C-13-depleted diamonds in Jericho eclogites: diamond formation from ancient subducted organic matter

The high-MgO, diamond-bearing eclogites from the Jericho kimberlite in the northern Slave craton are unique in their high Mg# (82-87), high incompatible element contents, radiogenic Sr isotope ratios, and abundant diamonds (up to 20 modal %) that contain lower-Mg garnet inclusions (Mg#~54). As first noted by [1], these diamonds have the lightest carbon isotope compositions ever reported for diamonds (δ¹³C = -37 to –41‰). Although upcoming SIMS δ13C analyses will help delineate the mechanism of diamond formation, we note that δ13C values of ca. –40‰ cannot reasonably be explained by Rayleigh-style fractional crystallization of diamond at mantle temperatures (T>800°C) from a parental fluid isotopically heavier than –35‰. Thus, these Jericho diamonds require the existence of a strongly 13C-depleted carbon reservoir beneath the northern Slave craton.

The #13C of organic carbon in modern marine sediments is ~ –20‰ [2], and therefore not a viable carbon source for the Jericho diamonds. Consequently, [1] invoked a heretofore unidentified, highly localized and 13C-depleted carbon source for the Jericho high MgO eclogitic diamonds. We suggest that this anomalous carbon source may have been organic carbon formed by methane fixation by methanogenic bacteria in the Neoarchean (ca. 2.7 Ga) or Paleoproterozoic (ca. 2.0 Ga) times, which are characterized by δ13C values as low as –50‰ [2, 3]. Interestingly, these time periods broadly correspond to hypothesized subduction events beneath the Slave Craton.

We propose that the protoliths of the high-MgO Jericho diamond eclogites were similar to those of common ‘basaltic’ eclogites, but melt depletion coupled with peridotite chemical equilibration produced the high-MgO compositions. Diamond formation was coincident with a semi-cryptic metasomatic event, which trapped some of the original, low-Mg eclogitic garnet and produced the incompatible-element-rich and radiogenic nature of the eclogites. The ca. -40‰ organic carbon may have been intrinsic to slab eclogitization during Neoarchean and/or Paleoproterozoic subduction events. Alternatively, the eclogites may have been older and metasomatized by slab-derived fluids generated during Neoarchean or Paleoproterozoic subduction events.