Continental crust recycling

Since hthium and boron isotope fractionations mainly occur during low temperature processes, Li and B isotopes may provide a robust tracer of surface material that is recycled to the mantle (Elhott et al. 2004). Heterogeneous distribution of subducted oceanic and continental crust in the mantle will thus result in variations in Li and B isotope ratios. Furthermore, dehydration processes active in subducdon zones appear to be of crucial importance in the control of Li and B isotope composition of different parts of the mantle. For the upper mantle as a whole Jeffcoate et al. (2007) gave an estimated 8 Li-value of 3.5%o. 

Differentiation of other terrestrial planets must have varied in important ways from that of the Earth, because of differences in chemistry and conditions. For example, in Chapter 13, we learned that the crusts of the Moon and Mars are anorthosite and basalt, respectively - both very different from the crust of the Earth. N either has experienced recycling of crust back into the mantle, because of the absence of plate tectonics, and neither has sufficient water to help drive repeated melting events that produced the incompatible-element-rich continental crust (Taylor and McLennan, 1995). The mantles of the Moon and Mars are compositionally different from that of the Earth, although all are ultramafic. Except for these bodies, our understanding of planetary differentiation is rather unconstrained and details are speculative. 

Figure 10.1 is a generalized model of the rock cycle. Both the sediments and the continental crust are recycled the former by processes of weathering, erosion, transportation, deposition, burial, uplift, and re-erosion and the latter by resetting... 

It is likely that the ancient crust of Mars is more mafic than the Earth s continental crust. Pervasive andesite may signal crustal fractionation, but the identity and significance of andesitic rocks is disputed. The martian crust is relatively more voluminous than the Earth s cmst, perhaps because it is not recycled. The cmst is characterized by high concentrations of incompatible hthophile elements, but fractionations are not as extreme as in terrestrial continental cmst, which has experienced repeated partial melting events over a protracted geologic history. 

McLennan S. M. (1988) Recycling of the continental crust. Pure Appl. Geophys. 128, 683-724. 

Sm/Nd requires mixing with less depleted material (lower Sm/Nd) at dilferent time periods, which could be either recycled continental crust or more primitive mantle. 

Green M. G., Sylvester P. I., and Buick R. (2000) Growth and recycling of early Archaean continental crust geochemical evidence from the Coonterunah and Warrawoona Groups, Pilbara Craton, Australia. Tectonophysics 322, 69-88. 

If delamination of dense lower crustal rocks has been essential to continental genesis, and the delaminated rocks constitute —20-40% of the mass of the continents, one might expect to see evidence for this component in magmas derived from the convecting mantle. However, continental crust comprises only —0.5% of the sihcate Earth, so that—even if it represents 40% of the original crustal mass— recycled lower cmst might comprise a very small fraction of the convecting mantle. 

Arndt N. T. and Goldstein S. L. (1989) An open boundary between lower continental crust and mantle its role in crust formation and crustal recycling. Tectonophysics 161, 201-212. 

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