The role of phosphorus in the incorporation of lithium into mantle olivine and potential implications for Li isotopic systematics

Guil Mallmann 1 , Hugh St.C. O'Neill 1 , Stephan Klemme 2

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 School of Geosciences, University of Edinburgh, Edinburgh, EH9 3JW, UK

The light elements Li, Be and B and their isotopes have the potential to constrain the extent to which subduction-related processes and recycled material contribute to the chemical heterogeneity of the Earth's mantle. Li isotopes, in particular, are being the focus of much work recently. However, the mechanism of Li incorporation into mantle minerals and rates of diffusion are still poorly known. We have found that olivine crystals from some peridotite xenoliths from the Anakies locality (SE Australia) have highly variable phosphorus (P) concentrations (50-250ppm), while all the other major and trace elements appear to be constant in composition. EMP X-ray maps revealed peculiar types of P zoning in these crystals, which we have interpreted as resulting from metasomatism followed by deformation at sub-solidus conditions; P heterogeneities are likely preserved due to sluggish diffusion of this element into olivine. The P contents were also found to covariate very well with Li (Fig. 1), suggesting that a coupled substitution mechanism similar to IV Mg 2+ + IV Si 4+ « VI Li + + IV P 5+ is responsible for the incorporation of Li into mantle olivine. Our findings have potential implications for the homogenisation of Li and fractionation of Li isotopes in mantle samples. These implications need to be further addressed.

Figure 1. Covariation plot between P and Li in olivine crystals from two Anakies peridotite xenoliths. Measurements were done with LA-ICP-MS in individual olivine crystals.