Regulating continent growth and composition by chemical weathering

doi:10.1073/pnas.0711143105

Regulating continent growth and composition by chemical weathering

*Department of Earth Science, MS-126, Rice University, 6100 Main Street, Houston, TX 77005;
^‡U.S. Geological Survey and Department of Earth Sciences, University of California, Riverside, CA 92521;
^§U.S. Geological Survey, 345 Middlefield Road (MS 937), Menlo Park, CA 94025;
^¶Department of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, RI 02912;
^‖Earth Science Division, National Science Foundation, 4201 Wilson Boulevard, Arlington, VA 22230; and
**Institut Universitaire Europeen de la Mer, Universite de Bretagne Occidentale, Domains Oceaniques, Unité Mixte de Recherche 6538, Place Nicolas Copernic, 29280 Plouzane, France

Edited by Norman H. Sleep, Stanford University, Stanford, CA, and approved January 28, 2008 (received for review November 25, 2007)

Abstract

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.

Footnotes

^†To whom correspondence should be addressed. E-mail: ctlee{at}rice.edu
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.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/cgi/content/full/0711143105/DC1.