LITHIUM ISOTOPE VARIATIONS IN THE PALABORA CARBONATITE COMPLEX: A TALE OF TWO HUMITES

The study of lithium isotopes can provide a useful tool for exploring/understanding processes in crustal environments. Many studies of Li isotopes in terrestrial upper mantle samples have been less informative, suggesting that the primitive mantle is quite homogeneous (d⁷Li ~+4‰). However, because of the large mass difference between the two lithium isotopes, kinetic fractionation of ⁶Li from ⁷Li has been observed experimentally and in natural samples, including a variety of materials from the upper mantle. Thus, depending on the timing, processes of overprinting (metasomatism) may be preserved. Perhaps more importantly, the study of carbonatites (less-commonly examined for Li isotopes than mantle-derived silicate peridotites) also has the potential to reveal other reservoirs of Li isotopes in the upper mantle. We present in-situ Li isotope analyses of humites (F, OH-bearing minerals with structural similarities to olivine) from the Precambrian Palabora Carbonatite Complex using secondary ion mass spectrometry (SIMS and NanoSIMS) that reveal (at the minimum) two generations of humite formation. Mg- and F-rich (more refractory) regions show d⁷Li from -2 to +3‰, while more Fe-rich, F-poor regions tend to show heavier ratios (d⁷Li = +6 to +11‰). The two types of humites are closely intertwined, with microanalyses (even NanoSIMS!) often sampling both populations, resulting in analytical precisions far worse than obtained on homogeneous standards. Because the refractory humite dominates the rims and patches within the humite grains, it appears that a metasomatizing agent (with d⁷Li close to normal mantle, given the analytical uncertainties) is overprinting a more positive, original isotopic signature, probably with minimal kinetic effects because of the (weak) positive correlation between d⁷Li and Li contents. The isotopically heavy nature of the earlier carbonatite magma responsible for initial humite growth may point to a reservoir heavily influenced by the addition of subduction-derived fluids or a previously unidentified mantle reservoir.