Sediment recycling

Eisele J., Sharma M., Galer S. J. G., Blichert-toft J., Devey C. W., and Hofmann A. W. (2002) The role of sediment recycling in EM-1 inferred from Os, Pb, Hf, Nd, Sr isotope and trace element systematics of the Pitcairn hotspot. Earth Planet. Set Lett. 196, 197-212. 

There are, however, two main reasons why this must be a minimum estimate of the amount of sediment recycled into the mantle over geological time. These are, first, reintroduction of CO2 into the atmosphere via arc volcanism, and second, the fact that purely clastic sediments are ignored in this mass balance. CO2 is reintroduced into the atmosphere as a result of decarbona-tion reactions in calc-silicate rocks (the reverse Urey reaction ), requiring further weathering and photosynthesis to remove it. Fluxes associated with these processes over Phanerozoic time have been reviewed by Berner et al. (1983) and Berner (1991), who concluded that such addition of CO2 by (mainly arc) volcanism and its drawdown by silicate weathering have been the major long-term fluxes over this period, and that drawdown has only slightly outstripped... 

What is unknown is the contribution to Archaean crust generation from sediment recycling, and the role, if any, which intracrustal fractionation played. 

The transport balance model of Kramers and Tolstikhin (1997) for the U-Th-Pb isotope system is particularly sensitive to the extent to which crustal material can be recycled into the mantle and offers some constraints on the mechanism of crustal recycling. They concluded that the rate of recycling of crustal material back into the Earth s mantle has increased with time, particularly since 2.0 Ga. We know from Pb-isotope studies that some sediment recycling took place during the Archaean (Halla, 2005), but it would seem as though the proportion was small. 

One of the more robust measures of plume versus arc contributions to the continental crust is the La/Nb ratio, for intraplate magmas typically have La/Nb < 1.0, whereas arc magmas have La/Nb > 1.0. This ratio was used by Rudnick (1995) to estimate that between 10 and 35% of the continents formed from an intraplate component. More recently, using revised crustal Nb concentrations, Barth et al. (2000) calculated that between 5 and 20% of the continental crust is of intraplate origin. However, Plank (2005) urges some caution in the acceptance of these proportions, for the impact of sediment recycling could overinflate the arc contribution to crustal growth. 

Plank, T., 2005. Constraints from Th/La on sediment recycling at subduction zones and the evolution of the continents. J. Petrol., 46, 921-44. 

Wilson, M. and Spencer, E.A., 2001. Carbonatite magmatism constraints on the history of carbonate sediment recycling in the upper mantle since the Archaean. J. Conf. Abs. (EUG XI), 6, 494. 

Provenance Cullers et al. (1988) showed that the immobile elements La and Th are more abundant in felsic than in basic rocks, hut that jhe opposite is true for Sc and Co. et a/., 1988) Ratios such as La/Sc, Th/Sc, Th/Co, La/Co, Ba/Sc and Ba/Co, therefore, in sand sized sediments allow a distinction to be made between a felsic and a mafic source. However, such ratios should be interpreted in terms of provenance with care, for the ratios can be firactionated during weathering and transport and may be strictly valid only for locally derived sediments. Recycled sediments and those from a mixed source are much more difficult to interpret... 

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