First Report of Majoritic-Garnet Diamond Inclusions From Yakutian Kimberlites

The presence of a majoritic component in mantle garnets is significant in that it indicates a deeper-than-normal origin for their host diamonds. We have discovered the first majoritic garnets both of peridotitic (P-/U-type) and eclogitic (E-type) parageneses, included in microdiamonds (<1 mm) from three Yakutian kimberlite pipes: Yubileynaya, Komsomolskaya, and Krasnopresnenskaya, all located in the Alakit kimberlite field of Upper Devonian age. Up until now, a considerable number of majoritic garnets have been recovered from placers. The new finds of majoritic garnets reported here practically double the number of kimberlitic pipes worldwide where such garnets have been detected. Multiple inclusions of garnet and olivine occur in single P-type diamond from Yubileynaya. Here, a CrCa-rich majoritic garnet coexists with a CrCa-rich non-majoritic garnet and olivine, but the 3 grains are not in contact. Positive identification of the majoritic garnet was obtained by single-crystal X-ray diffraction: space group Ia3d; a = 11.775 (1) Å; V = 1632.6 (2) ų. All garnets were analyzed extensively by electron microprobe. The specific features of the compositions of `coexisting' majoritic and non-majoritic garnets, resp., are: Si (pfu) = 3.22 and 3.02: Cr₂O₃ (wt %) = 10.2 and 13.7; CaO (wt %) = 20.8 and 12.7; Mg# 77.6 and 69.9. Coexisting olivine is Fo 91.5, which is consistent with the relatively low Mg# of the majoritic garnet. This Yubileynaya majoritic garnet diamond inclusion (DI) represents the first find of a garnet, containing solid solution pyroxene, from a wehrlitic paragenesis. Furthermore, its CaCr-component (uvarovite) content is unusually high (~50%). The chemical differences of the wehrlitic garnets in this one Yubileynaya diamond testifies directly to the complex history of this diamond, specifically to a large range of pressures. Majoritic garnet DIs from the Komsomolskaya and Krasnopresnenskaya pipes are both of E-type and are characterized by the following compositional features respectively: Si (pfu) 3.13 and 3.05; TiO₂ (wt %) 1.9 and 0.4 ; CaO (wt %) 12.2 & 12.9; Na₂O (wt %) 0.93 and 0.43; Mg# 49.6 and 54.2. In both garnets Na (pfu) exceeds Ti (0.028 and 0.039, resp.). It is of major significance that majoritic garnets have been found to occur as DIs from all known parageneses of P-/U-type garnets (harzburgitic, lherzolitic, and wehrlitic). These new results confirm the unusual character of DIs in 10-20% of the microdiamonds from Yakutian kimberlites [e.g., Sobolev et al., 2001, EOS]. A significantly deeper source and petrogenesis is indicated for these special microdiamonds.