A compilation of more than 300,000 rock compositions provides crucial input into a 100-year-old debate on how the continental crust formed, and provides new constraints for theories of continental-crust development. See Article p.301

Earth is the only planet in our Solar System known to have a buoyant continental crust. The origin of this crust has been long debated because its compositional heterogeneity precludes the straightforward testing of formation models. On page 301 of this issue, Keller et al.1 present perhaps the most convincing data so far on this issue. They demonstrate that most of the continental crust's igneous rocks — those formed by the solidification of lava or magma — are formed through progressive crystallization of melted material extracted from Earth's interior, followed by the return of the most dense crystals to the mantle.

Two competing hypotheses that arose in the pioneering experimental geochemistry laboratories of the early 1900s have influenced nearly all models for the formation of the continental crust. In 1915, Norman L. Bowen proposed that the continental crust formed through the progressive crystallization and distillation of magmas generated in Earth's mantle2 (Fig. 1a). In this theory, the first minerals to crystallize are those with the highest abundance of iron and magnesium (mafic minerals), leaving the remaining melt enriched in silica (SiO 2 ). Mantle-derived magma that originally contains 50% silica can thus evolve through continuous crystallization to form a composition that matches that of the bulk continental crust (approximately 61% silica). In 1933, Bowen's mentor, Reginald A. Daly, proposed that assimilation was at least as important in forming the modern continental crust — ascending mantle-derived magmas drive the melting of the pre-existing continental crust, and mixing of the two, to produce the observed silica content of the bulk continental crust3 (Fig. 1b). Figure 1: Models of continental-crust formation. a, In the crystallization model, melted material within the mantle rises to the surface, inducing the crystallization of minerals from the melt as the temperature of the surrounding environment falls. Mafic minerals (those with the highest abundance of iron and magnesium) are the first to crystallize and fall to the bottom of the melt, where they form rocks known as mafic cumulates. The remaining melt is enriched in silica and forms the continental crust. b, In the assimilation model, ascending mantle-derived magmas drive the melting of the pre-existing continental crust, which mixes into the magma. The resulting mixture goes on to form new continental crust. Keller and colleagues' compilation1 of geochemical data from around the globe supports the idea that crystallization is the dominant process in modern continental-crust formation. Full size image

As the problems and costs of geochemical analyses and computational time decrease, geoscientists can use ever-larger data sets to answer fundamental questions about our planet. Keller and colleagues1 report one of the most compelling examples of this so far. They compiled more than 300,000 existing geochemical analyses of igneous rocks from around the world to calculate the average composition of magmas in areas where continental plates converge or are pulled apart (rifted).

By comparing the chemical compositions of plutonic rocks (which formed from magmas that slowly cooled within the Earth) with those of volcanic rocks (formed when magmas erupted on Earth's surface), the researchers assessed the dominant processes that contributed to the rocks' formation. If Daly's assimilation theory is correct, there should be a linear relationship between the concentration of oxides, such as magnesium oxide, and silica. However, Keller and co-workers observe a nonlinear relationship, which confirms the dominance of Bowen's crystallization hypothesis.

One long-standing issue with Bowen's hypothesis is that mafic crystal residues should have accumulated in the lower crust over time, forming cumulate rock. Several lines of evidence — including data from geological samples, seismic imaging, elemental mass-balance calculations and numerical simulations — support a model in which mafic crystal residues can reach a density greater than that of the uppermost mantle, causing them to detach from the base of the continental crust and sink back into the mantle, a process called delamination4,5,6,7,8. Keller et al. identify key differences between the average plutonic and volcanic rock compositions from convergent and rift environments. These differences are consistent with predicted cumulate compositions, based on the authors' extensive calculations, and demonstrate that mafic-crystal cumulates are present in plutonic rocks, but not in large abundances. The authors' observation that most continental plutonic-rock compositions have higher silica content than expected for mafic cumulates substantiates both Bowen's hypothesis and the delamination model.

Keller et al. also conclude that plutonic rocks that formed in continental-rift environments exhibit characteristics consistent with the presence of water during their formation. Such environments were previously thought to be dry. The presence of water could explain why these magmas stalled and crystallized in the continental crust instead of erupting at the surface: water-containing magmas approach the temperature at which crystals start to form as they ascend, whereas dry magmas do not. Ultimately, the bulk continental crust has geochemical signatures consistent with formation in the presence of small amounts of water9. The observation that both convergent and rift plutonic environments produce such signatures, and that they preferentially crystallize in the crust rather than erupt, therefore provides new constraints on theories of how the continental crust developed.

Two notable questions relating to Keller and colleagues' findings are when continental-crust formation started, and whether crustal formation processes have varied over Earth's history — topics that two of the authors of this work wrestled with in an earlier paper10. Statistical evidence in the present data set suggests that the composition of the population of rocks more than 100 million years old does not vary substantially from that of the younger population. But the data set for older rocks is much smaller than that for younger ones, and many of the older rocks have been lost through recycling processes such as subduction and erosion; those that survived recycling may not be representative of the diversity that once existed. The observations now presented by Keller et al. are therefore most applicable to modern continental-crust formation.

In an era in which breakthroughs in geochemistry are often attained by probing rocks and minerals at the nanometre and atomic scales11, Keller et al. are pushing the boundaries in the other direction, reminding us of the value of investigations averaged over large scales of time and space. Since the pioneering days of Bowen and Daly, the field of geochemistry has evolved to incorporate observations ranging from the microscopic to the global in scale. Future work will require the integration of such geochemical observations with those from a wide range of other disciplines to explore questions such as whether the formation of the continental crust played a primary part in the rise of oxygen in Earth's atmosphere.Footnote 1

Notes

References 1 Keller, C. B., Schoene, B., Barboni, M., Samperton, K. M. & Husson, J. M. Nature 523, 301–307 (2015). 2 Bowen, N. L. J. Geol. 23 (suppl.), 1–91 (1915). 3 Daly, R. A. Igneous Rocks and the Depths of the Earth (McGraw-Hill, 1933). 4 Kay, R. W. & Kay, S. M. Tectonophysics 219, 177–189 (1993). 5 Jull, M. & Kelemen, P. B. J. Geophys. Res. 106, 6423–6446 (2001). 6 Gao, S. et al. Nature 432, 892–897 (2004). 7 Plank, T. J. Petrol. 46, 921–944 (2005). 8 Levander, A. et al. Nature 472, 461–465 (2011). 9 Rudnick, R. L. Nature 378, 571–578 (1995). 10 Keller, C. B. & Schoene, B. Nature 485, 490–493 (2012). 11 Valley, J. W. et al. Nature Geosci. 7, 219–223 (2014). Download references

Author information Affiliations Christy Till is at the School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85281, USA. Christy Till Authors Christy Till View author publications You can also search for this author in PubMed Google Scholar Corresponding author Correspondence to Christy Till.

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About this article Cite this article Till, C. Big geochemistry. Nature 523, 293–294 (2015). https://doi.org/10.1038/523293a Download citation Published: 15 July 2015

Issue Date: 16 July 2015

DOI : https://doi.org/10.1038/523293a