Light oxygen isotopes in mantle-derived magmas reflect assimilation of sub-continental lithospheric mantle material.

Oxygen isotope ratios in mantle-derived magmas that differ from typical mantle values are generally attributed to crustal contamination, deeply subducted crustal material in the mantle source or primordial heterogeneities. Here we provide an alternative view for the origin of light oxygen-isotope signatures in mantle-derived magmas using kimberlites, carbonate-rich magmas that assimilate mantle debris during ascent. Olivine grains in kimberlites are commonly zoned between a mantle-derived core and a magmatic rim, thus constraining the compositions of both mantle wall-rocks and melt phase. Secondary ion mass spectrometry (SIMS) analyses of olivine in worldwide kimberlites show a remarkable correlation between mean oxygen-isotope compositions of cores and rims from mantle-like ¹⁸O/¹⁶O to lower 'crustal' values. This observation indicates that kimberlites entraining low-¹⁸O/¹⁶O olivine xenocrysts are modified by assimilation of low-¹⁸O/¹⁶O sub-continental lithospheric mantle material. Interaction with geochemically-enriched domains of the sub-continental lithospheric mantle can therefore be an important source of apparently 'crustal' signatures in mantle-derived magmas. Isotopically light oxygen in mantle derived magmas is generally attributed to the occurrence of deeply subducted crustal material in the mantle. New analyses of olivine in kimberlites worldwide suggest that assimilation of lithospheric mantle during magma ascent is the source of isotopically light oxygen in these, and, potentially other continental magmas. [ABSTRACT FROM AUTHOR]