Micron-scale δ¹³C determination by NanoSIMS in a Juina diamond with a carbonate inclusion

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Juina diamonds present a wide spectrum of mineral inclusions, covering both peridotitic and eclogitic composition. Among the most rare inclusions, carbonates are interpreted as an evidence of deep recycling of sedimentary carbon into the transition zone or the lower mantle. Yet, the δ¹³C values measured in three FIB-TEM foils by NanoSIMS 50 of an alluvial diamond with a carbonate inclusion range between –8.85 ± 1.32‰ and –2.31 ± 1.88‰ with a mean total value of –5.0 ± 2.3‰. These values are in the range of typical mantle carbon, as measured in diamonds of peridotitic paragenesis. Similar δ¹³C values from –8.5 to –4.4‰ are reported in literature for five other Juina diamonds with carbonate inclusions. We can postulate either that the diamond is peridotitic and carbonate precipitated from fluids and survived to the reduction to diamond or that formed from reduction of carbonatitic melts in the upper mantle, percolating through eclogite.