Multiphase inclusions associated with residual carbonate in a transition zone diamond from Juina (Brazil)

Super-deep diamonds and their mineral inclusions preserve very precious information about Earth's deep mantle. In this study, we examined multiphase inclusions entrapped within a diamond from the Rio Vinte e um de Abril, São Luiz area (Juina, Brazil), using a combination of non-destructive methods. Micro-Computed X-ray Tomography (μ-CXRT) was used to investigate the size, shape, distribution and X-Ray absorption of inclusions and mapping by micro X-ray Fluorescence (μ-XRF), μ-Raman Spectroscopy and micro-Fourier Transform Infrared Spectroscopy (μ-FTIR) were used to determine the chemical and mineralogical composition of the inclusions. Four large inclusions enclosed in the N-rich diamond core consist of dominant ferropericlase–magnesiowüstite and locally exsolved magnesioferrite. FTIR maps, obtained integrating the band at 1430 cm−1, show also the presence of carbonates. A fifth large inclusion (ca 100 μm) was remarkable because it showed a very unusual flask shape, resembling a fluid/melt inclusion. Based on μCXRT tomography and μ-Raman mapping, the flask-shaped inclusion is polyphase and consists of magnetite and hematite partly replacing a magnesiowüstite core and small-volume of gas/vacuum. μ-Raman spectra on the same inclusion revealed local features that are ascribed to post-spinel polymorphs, such as maohokite or xieite, which are stable at P ≥ 18 GPa, and to huntite, a carbonate with formula CaMg3(CO3)4. This represents the first finding of maohokite and huntite in diamond. We interpret the composition of the inclusions as evidence of formation of ferropericlase–magnesiowüstite and diamond in a carbonate-rich environment at depths corresponding at least to the Transition Zone, followed by oxidation of ferropericlase–magnesiowüstite by reaction with relatively large-volume entrapped melt during diamond ascent.