Subducted oceanic crust as diamond hosts revealed by garnets of mantle xenoliths from Nyurbinskaya, Siberia

The ~ 380 Ma Nyurbinskaya kimberlite pipe, Yakutia, Siberia, sampled a highly-diamondiferous and unusual mantle xenolith population dominated by eclogites. New in-situ major- and trace-element data for garnets previously analyzed for oxygen isotope compositions show that Group A eclogitic garnets have Mg# > 68 and are LREE-depleted. Group B and Group C eclogitic garnets cover a range of Mg#, and are each divided into two types based on their trace-element characteristics. Type B1 and C1 eclogitic garnets are dominant and are LREE-depleted. Less common Type B2 and C2 garnets generally have Mg# > 60 and convex-upward REE-profiles. Harzburgitic garnets are a minor component of the Nyurbinskaya xenolith suite, and have high Mg# (~ 84), high Cr contents (~ 11 wt.% Cr2O3), and sinusoidal REE-patterns. Group A, Type B1, and C1 eclogitic garnets define a broad negative correlation between Mg# and Yb abundances consistent with a shallow origin as basaltic and gabbroic portions of oceanic crust. Harzburgitic, Type B2, and C2 eclogitic garnets have trace-element characteristics indicative of interaction with a C–O–H–N–S-rich fluid in lithospheric environments. These results provide clear evidence for the presence of subducted crustal materials in the Siberian mantle lithosphere and support models of craton formation by subduction zone stacking.

Research Highlights►The Nyurbinskaya diamond pipe is linked with an eclogite-rich xenolith population. ►All Nyurbinskaya eclogites may be of low-pressure origin. ►This region may have experienced metasomatism by C–O–H–N–S-rich fluids.