Regulating continent growth and composition by chemical weathering

Continents ride high above the ocean floor because they are underlain by thick, low-density, Si-rich, and Mg-poor crust. However, the parental magmas of continents were basaltic, which means they must have lost Mg relative to Si during their maturation into continents. Igneous differentiation follow...

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Published inProceedings of the National Academy of Sciences - PNAS Vol. 105; no. 13; pp. 4981 - 4986
Main Authors Lee, Cin-Ty Aeolus, Morton, Douglas M, Little, Mark G, Kistler, Ronald, Horodyskyj, Ulyana N, Leeman, William P, Agranier, Arnaud
Format Journal Article
LanguageEnglish
Published United States National Academy of Sciences 01.04.2008
National Acad Sciences
Subjects
Basalt
Batholiths
Biological Sciences
Chemical composition
Chemical weathering
Continental crust
Continents
Earth
Earth Sciences
Environmental Sciences
Geochemistry
Geology
Global Changes
Igneous rocks
Land
Lithium
Mantle
Physical Sciences
Recycling
Sciences of the Universe
Subduction
Weathering processes
continent
lithium
geochemistry
igneous
crust
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Summary:Continents ride high above the ocean floor because they are underlain by thick, low-density, Si-rich, and Mg-poor crust. However, the parental magmas of continents were basaltic, which means they must have lost Mg relative to Si during their maturation into continents. Igneous differentiation followed by lower crustal delamination and chemical weathering followed by subduction recycling are possible solutions, but the relative magnitudes of each process have never been quantitatively constrained because of the lack of appropriate data. Here, we show that the relative contributions of these processes can be obtained by simultaneous examination of Mg and Li (an analog for Mg) on the regional and global scales in arcs, delaminated lower crust, and river waters. At least 20% of Mg is lost from continents by weathering, which translates into >20% of continental mass lost by weathering (40% by delamination). Chemical weathering leaves behind a more Si-rich and Mg-poor crust, which is less dense and hence decreases the probability of crustal recycling by subduction. Net continental growth is thus modulated by chemical weathering and likely influenced by secular changes in weathering mechanisms.
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Author contributions: C.-T.A.L. designed research; C.-T.A.L. and U.N.H. performed research; C.-T.A.L., D.M.M., and R.K. contributed new reagents/analytic tools; C.-T.A.L., M.G.L., W.P.L., and A.A. analyzed data; and C.-T.A.L. wrote the paper.
Edited by Norman H. Sleep, Stanford University, Stanford, CA, and approved January 28, 2008
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.0711143105