Hf isotope compositions of komatiites

Abstract

Many of the oldest ∼3.5 Ga komatiites have low Al2O3/TiO2 and depleted HREE, features that are attributed to garnet fractionation at some stage during their formation. It is not known, however, whether this fractionation took place at the time of melting, or well before, perhaps during the crystallization of an early magma ocean. We report here the Hf isotope compositions of four Al-depleted komatiites from the 3.45 Ga Barberton Greenstone Belt in South Africa and six Al-undepleted komatiites from 2.7 to 1.9 Ga belts in Canada and Zimbabwe. We also measured the Hf isotope compositions of five tholeiites from Barberton and two tholeiites from the 1.9 Ga Canadian Belt, to provide information about the composition of ambient Precambrian mantle. All the komatiites have high initial 176Hf/177Hf ratios. All initial ɛHf values are positive, in the range of +2.6 to +7.8. Almost this entire range is also present in the 3.45 Ga Barberton komatiites (+2.6 to +7.3). ɛHf(T) correlates with ɛNd(T) and defines a crude linear array that coincides with compositions of both modern oceanic basalts and of Phanerozoic to Archean juvenile crustal rocks. The composition of Bulk Silicate Earth [J. Blichert-Toft, F. Albarède, Earth Planet. Sci. Lett. 148 (1997) 243–258] plots below this array, at slightly lower ɛHf. To explain the positive initial ɛHf values of 3.45 Ga Barberton komatiites, the source must have had high Lu/Hf. This implies a source enriched in garnet, a result contrary to earlier models in which the low Al2O3/TiO2 and low Lu/Hf of this type of komatiite was thought to be inherited directly from mantle depleted in garnet. The low Al2O3/TiO2 and low Lu/Hf are features of the primary komatiite magmas, not of their mantle source, and result from garnet fractionation during komatiite formation. To explain the bias to high initial ɛHf values of the komatiites and other terrestrial rocks, we consider two alternatives: (a) a reservoir with low Lu/Hf formed very early in Earth history; (b) accepted values for the Hf isotopic composition of Bulk Silicate Earth are inappropriate and the actual composition has higher 176Hf/177Hf and lower 176Lu/177Hf. A problem with the latter interpretation is that the proposed composition lies off the achondrite meteorite isochron, which implies that the Earth has a different age, or accreted from material with composition different from that of meteorites.