Helium isotopic evidence for episodic mantle melting and crustal growth

• Published in: Nature Vol. 446; no. 7138; pp. 900 - 903
• Main Author: Parman, S. W.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 19.04.2007; Nature Publishing; Nature Publishing Group
• Subjects: Basalt; Continental crust; Convection; Correlation; Crusts; Crystalline rocks; Depletion; Earth; Earth sciences; Earth, ocean, space; Exact sciences and technology; Geochemistry; Geology; Helium; Humanities and Social Sciences; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Internal geophysics; Isotope geochemistry; Isotope geochemistry. Geochronology; Isotopes; letter; Mantle; Melting; multidisciplinary; Neodymium; Oceanic crust; Preserves; Reservoirs; Science; Science (multidisciplinary); Solid-earth geophysics, tectonophysics, gravimetry; Strontium; Mid Ocean Ridge Basalt; degassing; ocean-island basalts; volcanic rocks; reservoirs; igneous rocks; helium; continental accretion; He-4/He-3; depleted mantle; continental crust; partial melting; convection; crustal growth; basalts; isotopes; noble gases; mantle; tholeiitic basalt; volatile elements
• ISSN: 0028-0836; 1476-4687; 1476-4687; 1476-4679
• DOI: 10.1038/nature05691

When on Earth? The timing of continental crust formation is a topic of heated debate. What is generally agreed is that the continental crust was extracted from the mantle by partial melting. That process should have left a chemical fingerprint in the isotopic composition of the mantle. Stephen Parman has now identified this fingerprint, in the shape of a spectrum of helium ratios in ocean island basalts that appears to preserve the mantle's depletion history, closely correlating with the ages of proposed continental growth pulses. 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. A study shows that the spectrum of helium ratios in ocean island basalts seems to preserve the mantle's depletion history, closely correlating with the ages of proposed continental growth pulses. The correspondence between the ages of mantle depletion events and pulses of crustal production implies that the formation of the continental crust was episodic and punctuated by large, potentially global melting events. The timing of formation of the Earth’s continental crust is the subject of a long-standing debate 1 , 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 melting 3 . 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 4 He/ 3 He ratios in ocean island basalts appears to preserve the mantle’s depletion history, correlating closely with the ages of proposed continental growth pulses 4 , 5 . The correlation independently predicts both the dominant 4 He/ 3 He peak found in modern mid-ocean-ridge basalts, as well as estimates of the initial 4 He/ 3 He ratio of the Earth 6 . 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.