LI ABUNDANCES IN INCLUSIONS IN DIAMONDS FROM THE UPPER AND LOWER MANTLE

Abstract

Lithium abundances in inclusions in diamonds of peridotitic, websteritic, eclogitic (including garnets with majoritic component) and lower mantle paragenesis were determined with the ion microprobe. The partitioning of Li both between olivine and clinopyroxene and clinopyroxene and garnet appears to be pressure-dependent in peridotites. More importantly, garnets containing a majorite component show a dramatic increase in Li solubility so that even for mildly majoritic garnets Dcpx/majorite is smaller than one. Lower mantle ferropericlase is a sink for lithium, containing several hundred times more Li than coexisting Mg-perovskite. Lithium decreases in olivine inclusions with increasing forsterite content, which we interpret as reflecting a depletion event in the protoliths of the inclusions. We also infer that lithium was not re-enriched in the silicate phases by a metasomatic agent during diamond formation unlike other trace elements like REE and HFSE. Depletion on partial melting and no re-enrichment also seems to hold for the eclogitic suite of inclusions where cpx's (and deduced bulk composition) have similar Li abundances as in the peridotitic suite and are significantly lower than in crustal eclogites. For the lower mantle bulk, lithium estimates give values four to five times higher than primitive mantle. This may relate either to local enrichment through fluid/melt metasomatism similarly as for spinel lherzolites or high Li abundances in lower mantle diamonds sources are inherited from crustal protoliths.