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Re-Os isotope evidence for the composition, formation and age of the lower continental crust

机译：Re-Os同位素证明下大陆壳的组成，形成和年龄

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摘要

Knowledge of the composition of the lower continental crust is important for understanding the formation and evolution of the crust as a whole, and the petrogenesis of continental basalts. Here we present rhenium-osmium isotope data for two well characterized suites of lower-crustal xenoliths from North Queensland, Australia, which have average major- and trace-element compositions similar to estimates of the bulk lower continental crust. Our data indicate that the lower crust has 1 to 2 times as much osmium, about half as much rhenium, and is less radiogenic than the upper continental crust. We interpret the rhenium-osmium isotope systematics to indicate that assimilation and fractional crystallization are important processes in the formation of the lower crust, and lead to dramatic changes in the osmium isotopic composition of basalts that pond and fractionate there. A consequence of this is that the rhenium-osmium isotopic system should not be relied on to yield accurate mantle extraction ages for continental rocks.
机译：了解下陆壳的组成对于了解整个地壳的形成和演化以及陆相玄武岩的成因非常重要。在这里，我们介绍了来自澳大利亚北昆士兰州的两套特征良好的低壳异岩的rh- is同位素数据，它们的平均主要元素和痕量元素组成与大块下陆壳的估计相似。我们的数据表明，下地壳的含量是上地壳的1-2倍，rh的一半左右，并且放射源性较低。我们对systematic- is同位素进行系统解释，以表明同化作用和分步结晶是下地壳形成的重要过程，并导致玄武岩的there同位素组成在该处沉积和分馏而发生重大变化。其结果是不应依赖ed--同位素系统来为大陆岩石产生准确的地幔提取年龄。

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