Links between mantle metasomatism and lithium isotopes: Evidence from glass-bearing and cryptically metasomatized xenoliths from Mongolia

Abstract

Lithium (Li) abundances and isotope compositions are presented for well-studied spinel peridotite xenoliths from Atsagin-Dush, Mongolia. The xenoliths range from fertile lherzolites, which have LREE-depleted patterns and experienced little or no metasomatism, to refractory harzburgites characterized by strong LREE-enrichments, marked negative anomalies of high-field-strength elements and elevated Zr/Hf; the latter rocks were likely metasomatized by carbonate-rich melts. The majority of the peridotites show small but resolvable differences in Li abundances (1–1.7 ppm) and δ 7Li (+ 3.8 to + 5.7‰), but these differences are not related to any chemical or isotopic indices of metasomatism. Two refractory xenoliths containing abundant melt pockets are significantly enriched in Li (~ 4 ppm) as a result of a metasomatic event that promoted recent incipient melting; those pockets have lower δ 7Li than coexisting olivines. We explain this relationship as a consequence of late-stage diffusive isotope fractionation with preferential 6Li re-distribution into the newly formed melt. Further support for this conclusion is provided by data for a xenolith from another Mongolian locality for which an even larger difference in δ 7Li is found between clinopyroxene and melt pockets, consistent with recent studies of Li isotope fractionation between mantle phases.