Small-scale Hf isotopic variability in the Peninsula pluton (South Africa): the processes that control inheritance of source 176Hf/177Hf diversity in S-type granites

The Hf-176/Hf-177 composition of inherited and magmatic zircon in the 538 Ma S-type Peninsula pluton (South Africa) has been determined at different scales. In the smallest rock samples investigated (<0.5 dm(3)), as well as within individual thin sections, magmatic zircon crystals exhibit the same wide range in epsilon(Hf(538)) as the pluton (8 epsilon units). In addition, across a significant range of bulk-rock compositions, both the range and average of the magmatic zircon Hf isotopic composition do not vary significantly with compositional parameters that are expected to scale with the proportion of mantle-derived magma addition (e.g., Mg# and Ca). At all scales, the epsilon(Hf) variability in the magmatic zircon fraction matches well with that portrayed by the time-evolved inherited zircon population [i.e., with the epsilon(Hf(538)) range of the inherited zircon cores]. This evidence suggests that the epsilon(Hf) heterogeneity of magmatic zircon is directly inherited from the source. However, the analysis of zircon core-rim pairs reveals that the Hf-176/Hf-177 composition of the inherited crystals does not directly transfer to their magmatic overgrowths. Small-scale modeling of zircon dissolution and re-precipitation in a static magma generates sub-mm melt domains having variable Zr content and Hf isotope composition. The composition of these domains is controlled by the size and isotope composition of the nearest dissolving zircon crystals and the cooling rate of the magma. These results suggest that in magma systems with a substantial inherited zircon load, zircon crystals within the same rock should record variable Hf-176/Hf-177 in the magmatic zircon fraction.