The origin of highly radiogenic Hf isotope compositions in 3.33Â Ga Commondale komatiite lavas (South Africa)

Drill core samples from the 3.33 Ga high-alumina-type Commondale komatiites from the southern Kaapvaal craton (South Africa) were analysed for major, trace elements and Hf- isotope compositions in order to place possible constraints on their mantle sources and the process and timing of mantle depletion. The initial εHf isotope values range from + 6.5 to + 9.1 indicating a strongly depleted mantle source. This contrasts with previously published initial εNd values on the same geologic unit that reflect only minor depleted mantle sources of + 2. Trace element modelling and modelling of the Hf-Nd isotope composition of the mantle source indicate that the strongly incompatible element-depleted trace element patterns may be explained by a mantle source that was depleted to high-degrees (ca. 40%) in the garnet stability field prior to high- degree melting of the restitic mantle in the spinel stability field. The depletion in the garnet-stability field may have been related to a plume event that produced the low-alumina Barberton-type komatiites. However, the decoupled Hf-Nd isotope compositions cannot be explained by such a mechanism. We discuss several possible scenarios that may explain the trace element distribution as well as the isotope composition of the Commondale komatiites. Based on trace and isotope modelling, we consider three different models (1) the decoupled Hf-Nd isotope compositions resulted from a depleted mantle source overprint by melt-depleted garnet-pyroxenite components that originated from older crustal differentiation events and were delaminated into the mantle; (2) the decoupling resulted from subduction fluids that overprinted a highly depleted mantle source and added Th, Nd and other light-rare-Earth elements (LREE) shortly before eruption of the komatiites and triggered the high degree of melting; (3) a possible origin from a mantle source that experienced fractionation processes in an early magma ocean, followed by at least three mantle depletion events. The trace element and Hf-Nd isotope composition of the Commondale komatiites reveal that parts of the Archean mantle must have been highly heterogeneous due to komatiite extraction and possible delamination processes of lower crustal restites.