The timing of formation of the Earth’s continental crust is the subject of a long-standing debate1,2, with models ranging from early formation with little subsequent growth, to pulsed growth, to steadily increasing growth. But most models do agree that the continental crust was extracted from the mantle by partial melting3. If so, such crustal extraction should have left a chemical fingerprint in the isotopic composition of the mantle. The subduction of oceanic crust and subsequent convective mixing, however, seems to have largely erased this record in most mantle isotopic systems (for example, strontium, neodymium and lead). In contrast, helium is not recycled into the mantle because it is volatile and degasses from erupted oceanic basalts. Therefore helium isotopes may potentially preserve a clearer record of mantle depletion than recycled isotopes. Here I show that the spectrum of 4He/3He ratios in ocean island basalts appears to preserve the mantle’s depletion history, correlating closely with the ages of proposed continental growth pulses4,5. The correlation independently predicts both the dominant 4He/3He peak found in modern mid-ocean-ridge basalts, as well as estimates of the initial 4He/3He ratio of the Earth6. The correspondence between the ages of mantle depletion events and pulses of crustal production implies that the formation of the continental crust was indeed episodic and punctuated by large, potentially global, melting events. The proposed helium isotopic evolution model does not require a primitive, undegassed mantle reservoir, and therefore is consistent with whole mantle convection.
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I thank G. Pearson, P. Martin and T. Grove for informal reviews, M. Kurz, S. Hart, M. Walter and J. Blundy for discussions, and A. Abedini for compiling the He isotope database. This research was supported by the Nuffield Foundation.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The author declares no competing financial interests.
The file contains a Supplementary Table 1 of the corresponding 4He/3He and zircon age peaks used in the regression in the text and in Figure 2. It also contains Supplementary Figure 1 which is a more comprehensive version of Figure 1, showing all of the ocean islands for which there are more than 10 analyses, separating out various ridge systems and also showing the underlying histograms for the probability distribution functions. (PDF 301 kb)
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Parman, S. Helium isotopic evidence for episodic mantle melting and crustal growth. Nature 446, 900–903 (2007). https://doi.org/10.1038/nature05691
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