Has continental crust always formed the same way?
In an introduction to new research published in Geology, Ali Polat (2013) summarizes some of the controversies related to the timing and processes of the formation of Earth’s continental crust. An important questions is whether crustal growth process were different (non-uniformitarian) during the Archean. Many folk suggest that about half of the Earth’s continental crust was emplaced by the end of the Archean (2.5 billion years ago), citing isotopic values that imply the mantle was strongly depleted in incompatible elements at this time. High Nb/Th and Nb/U ratios of Archean komatiites further supports this hypothesis (Th and U are incompatible and concentrate in oceanic crust during partial melting of the mantle). Another question of consequence of this manner concerns the origin of tonalite-trondhjemite-granodiorite (TTG) intrusive suites, which may comprise up to 80% of remaining Archean crust. TTGs are characterized by strongly fractionated REE patterns, typically attributed to residual garnet and are often thought to be derived by melting oceanic crust as it subducts into the mantle. The Earth was much hotter in the Archean, and capable of this process, unlike the planet's later years. TTGs may not be exclusive to slab melting, however, as recent research of Greenland rocks by Nagel et al., 2013 supports an origin by melting thickened oceanic crust, on the upper plate of an island arc. Adam et al. (2013) reach a similar conclusion for Archean rocks in Canada. Key to the geochemical distinction between the two models is the evidence of water (bound in amphibole) in many TTGs, which would be present beneath an island arc crust, but absent in the dehydrated subducting slab. Upper plate melting is a process that continues today, and is consistent with uniformitarian models of crustal formation.
In an introduction to new research published in Geology, Ali Polat (2013) summarizes some of the controversies related to the timing and processes of the formation of Earth’s continental crust. An important questions is whether crustal growth process were different (non-uniformitarian) during the Archean. Many folk suggest that about half of the Earth’s continental crust was emplaced by the end of the Archean (2.5 billion years ago), citing isotopic values that imply the mantle was strongly depleted in incompatible elements at this time. High Nb/Th and Nb/U ratios of Archean komatiites further supports this hypothesis (Th and U are incompatible and concentrate in oceanic crust during partial melting of the mantle). Another question of consequence of this manner concerns the origin of tonalite-trondhjemite-granodiorite (TTG) intrusive suites, which may comprise up to 80% of remaining Archean crust. TTGs are characterized by strongly fractionated REE patterns, typically attributed to residual garnet and are often thought to be derived by melting oceanic crust as it subducts into the mantle. The Earth was much hotter in the Archean, and capable of this process, unlike the planet's later years. TTGs may not be exclusive to slab melting, however, as recent research of Greenland rocks by Nagel et al., 2013 supports an origin by melting thickened oceanic crust, on the upper plate of an island arc. Adam et al. (2013) reach a similar conclusion for Archean rocks in Canada. Key to the geochemical distinction between the two models is the evidence of water (bound in amphibole) in many TTGs, which would be present beneath an island arc crust, but absent in the dehydrated subducting slab. Upper plate melting is a process that continues today, and is consistent with uniformitarian models of crustal formation.
Figure 1 of Polat (2013) showing hydrous melting of mafic
lower crust as the key producer of Archean TTGs, as opposed to dry melting of
subducted oceanic crust. The image was modified after Davidson and Arculus,
2006)
Source: Polat, A., 2013, Growth of Archean continental crust
in oceanic island arcs, Geology, v. 40, p. 383-384.
Nagel, T.J., Hoffmann, J.E., and Münker, C., 2012,
Generation of Eoarchean tonalitic-trondhjemitic-granodioritics from thickened
mafic arc crust: Geology, v. 40, p. 375–378, doi:10.1130/G32729.1.
Adam, J., Rushmer, T., O’Neil, J., and Francis, D., 2012,
Hadean greenstones from the Nuvvuagittuq fold belt and the origin of the
Earth’s early continental crust: Geology, v. 40, p. 363–366,
doi:10.1130/G32623.1.
Davidson, J.P., and Arculus, R.J., 2006, The significance of
Phanerozoic arc magmatism in generating continental crust, in Brown, M., and
Rushmer, T.,eds., Evolution and Differentiation of the Continental Crust: New
York, Cam- bridge University Press, p. 135–172.
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