Continental crust origin

O, H, C, S, and N isotope compositions of mantle-derived rocks are substantially more variable than expected from the small fractionations at high temperatures. The most plausible process that may result in variable isotope ratios in the mantle is the input of subducted oceanic crust, and less frequent of continental crust, into some portions of the mantle. Because different parts of subducted slabs have different isotopic compositions, the released fluids may also differ in the O, H, C, and S isotope composition. In this context, the process of mantle metasomatism is of special significance. Metasomatic fluids rich in Fe +, Ti, K, TREE, P, and other large ion lithophile (LIE) elements tend to react with peridotite mantle and form secondary micas, amphiboles and other accessory minerals. The origin of metasomatic fluids is likely to be either (1) exsolved fluids from an ascending magma or (2) fluids or melts derived from subducted, hydrothermally altered crust and its overlying sediments. 

In order to trace the migration of basalt-derived REE in the salt, REE distribution patterns (Fig. 7) and Nd isotopic compositions (Fig. 8) have been determined in a salt horizon adjacent to a basalt dyke (Fig. 2). The flat REE distribution patterns and the almost basaltic Nd isotopic composition of the salt samples collected at the basalt-salt contact point to a basaltic origin of the REE for this sample. With increasing distance from the contact, the patterns are more and more depleted in Ce, Pr, Nd, Sm, and Eu and the Nd isotopic compositions are slightly shifted towards lower eNd values, which, however, still remain above values typical for continental crust or Permian seawater (Stille et al. 1996, and citations therein). This evolution of the REE distribution patterns and the Nd isotopic compositions could basically be due to mixing between a basalt and a salt end member or, alternatively, it could have been fractionation of the REE during migration in the salt that modified the REE patterns. 

Sims, K.W.W., Newsom, H.E. and Gladney, E.S. (1990) Chemical fractionation during formation of the Earth s core and continental crust clues from As, Sb, W, and Mo, in Origin of the Earth (eds H.E. Newson and J.H. Jones), Oxford University Press, New York Lunar and Planetary Institute, Houston, pp. 291-317. 

These basalts represent the oceanic subclass of so-called intraplate basalts, which also include continental varieties of flood and rift basalts. They will be collectively referred to as OIE, even though many of them are not found on actual oceanic islands either because they never rose above sea level or because they were formed on islands that have sunk below sea level. Continental and island arc basalts will not be discussed here, because at least some of them have clearly been contaminated by continental crust. Others may or may not originate in, or have been contaminated by, the subcontinental lithosphere. For this reason, they are not considered in the present chapter, which is concerned primarily with the chemistry of the sublitho-spheric mantle. 

Noll P. D., Jr., Newsom H. E., and Leeman W. P. (1993) Lead and tin in subduction related lavas fluid mobility, cross-arc variations and the origin of Pb enriments in the continental crust. EOS, Trans., AGU Suppl. 74, 674. 

Bohlen S. R. and Mezger K. (1989) Origin of granulite terranes and the formation of the lowermost continental crust. 

Gallet S., Jahn B.-M., van Vliet Lanoe B., Dia A., and Rossello E. (1998) Loess geochemistry and its implications for particle origin and composition of the upper continental crust. Earth Planet. Sci. Lett. 156, 157-172. 

Mattie P. D., Condie K. C., Selverstone J., and Kyle P. R. (1997) Origin of the continental crust in the Colorado Plateau geochemical evidence from mafic xenoliths from the Navajo volcanic field, southwestern USA. Geochim. Cosmochim. Acta 61(10), 2007-2021. 

Condie K. C. (1999) Mafic crustal xenoliths and the origin of the lower continental crust. Lithos 46, 95-101. 

The balance of evidence now favors the hypothesis of in situ UHP metamorphism. The shift towards acceptance of an in situ origin has been accelerated by the development of physically plausible models of the subduction and exhumation of continental crust. The current focus of debate has, in fact, shifted towards evaluation of evidence of super ultrahigh pressure metamorphism. These topics will be discussed below. 

The abundant crust that was produced in the 1.9-1.7 Ga interval has been the subject of numerous studies, and the evolution of that work illustrates important elements in study of the continental crust. Following the demonstration of a juvenile origin for the —1.8 Ga crustal assemblage in Colorado (DePaolo, 1981), there followed a period in which neodymium isotopic data were gathered for numerous terrains in the northern continents (Figure 4). In North America, studies by Nelson and DePaolo (1984, 1985), Bennett and DePaolo (1987),... 

Patchett P. J. and Bridgwater D. (1984) Origin of continental crust of 1.9-1.7Ga age defined by Nd isotopes in the Ketilidian terrain of South Greenland. Contrib. Mineral. Petrol. 87, 311-318. 

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