350 Copenhagen K, Denmark,Nordic Center for Earth Evolution (NordCEE), Natural History Museum of Denmark, University of Copenhagen, Oster Voldgade 5-7,1350 Copenhagen K, Denmark;

    Department of Geology, Lund University, Solvegatan 12,223 62 Lund, Sweden;

    Nordic Center for Earth Evolution (NordCEE), Natural History Museum of Denmark, University of Copenhagen, Oster Voldgade 5-7,1350 Copenhagen K, Denmark;

    Centre for Exploration Targeting, School of Earth and Environment, University of Western Australia, Crawley, WA 6009 Australia,School of Earth and Environmental Science, James Cook University, Townsville, QLD 4811, Australia;

    Institut fuer Geologie und Mineralogie, Universitaet zu Kbln, Zulpicher Strasse 49a, 50674 Koeln, Germany,Steinmann Institut fuer Geologie, Mineralogie & Palaontologie, Rheinische Wilhelms-Universitaet, Poppelsdorfer Schloss,53115 Bonn, Germany;

    Geological Survey of Denmark and Greenland (GEUS), Oster Voldgade 10,1350 Copenhagen K, Denmark;

    Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden;

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