The Mg Isotopic Systematics of Granitoids in Continental Arcs and Implications for the Role of Chemical Weathering in Crust Formation

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. Howev...

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Bibliographic Details
Published in:Proceedings of the National Academy of Sciences - PNAS 2009-12, Vol.106 (49), p.20652-20657
Main Authors: Shen, Bing, Jacobsen, Benjamin, Lee, Cin-Ty A., Yin, Qing-Zhu, Morton, Douglas M., Sleep, Norman H.
Format: Article
Language:English
Subjects:
Chemical composition
Chemical weathering
Continental crust
Fractionation
Granite
Igneous rocks
Isotopes
Magma
Magnesium
Mantle
Physical Sciences
Sedimentary soils
Stratigraphy
Weathering processes
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Summary: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 f ractionations 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.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.0910663106