Age dependence of the composition of continental crust: evidence from Nd isotopic variations in granitic rocks
Age dependence of the composition of continental crust: evidence from Nd isotopic variations in granitic rocks
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DOI：
10.1016/0012-821x(88)90130-6
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发表时间：
1988-11
影响因子：
5.3
通讯作者：
D. DePaolo
中科院分区：
文献类型：
--
作者：
D. DePaolo
地壳花岗岩来源的 Nd 同位素系统学用于估计大陆地壳的 Sm/Nd 比值作为其年龄的函数。研究发现，从早太古代到晚元古代，随着年龄的降低，大陆地壳花岗岩岩浆源的Sm/Nd值从球粒陨石值的0.47倍增加到0.64倍。这一趋势与根据沉积岩中稀土元素模式推断的地壳趋势相反。观察到的趋势可能严格地仅适用于地壳的长英质部分，但除非较老的地壳含有比年轻的地壳高得多的镁铁质材料百分比（50％对0％），否则趋势的方向也适用于大块地壳。由于地球上最古老的地壳的某些部分可能已被随后的过程所破坏，因此可以想象，这种趋势可能是保存偏差的结果，而不是地壳成分随时间的真正变化的结果。同位素数据结合地壳年龄分布表明，大块大陆地壳的Sm/Nd值不低于球粒陨石值的0.60倍。对地壳 Nd 浓度的这一限制和估计与将大陆中的 Nd 等同于从上地幔到约 700 公里深度的缺失的质量平衡是一致的。
The Nd isotopic systematics of the sources of crustal granitic rocks are used to estimate the Sm/Nd ratio of the continental crust as a function of its age. It is found that the Sm/Nd value of granite magma sources in continental crust increases from about 0.47 to 0.64 times the chondritic value with decreasing age from the Early Archean to the Late Proterozoic. This trend is opposite to that inferred for the crust from rare earth element patterns in sedimentary rocks. The observed trend may apply strictly only to the felsic portions of the crust, but unless older crust contains a much higher percentage of mafic material than young crust (50% versus 0%), the direction of the trend also applies to the bulk crust. Because some portion of the earth's oldest crust has probably been destroyed by subsequent processes, the trend could conceivably be the result of preservational bias rather than a real shift in crustal composition with time. The isotopic data, combined with the crustal age distribution, indicate that the Sm/Nd value of the bulk continental crust is not lower than 0.60 times the chondritic value. This limit and estimates of the Nd concentration of the crust are consistent with the mass balance that equates the Nd in the continents to that missing from the upper mantle down to a depth of about 700 km.