Direct assessment of mantle boron and lithium contents and distribution by SIMS analyses of peridotite minerals

The importance of Li and B geochemistry has long been recognised, owing specifically to their characteristic behaviours during processes involving fluid phases. However, the lack of a set of validated reference data for the "normal" Earth mantle has hampered the development of models for Li and B metasomatic effects on mantle rocks. In particular, the concentration of B in the mantle is still a matter of debate. An estimate of 0.1 ppm B seems to be consistent as a source for non-arc basalts, but to date such data have not been directly confirmed. Li and B literature contents for peridotites are derived from samples whose non-metasomatized character has not been established for both elements, due to the aforementioned lack of a comprehensive metasomatism model for Li and B.

We have looked at two groups of mantle rocks, with and without clear compositional evidence that they are metasomatized. The latter rocks provide the best constraints on "normal" mantle B and Li contents. We propose a diagnostic diagram based on (Ce/B) vs. (Li/Yb) as measured by SIMS in peridotite clinopyroxenes, which is useful in identifying metasomatized samples. After discovering samples with no metasomatic alteration, which are considered representative of the "normal" mantle, we derived for each mantle mineral phase (ol, opx, cpx and sp) the Mg#, Li and B partial-melting evolution trends. Additionally, considering that the "normal" mantle rocks have evolved through partial melting only, we assessed Li and B contents in the parental mantle of our samples, and assumed that the calculated values (1.6-1.8 ppm Li and 0.07-0.10 ppm B) are representative of the contents in MORB mantle sources. These new data are consistent with current melting models of fertile peridotites.