Potassic, high-silica Hadean crust

Patrick Boehnke, Elizabeth A. Bell, Thomas Stephan, Reto Trappitsch, C. Brenhin Keller, Olivia S. Pardo, Andrew M. Davis, T. Mark Harrison, and Michael J. Pellin
PNAS June 19, 2018 115 (25) 6353-6356; published ahead of print June 4, 2018 https://doi.org/10.1073/pnas.1720880115

  1. Contributed by T. Mark Harrison, May 4, 2018 (sent for review November 30, 2017; reviewed by Tamara L. Carley and Stein B. Jacobsen)

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Significance

To understand early Earth’s habitability, we need to know when the continental crust first formed. However, due to the combined actions of plate tectonics and erosion, most of the evidence of the early crust has been destroyed. To shed light on this debate, we analyzed the strontium isotopic composition of apatite inclusions in zircons from Nuvvuagittuq, Canada, where independent evidence suggests a crust-forming event prior to 4.2 Ga, possibly as early as 4.4 Ga. Our results show that this early crust had a high Rb/Sr ratio and therefore a high silica content. This suggests that the early Earth was capable of forming continental crust within <350 million y of solar system formation.

Abstract

Understanding Hadean (>4 Ga) Earth requires knowledge of its crust. The composition of the crust and volatiles migrating through it directly influence the makeup of the atmosphere, the composition of seawater, and nutrient availability. Despite its importance, there is little known and less agreed upon regarding the nature of the Hadean crust. By analyzing the 87Sr/86Sr ratio of apatite inclusions in Archean zircons from Nuvvuagittuq, Canada, we show that its protolith had formed a high (>1) Rb/Sr ratio reservoir by at least 4.2 Ga. This result implies that the early crust had a broad range of igneous rocks, extending from mafic to highly silicic compositions.

Footnotes

  • ↵1To whom correspondence may be addressed. Email: pboehnke{at}gmail.com or tmark.harrison{at}gmail.com.

  • ↵2Present address: Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550.

  • Author contributions: P.B., E.A.B., and T.M.H. designed research; P.B., E.A.B., T.S., R.T., C.B.K., and O.S.P. performed research; P.B., T.S., R.T., O.S.P., A.M.D., and M.J.P. contributed new reagents/analytic tools; P.B., E.A.B., C.B.K., and T.M.H. analyzed data; and P.B., E.A.B., T.S., C.B.K., O.S.P., A.M.D., T.M.H., and M.J.P. wrote the paper.

  • Reviewers: T.L.C., Lafayette College; and S.B.J., Harvard University.

  • The authors declare no conflict of interest.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1720880115/-/DCSupplemental.

Published under the PNAS license.

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Potassic, high-silica Hadean crust
Patrick Boehnke, Elizabeth A. Bell, Thomas Stephan, Reto Trappitsch, C. Brenhin Keller, Olivia S. Pardo, Andrew M. Davis, T. Mark Harrison, Michael J. Pellin
Proceedings of the National Academy of Sciences Jun 2018, 115 (25) 6353-6356; DOI: 10.1073/pnas.1720880115
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