Alexander Rocholl

Nanocrystalline diamond from the Earth’s mantle underneath Hawaii

Abstract Melt inclusions in a mantle-derived garnet pyroxenite xenolith from Salt Lake Crater (Hawaiian island of Oahu) have been analysed by means of transmission electron microscopy, including energy dispersive X-ray analysis, electron energy-loss spectroscopy and electron diffraction. The melt inclusions represent trapped low-degree melt that metasomatised the Hawaiian mantle lithosphere before the eruption of the host magma. The inclusions consist of a C–Cl–S-rich glass of basaltic composition, an exsolved gas–fluid phase (mainly CO2 with minor amounts of CO, H2O, and some unidentified C–H and H–S species) and/or carbonate. Diamond is the major phase in the glass, ranging in size between a few and several hundred nanometres. Two varieties of nanocrystalline diamonds are identified. Polycrystalline aggregates of nanometre-size diamonds are tentatively explained as pseudomorphs after carbonate, while single crystals are suggested to have crystallised from the melt. Other nanocrystalline phases include native Fe and Cu, FeS, FeS2, ZnS, AgS and several Ti, Nb, Zr, Ir, In, Pd-rich phases of unknown composition. Overall, the observed mineral assemblage suggests that the melt was highly reduced and strongly enriched in volatiles and trace elements of contrasting geochemical affinity. Our findings are the first evidence for a metasomatic impregnation of the Hawaiian mantle by low-degree melts. Here, we report the first occurrence of diamond in the modern oceanic mantle.

Biologically mediated crystallization of buddingtonite in the Paleoproterozoic: Organic-igneous interactions from the Volyn pegmatite, Ukraine

Abstract The Volyn pegmatites from Volodarsk-Volynskyi in the Zhytomyr Oblast, NW Ukraine, are associated with granites genetically related to the Paleoproterozoic Korosten pluton. Their late-stage evolution is characterized by the formation of opal-cemented breccia. A polymineralic pseudomorph after beryl within the breccia includes bertrandite (±euclase) + F-muscovite (with tobelite component) + buddingtonite + organic matter (OM) + opal (+ traces of K-feldspar, albite, columbite, FeS2, barite, REE-minerals). Sector-zoned and platy to fibrous buddingtonite has variable (K+Na)-vs. NH4,-contents (electron microprobe analyses) and some H2O or H3O+, as indicated by microscope infrared spectroscopy. We suggest that ammonium was produced by decay of OM, which is partly preserved in the pseudomorph. Energy-dispersive electron microprobe data of the OM show with increasing O–decreasing C-N-content due to degassing; the OM contains the high field strength elements Zr (≤7 at%), Y (≤3 at%), Sc (≤0.8 at%), REE (≤0.3 at%), Th (≤0.2 at%), and U (≤1.25 at%), which increase with increasing O-content. Transmission electron microscopy of the OM confirms the presence of N; Zr, Si, and O (with other HFSE) are concentrated in nanometer-sized areas and at the transition from OM to opal in nanometer-sized platy Zr-Si-O crystals. C-rich areas are amorphous but show poorly developed lattice fringes. OM is present in the pseudomorph also as brown pigmentation of opal and in pegmatitic beryl from Volyn as a component in late stage fluid inclusions, identified by C-H vibrational bands in infrared spectra. Stable isotope investigations of C and N of buddingtonite, black opal and kerite (fibrous OM known from the literature to occur in the Volyn pegmatites and interpreted as microfossils) indicate a biogenic origin of the OM. We propose that OM in the pseudomorph is condensed kerite, which achieved the high concentrations of high field strength elements via fluid-pegmatite interaction. Although no age determination of minerals in the pseudomorph is available, textural arguments and phase equilibria indicate its formation in a late stage of the pegmatite evolution, at P-T conditions below ~100 MPa/150 °C. We favor a conceptual model for the formation of the Volyn buddingtonite in analogy to Phanerozoic occurrences of buddingtonite, where over and around the shallow anorthosite-granite Korosten pluton hydrothermal convection cells introduced N-bearing hydrocarbons and its precursors into the cooling igneous rocks. Due to the elevated temperature, the OM disintegrated into degassing volatile and non-volatile residual components analogous to petroleum maturation. Organic N, released as NH4, was then incorporated into buddingtonite.