In situ oxygen-isotope, major-, and trace-element constraints on the metasomatic modification and crustal origin of a diamondiferous eclogite from Roberts Victor, Kaapvaal Craton

Modelling of the buffering effect of mantle peridotite on CO2-rich and H2O-rich metasomatic fluids at temperatures within the diamond stability field indicates that the likelihood of a metasomatic fluid with exotic oxygen isotopic composition arriving at a mantle eclogite body with its isotopic composition unmodified, after percolative flow through dominantly peridotitic mantle at great depth, is very low. As we find no evidence of metasomatically induced garnet oxygen isotope variations in the studied diamondiferous eclogite xenolith we conclude that the most likely origin for the elevated garnet δ18O-values is via inheritance from a crustal protolith altered at relatively low temperatures. These results have broader relevance and support the hypothesis of a low-pressure protolith for mantle eclogite xenoliths, demonstrating the robust nature of garnet oxygen isotope compositions - even in diamond-bearing eclogites.