The problem of compositional heterogeneity in the Archaean upper mantle is evaluated from the existing data of isotopic compositions (Sr, Pb and Nd), trace element abundances (rare earth elements (r.e.e.)) and some critical major element chemistry in Archaean igneous rocks. To assess a large-scale mantle heterogeneity, we have emphasized the chemical characteristics of undoubtedly mantle-derived komatiites. The initial isotopic compositions of Sr and Pb (
I
Sr
and
I
Pb
) in Archaean igneous rocks show a significant variation, suggesting that a long-term heterogeneity in Rb/Sr and U /Pb ratios might have existed in their mantle sources. In contrast, the available
I
Nd
values show little variation from the expected evolution in a chondritic uniform reservoir. These contrasting results, i.e. the variable
I
Sr
and
I
Pb
values as against the uniform
I
Nd
values, are in part attributed to the fact that mobile-refractory pairs in the Rb/Sr and the U /Pb systems but only the refractory-refractory pair in the Sm/Nd system have been involved in geochemical differentiation. The r.e.e. distributions in komatiitic rocks are not uniform. Many show strong depletion in light r.e.e. The degree of light r.e.e. depletion in these komatiitic rocks is comparable with that found in present mid-ocean ridge basalts. Regarding the major element chemistry, we have demonstrated that the CaO / Al
2
O
3
ratios are significantly different in the komatiitic suites of two adjacent cratons (Kaapvaal and Rhodesia) in southern Africa. Both trace and major elements data suggest that the Archaean upper mantle was heterogeneous, but whether the heterogeneity truly represents a long-term phenomenon remains to be resolved by more isotopic analyses.
An assessment of upper mantle heterogeneity based on abyssal peridotite isotopic compositions
