Nature of slab-mantle interactions recorded by coupled δ13C-δ15N-δ18O signatures and elemental compositions of Koidu diamonds and their inclusions

Eclogitic diamond formation can be associated with interactions between subducted slabs and ambient mantle. To gain a better understanding of the extent of chemical exchange between slabs and ambient mantle and the mechanisms of coeval diamond formation, we examined 16 diamonds with eclogitic gar-net inclusions from the Koidu kimberlite complex in Sierra Leone (West African Craton). We analyzed the 16 garnet inclusions for major element, trace element and oxygen isotope (delta O-18) compositions and their host diamonds for carbon (delta C-13) and nitrogen (delta N-15) isotope compositions. The garnet inclusions have delta O-18 values ranging from +5.4 to +12.1 parts per thousand (median = +11.3 parts per thousand) and all but two have delta O-18 values >=+9.9 parts per thousand. Such high delta O-18 values indicate a link to protoliths that had undergone exten-sive low-temperature alteration by seawater, which occurs in the uppermost basaltic layer of oceanic crust, prior to subduction. Diamonds hosting the high delta O-18 garnets have a crustal delta C-13 signature (-29.6 to-19.4 parts per thousand) paired with delta N-15 values (-5.6 to +1.3 parts per thousand with one outlier at +9.9 parts per thousand) implying typically only minor N-15 enrichment. This apparent decoupling of delta O-18 and delta C-13 from delta N-15 indicates diamond for-mation in an eclogitic substrate where garnet oxygen isotope compositions are inherited from altered low-pressure protoliths, diamond carbon is principally derived from biogenic carbonate and organic mat -ter, and diamond nitrogen is variably added by an external, mantle-derived fluid that prompted diamond formation. Of the two remaining garnet inclusions, one has mantle-like delta O-18 (+5.4 parts per thousand) and a positive Eu anomaly, suggesting derivation from gabbroic protoliths originally located deep in the oceanic crust where significant alteration by seawater did not occur. Based on mantle-like delta C-13 (-4.7 parts per thousand) and delta N-15 (-6.9 parts per thousand), the associated diamond formed from mantle-derived fluids/melts. Relative to the other Koidu garnet inclusions, the final garnet has a small majorite component (formation pressure similar to 8 GPa), high Mg# (79.0), elevated Cr# (0.90), and low delta O-18 (+6.3 parts per thousand), suggesting encapsulation during infiltration of slab-derived melts into surrounding asthenospheric peridotite, associated with a high degree of chemical exchange between slab-and mantle-derived components. (c) 2023 Elsevier Ltd. All rights reserved.

Automated summary

Through examining 16 diamonds with eclogitic garnet inclusions from the Koidu kimberlite complex in Sierra Leone, it was found that the garnet inclusions have high oxygen isotope values, indicating a link to protoliths that had undergone extensive low-temperature alteration by seawater. The carbon and nitrogen isotope compositions of the host diamonds suggest that diamond formation is associated with an external, mantle-derived fluid.