Abstract

A careful consideration of the heterogeneities in the trace-element concentrations and lead and strontium isotope ratios for primary alkali basalt and nephelinite shows that the isotopic heterogeneities reflect real geochemical heterogeneities of their mantle source that have existed variously from about 1,000 to 3,000 m.y. ago. It is suggested that at present there is not mantle-wide convection and that the mantle source for alkali basalt is deeper than the presently convecting mantle. During Archean time, mantle convection was much deeper than at the present time, and this deeper convection led to hot-line tectonics that may have determined the tectonic style found in Archean greenstone belts.

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