NB-TH-LA IN KOMATIITES AND BASALTS - CONSTRAINTS ON KOMATIITE PETROGENESIS AND MANTLE EVOLUTION

New spark-source mass spectrometric analyses of Nb, Th, REE and other trace elements in Archaean to Tertiary komatities and basalts were undertaken to test the model of Hofmann et al. [1] of secular variation of the composition of the upper mantle. Most 3.4 Ga komatites and basalts have Nb/Th between 7 and 9, values that straddle the primitive mantle ratio of approximately 8. Several 2.7 Ga komatiites also have Nb/Th = 7-9, but most samples of this age have higher Nb/Th, in the range 10-15. Cretaceous-Tertiary komatiites and basalts from Gorgona Island (Colombia) have Nb/Th between 11 and 24, values that approach those of modern oceanic basalts (approximately 15-34). Although these results generally support the model of Hofmann et al., there are several complicating factors: (1) most of the Cretaceous-Tertiary Gorgona komatiites have Nb/Th ratios little higher than those of 2.7 Ga komatiites and primitive mantle, which suggests that low Nb/Th may be a peculiarity of komatiites and not a feature of the Archaean mantle; (2) many Archaean komatiites are depleted in both Nb and Th relative to the REE, a feature that is inconsistent with their derivation from primitive mantle. We speculate that komatiites come from a source that evolved independently from normal upper mantle, and that the depletion of Nb and Th resulted from fractionation of an unknown phase during the deep melting. Certain tholeiitic basalts do not show unusual Nb-Th-La fractionation but nonetheless show a secular increase in Nb/Th. This variation may indicate a change in upper mantle compositions resulting from progressive withdrawal of continental crust during the past 3 Ga.