No evidence for high-pressure melting of Earth’s crust in the Archean

• Published in: Nature communications Vol. 10; no. 1; pp. 5559 - 12
• Main Authors: Smithies, Robert H., Lu, Yongjun, Johnson, Tim E., Kirkland, Christopher L., Cassidy, Kevin F., Champion, David C., Mole, David R., Zibra, Ivan, Gessner, Klaus, Sapkota, Jyotindra, De Paoli, Matthew C., Poujol, Marc
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 05.12.2019; Nature Publishing Group; Nature Portfolio
• Subjects: 704/2151/209; 704/2151/210; 704/2151/562; Chemical composition; Continental crust; Earth crust; Earth Sciences; Fractionation; Geochemistry; Humanities and Social Sciences; Magma; Melting; multidisciplinary; Organic chemistry; Plate tectonics; Pressure; Science; Science (multidisciplinary); Sciences of the Universe; Tectonics
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-019-13547-x

Much of the present-day volume of Earth’s continental crust had formed by the end of the Archean Eon, 2.5 billion years ago, through the conversion of basaltic (mafic) crust into sodic granite of tonalite, trondhjemite and granodiorite (TTG) composition. Distinctive chemical signatures in a small proportion of these rocks, the so-called high-pressure TTG, are interpreted to indicate partial melting of hydrated crust at pressures above 1.5 GPa (>50 km depth), pressures typically not reached in post-Archean continental crust. These interpretations significantly influence views on early crustal evolution and the onset of plate tectonics. Here we show that high-pressure TTG did not form through melting of crust, but through fractionation of melts derived from metasomatically enriched lithospheric mantle. Although the remaining, and dominant, group of Archean TTG did form through melting of hydrated mafic crust, there is no evidence that this occurred at depths significantly greater than the ~40 km average thickness of modern continental crust. Some of Earth’s earliest continental crust has been previously inferred to have formed from partial melting of hydrated mafic crust at pressures above 1.5 GPa (more than 50 km deep), pressures typically not reached in post-Archean continental crust. Here, the authors show that such high pressure signatures can result from melting of mantle sources rather than melting of crust, and they suggest there is a lack of evidence that Earth’s earliest crust melted at depths significantly below 40 km.