Mantle fluxes

Sano (1986) and Sano et al. (1986) found He isotopic variations with depth, 3He/4He decreasing toward the surface, in two natural gas wells in northern Taiwan. This relation is interpreted as a mantle flux to the bottom of the well, progressively diluted by radiogenic He released from the surrounding sediment as the gas migrates upward. With a simple mixing model, they obtained mantle He fluxes close to the mean oceanic value (Table 6.4), but the situation in a gas well is rather complicated, and it remains to be seen whether or not the coincidence with the oceanic value is accidental. 

Davies G. F. (1998) Topography a robust constraint on mantle fluxes. Chem. Geol. 145, 479-489. 

FIGURE 3.21 Mantle fluxes from Davies (1998) and Schubert et al. (2001). 

The paradox of modern mantle fluxes One of the most profound problems in understanding the origin of the Earth s continental crust is the discrepancy between the composition of the continental crust, which is andesitic, and the composition of the present-day flux across the Moho, which is basaltic. Two types of solution have been proposed. On the one hand it has been argued that the composition of the continental crust has been modified after it formed in order to adjust its composition from a basaltic protolith to andesite. Alternatively, the balance of fluxes across the Moho has changed over time so that modern processes provide an incomplete explanation for the time-averaged composition of the continental crust. These two models will be examined in turn. 

Here we explore the idea that the discrepancy between the average composition of the continental crust, which is andesitic, and the modern flux from the mantle to the continental crust, which is basaltic, can be explained in terms of a change in the composition of the crust-mantle flux over time. The hypothesis adopted here is that Archaean crust had a TI G composition, formed from a TTG melt, and was not fractionated into lower basaltic and upper felsic components. Modern crust on the other hand has a basaltic bulk composition but has been modified to andesitic through the fractionation and the removal of a mafic lower crustal component (Rudnick Taylor, 1987). Evidence for the absence of a mafic lower crust in the Archaean comes from Archaean lower crust preserved as granulite terrains, such as the Lewisian (Rollinson Tarney, 2005), the Limpopo Belt (Berger Rollinson, 1997), and the lower crust of the Kaapvaal Craton... 

The mantle fluxes that can be most easily identified are those of He. Also, concentrations in the mantle source regions from which He is lost through melting can be obtained where the source volume involved can be quantified. Estimated fluxes are given in Table 3. 

Davies GF (1995) Punctuated tectonic evolution of the earth. Earth Planet Sci Lett 136 363-379 Davies GF (1998) Topography a robust constraint on mantle fluxes. Chem Geol 145 479-489 Davies GF (1999) Geophysically constrained mantle mass flows and the Ar-40 budget a degassed lower mantle Earth Planet Sci Lett 166 149-162 Davies GF, Richards MA (1992) Mantle convection. J Geol 100 151-206... 

Coupled Sulfur and Carbon Cycles and the Net Mantle Flux... 

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