CARBONATITIC MELT INCLUSIONS WITH CO2 AND CH4 IN HEALED CRACKS IN A DIAMOND

Mantle fluids and melts are key agents for mass transfer, metasomatism, diamond growth, as well as for mantle-sourced volcanism, such as kimberlites. Despite their broad influence, these agents are seldom preserved in physical samples that reach Earth’s surface. We report the finding of volatile-rich carbonatitic inclusions trapped within healed cracks in a 3.0 carat, facetted, type IIa colorless diamond. The inclusions contain dolomite, magnesite, nahcolite (NaHCO₃), eitelite Na₂Mg(CO₃)₂, with coexisting fluid CO₂ and CH₄, and are considered to be samples of a mantle-derived carbonatitic melt that became trapped in a pre-existing diamond. The coexisting CO₂ and CH₄ require that the inclusions are hydrous, even in the absence of clear water peaks in Raman spectroscopy, and indicates fO₂ conditions near the CHO water maximum. As it is trapped in healed cracks the melt is unrelated to diamond growth, but serves as a well-preserved example of hydrous carbonatitic mantle melt.

Since the diamond is type IIa with pronounced dislocation network patterns (visible in cathodoluminescence imaging), we tentatively link this sample to the sublithospheric diamond variety known as CLIPPIR diamonds. The carbonatitic melt inclusions could have been trapped later at shallower depths, but since the dislocation networks conform to the healed cracks, the melt may have been trapped in sublithospheric mantle before the final stages of dislocation network formation. At the very least, the melt being sealed into healed cracks attests to it being trapped at diamond-stable depths in the mantle. The fact that the inclusions are trapped in cracks suggests the melt was associated with potentially violent stresses, overstepping the typical plastic high-temperature deformation behavior of diamond, perhaps associated with ascent of the diamond and/or early stages of kimberlite volcanism. Melt inclusions of similar carbonatitic composition have previously been documented in healed cracks in mantle-derived olivine xenocrysts from the Koala kimberlite (Ekati mine, Canada). These melt inclusions may be valuable, direct evidence of the nature of carbonatitic melts at mantle depths.