Micro-Raman study of crichtonite group minerals enclosed into mantle garnet

We report the first comprehensive micro-Raman study of crichtonite group minerals (CGM) as inclusions in pyropic garnet grains from peridotite and pyroxenite mantle xenoliths of the Yakutian kimberlites as well as in garnet xenocrysts from the Aldan shield lamprophyres (Russia). The CGM form (i) morphologically oriented needles, lamellae, and short prisms and (ii) optically unoriented subhedral to euhedral grains, either single or intergrown with other minerals. We considered common mantle-derived CGM species (like loveringite, lindsleyite, and their analogues), with Ca, Ba, or Sr dominating in the dodecahedral A site and Zr or Fe in the octahedral B site. The Raman bands at the region of 600–830 cm−1 are indicative of CGM and their crystal-chemical distinction, although the intensity and shape of the bands appear to be dependent on laser beam power and wavelength. The factor-group analysis based on the loveringite crystal structure showed the octahedral and tetrahedral cation groups with 18f and 6c Wyckoff positions, namely, dominantly TiO6 and to a lower extent CrO6, MgO4, and FeO4 groups, to be the major contributors to the Raman spectral features. The ionic groups with dodecahedral (M0) and octahedral (M1) coordination are inactive for Raman scattering while active in infrared absorption. A number of observed Raman modes in the CGM spectra are several times lower than that predicted by the factor group analysis. The noticed broadening of modes in the CGM Raman spectra may result from a combining of bands at the narrow frequency shift regions. Solid solution behavior, luminescence, and partial metamictization of the CGM may exert additional influence on the Raman band shape. The Raman spectral features showed CGM to be accurately identified and distinguished from other Ti-, Fe-, Cr-, and Zr-containing oxides (e.g., ilmenite or those of spinel and magnetoplumbite groups) occurring as accessory mantle minerals. © 2020 The Authors. Journal of Raman Spectroscopy published by John Wiley & Sons LtdRussian Science Foundation, RSF: 18‐77‐10062Council on grants of the President of the Russian FederationThis study was supported by the Russian Science Foundation (Grant 18‐77‐10062). The equipment of the Ural Center for Shared Use «Modern Nanotechnology», Ural Federal University, and the Analytical Center for Multi‐elemental and Isotope Research, IGM, was used. Sampling was supported by the Russian Federation state assignment project of IGM. We are grateful to Nikolai V. Sobolev for Samples O‐173, O‐39, and O‐264. Vladimir N. Korolyuk, Elena N. Nigmatulina (IGM), and Allan Patchen (UT) are highly appreciated for the help with EMP analyses. We express our sincere thanks to F. Nestola and an anonymous reviewer for their thorough reviews and helpful suggestions, and to C. Marshall for regardful editorial handling of the manuscript on every stage of its revision

Late metasomatic addition of garnet to the SCLM : Os-isotope evidence

Archean cratons are underlain by highly depleted subcontinental lithospheric mantle (SCLM). However, xenolith and xenocryst data [1; references therein] suggest that Archean SCLM has been extensively refertilized by metasomatic processes, with the addition of Fe, Ca, and Al to depleted protoliths. The distribution of sub-calcic garnets in the SCLM beneath the Siberian craton suggests (1) sub-calcic garnets and diamonds are metasomatic phases in the cratonic SCLM; (2) the distribution of both phases is laterally heterogeneous on relatively small scales and related to ancient structural controls [2]. Re-Os isotopic compositions of sulfide inclusions in lherzolitic Cr-pyropes from four Siberian middle Paleozoic diamond mines have been determined by laser ablation MC-ICP-MS: Mir (n=17) and Internationalnaya (n=109), Malobotuobiya field, Udachnaya (n=17), Daldyn field, and Nyurbinskaya (n=12), Nakyn field. Most analysed sulfides ( approximately 84%) have very low Re/Os ratios (<0.07), and their Re-depletion ages (T (sub RD) ) fall between 2.2 and 3.0 Ga (+ or -0.03 Ga, mean 2s analytical uncertainty). 10 to 15% of the sulfides give younger T (sub RD) down to 600 Ma. Our previous study [3] of sulfide inclusions in megacrystic olivines from the Udachnaya pipe suggests that most of the SCLM beneath the Daldyn kimberlite field formed at 3-3.5 Ga, and that lithosphere formation culminated at ca 2.9 Ga. Our new data suggest that refertilization of the highly depleted SCLM and the introduction of Cr-pyrope garnet occurred between 2.2 and 3.0 Ga; little garnet was present before 3 Ga. Pyropes with young sulfides (between approximately 1.9 and approximately 2.2 Ga) may have crystallised during the amalgamation of the Siberian craton in Paleoproterozoic time.1 page(s

Cr-rich rutile : a powerful tool for diamond exploration

Mineralogical studies and U–Pb dating have been carried out on rutile included in peridotitic and eclogitic garnets from the Internatsionalnaya pipe, Mirny field, Siberian craton. We also describe a unique peridotitic paragenesis (rutile + forsterite + enstatite + Cr-diopside + Cr-pyrope) preserved in diamond from the Mir pipe, Mirny field. Compositions of rutile from the heavy mineral concentrates of the Internatsionalnaya pipe and rutile inclusions in crustal almandine-rich garnets from the Mayskaya pipe (Nakyn field), as well as from a range of different lithologies, are presented for comparison. Rutile from cratonic mantle peridotites shows characteristic enrichment in Cr, in contrast to lower-Cr rutile from crustal rocks and off-craton mantle. Rutile with Cr₂O₃ > 1.7 wt% is commonly derived from cratonic mantle, while rutiles with lower Cr₂O₃ may be both of cratonic and off-cratonic origin. New analytical developments and availability of standards have made rutile accessible to in situ U–Pb dating by laser ablation ICP-MS. A U–Pb age of 369 ± 10 Ma for 9 rutile grains in 6 garnets from the Internatsionalnaya pipe is consistent with the accepted eruption age of the pipe (360 Ma). The equilibrium temperatures of pyropes with rutile inclusions calculated using Ni-in-Gar thermometer range between ~ 725 and 1030°C, corresponding to a depth range of ca ~ 100–165 km. At the time of entrainment in the kimberlite, garnets with Cr-rich rutile inclusions resided at temperatures well above the closure temperature for Pb in rutile, and thus U–Pb ages on mantle-derived rutile most likely record the emplacement age of the kimberlites. The synthesis of distinctive rutile compositions and U–Pb dating opens new perspectives for using rutile in diamond exploration in cratonic areas.8 page(s