Department of Earth Science, Rice University, Houston, TX 77005, USA.
Proc Natl Acad Sci U S A. 2009 Dec 8;106(49):20652-7. doi: 10.1073/pnas.0910663106. Epub 2009 Nov 17.
Continental crust is too Si-rich and Mg-poor to derive directly from mantle melting, which generates basaltic rather than felsic magmas. Converting basalt to more felsic compositions requires a second step involving Mg loss, which is thought to be dominated by internal igneous differentiation. However, igneous differentiation alone may not be able to generate granites, the most silicic endmember making up the upper continental crust. Here, we show that granites from the eastern Peninsular Ranges Batholith (PRB) in southern California are isotopically heavy in Mg compared with PRB granodiorites and canonical mantle. Specifically, Mg isotopes correlate positively with Si content and O, Sr, and Pb isotopes and negatively with Mg content. The elevated Sr and Pb isotopes require that a component in the source of the granitic magmas to be ancient preexisting crust making up the prebatholithic crustal basement, but the accompanying O and Mg isotope fractionations suggest that this prebatholithic crust preserved a signature of low-temperature alteration. The protolith of this basement rock may have been the residue of chemical weathering, which progressively leached Mg from the residue, leaving the remaining Mg highly fractionated in terms of its isotopic signature. Our observations indicate that ancient continental crust preserves the isotopic signature of compositional modification by chemical weathering.
大陆地壳的硅含量过高，镁含量过低，不可能直接来源于地幔熔融，因为地幔熔融产生的是玄武质岩浆，而不是长英质岩浆。要将玄武岩转化为长英质成分，需要经历第二个阶段，即镁的损失，这一过程被认为主要是由内部火成分异作用导致的。然而，单凭火成分异作用可能无法产生花岗岩，因为花岗岩是构成上地壳的硅含量最高的端元。在这里，我们表明加利福尼亚半岛山脉东部岩基（PRB）的花岗岩的镁同位素相对于 PRB 花岗闪长岩和典型地幔而言偏重。具体来说，镁同位素与硅含量以及氧、锶和铅同位素呈正相关，而与镁含量呈负相关。锶和铅同位素的升高表明，花岗岩浆源的一个组成部分是构成前岩基地壳基底的古老原有地壳，但伴随的氧和镁同位素分馏表明，这种前岩基地壳保留了低温蚀变的特征。这种基底岩石的原岩可能是化学风化的残留物，化学风化逐渐从残留物中淋滤出镁，使剩余的镁在同位素特征上高度分馏。我们的观察结果表明，古老的大陆地壳保留了化学风化导致的成分变化的同位素特征。
