Hf isotopic measurements on Barberton komatiites: effects of incomplete sample dissolution and importance for primary and secondary magmatic signatures

Komatiites from a single lava flow, Carlʹs Flow, in the Schapenburg Greenstone Remnant, near Barberton in South Africa, yielded very different Hf isotope compositions, depending on the manner in which the samples were treated analytically. Data from samples dissolved in Savillex beakers define a statistically significant (MSWD=0.77) isochron with an age of 3.4±0.2 Ga, which is identical within error to the probable eruption age, and an initial εHf of −1.3. Analyses of samples dissolved in bombs, on the other hand, plot on a linear array which, if interpreted as an isochron, gives an age of 2.9 Ga. Their initial 176Hf/177Hf, calculated using the probable eruption age of 3.45 Ga, are positive and variable, ranging from εHf of +1.3 to +6.3. Hf contents are much lower and Lu/Hf ratios systematically higher in the Savillex beaker-dissolved batch of komatiites, indicating that a Hf-rich phase (zircon or baddeleyite?) remained undissolved in these samples. The negative εHf of the Savillex beaker-dissolved samples is attributed to interaction with crust-derived fluids that affected the komatiite soon after its eruption. The anomalously low isochron age of the bomb-dissolved samples is due to the presence of a single olivine cumulate sample that appears to have lost Hf during post-emplacement alteration. Without this sample, initial εHf values cover a much narrower range between +4.3 and +6.3, which is believed to be that of the mantle source of the komatiites. The combination of highly positive εHf and lower-than-chondritic Lu/Hf indicates that the mantle source of the Carlʹs Flow was garnet-enriched for a substantial period of time prior to komatiite formation and that garnet was retained in the source during the melting event that eventually produced the komatiite magma.