Hafnium isotope evidence for a transition in the dynamics of continental growth 3.2 Gyr ago
Hafnium isotope evidence for a transition in the dynamics of continental growth 3.2 Gyr ago
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DOI：
10.1038/nature11140
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发表时间：
2012-05
期刊：
影响因子：
64.8
通讯作者：
T. Næraa;A. Schersten;M. Rosing;A. Kemp;J. Hoffmann;T. Kokfelt;M. Whitehouse
中科院分区：
文献类型：
--
作者：
T. Næraa;A. Schersten;M. Rosing;A. Kemp;J. Hoffmann;T. Kokfelt;M. Whitehouse
由于地幔温度的长期变化，地球岩石圈可能经历了向现代板块构造体制的演化。放射性同位素变化被解释为大陆地壳生长速率随时间推移而下降的证据，，一些估计表明，目前大陆地壳储层的70%以上是在太古代末期开采的。地壳的生长和改造的岩石年龄小于30亿年（Gyr）的模式被认为是反映了组装和分解的超大陆威尔逊循环过程，标志着岩石圈动力学的一个重要变化。然而，在西格陵兰南部，许多研究认为俯冲环境和地壳生长的弧增生可以追溯到3.8 Gyr前，这表明现代构造体制在最早的地壳岩石记录形成期间运作。在这里，我们在西格陵兰岛南部的基底岩石的锆石的situuturanium-lead，铪和氧同位素数据在整个关键的时间段，在此期间，现代的构造体制可能已经启动。我们的数据显示，在这个时间间隔的铪同位素时间模式的显着差异，需要在岩浆原岩的特性的变化。这些观察结果表明，3.9-3.5 Gyr的岩石是从一个年龄大于3.9 Gyr的烃源岩储层中分化出来的，其铪同位素组成为贫铪到轻微亏损。相比之下，3.2 Gyr ago之后形成的岩石记录了3.9 Gyr ago以来第一次增加的幼年贫化物质（即新的幔源地壳），其特征是铪同位素比值的显著变化，类似于古生代俯冲相关造山带的变化。这些数据表明，3.5-3.2 Gyr前的过渡期从一个古老的（3.9-3.5 Gyr老）地壳演化制度不同于现代板块构造的地球动力学设置后，3.2 Gyr前，涉及年轻地壳生成的板块构造过程。
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 ago,,, suggesting that modern-day tectonic regimes operated during the formation of the earliest crustal rock record. Here we reportin situuranium–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 magmatic 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.