4.7 Article

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

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GEOCHIMICA ET COSMOCHIMICA ACTA
卷 347, 期 -, 页码 16-27

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PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.gca.2023.02.016

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Eclogitic diamond; Carbon isotope; Nitrogen isotope; Oxygen isotope; Slab-mantle interaction

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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.
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.

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