DO OCEANIC LIPS RECORD THE GROWTH OF CONTINENTAL CRUST?

Basalts from oceanic LIPS exhibit compositional changes near the end of the Archean. The best documented are in rare earth and high field strength element ratios that fall into two groups: 1) those that record a decrease in the temperature of mantle plumes such as increases in La/Yb, Zr/Hf, Zr/Y, La/Sm, Gd/Yb and Nb/Yb; and 2) those that record the extraction of continental crust from the mantle such as increases in Nb/Th, Nb/La, Nb/Nb* and Nb/Ta. Changes are apparent in both mean and maximum values of the ratios. The element ratio changes occur during a probable global magmatic and plate tectonic slowdown between about 2.4 and 2.2 Ga (Condie et al. 2009) and emphasize the uniqueness of this period of time during evolution of the crust/mantle system. But why should mantle plumes track growth of continental crust? Perhaps depleted slabs that accumulate in mantle plume sources (D” layer) beginning around 2.2 Ga carry the depleted restite signature of rapid continental growth in the late Archean. A possible scenario to explain these geochemical changes involves three stages: 1) modern plate tectonics becomes widespread in the late Archean and lithospheric slabs collect in D” as one or more supercratons grow; the arrival of cool slabs promotes an increase in core heat flux and results in a period of enhanced plume production in the late Archean; 2) plate tectonics slows at 2.4-2.2 Ga, thus slab accumulation in D” slows resulting in a decrease in plume production rate; plume sources have been significantly cooled by enhanced plume production in the late Archean (also supported by a decrease in komatiites and komatiite Mg # after 2.2 Ga); and 3) after 2.2 Ga, plate tectonics is permanently established and the mantle continues to cool in a more predictable manner.