Eclogites subduction slabs

Orogen have a continental affinity and could not have been produced from a subducted Tethyan oceanic slab. The very low 8Nd(0 values further require the protoliths of these eclogites to have formed long before the Triassic collision, probably in mid-Proterozoic times. It appears that, in... 

Mass balance calculations suggest that this eclogitic reservoir is of considerable size. Kamber and Collerson (2000) estimated the mass of deeply subducted ocean crust over 4.3 Ga to be about 1.4 X 1026 g, that is, about 3% of the mass of the silicate Earth, greater than the mass of the continental crust, and equal to about 20% of the mass of oceanic crust subducted over time. Where this reservoir is located is not precisely known. However, Kamber and Collerson (2000) argued that the Nb and Ta contents of MORB worldwide are very well correlated, implying that, for these elements, the depleted mantle is a well-mixed reservoir, and unlikely therefore to contain high Nb/Ta slabs. For this reason they propose that the eclogitic slab reservoir is located in the lower mantle. 

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