Abstract Ingashi lamproite dykes are the only known primary sources of diamond in the Irkutsk district (Russia) and the only non-kimberlitic one in the Siberian craton. The Ingashi lamproite field is situated in the Urik-Iya graben within the Prisayan uplift of the Siberian craton. The phlogopite-olivine lamproites contain olivine, talc, phlogopite, serpentine, chlorite, olivine, garnet, chromite, orthopyroxene, clinopyroxene as well as Sr-F-apatite, monazite, zircon, armolcolite, priderite, potassium Mg-arfvedsonite, Mn-ilmenite, Nb-rutile and diamond. The only ultramafic lamprophyre dyke is composed mainly of serpentinized olivine and phlogopite in the talc-carbonate groundmass and is similar to Ingashi lamproites accessory assemblage with the same major element compositions. Trace element and Sr-Nd isotopic relationships of the Ingashi lamproites are similar to classic lamproites. Different dating methods have provided the ages of lamproites: 1481 Ma (Ar-Ar phlogopite), 1268 Ma (Rb-Sr whole rock) and 300 Ma (U-Pb zircon). Ingashi lamproite ages are controversial and require additional study. The calculated pressure of 3.5 GPa max for clinopyroxenes indicates that lamproite magma originated deeper than 100 km. A Cr-in-garnet barometer shows a 3.7–4.3 GPa min and derivation of Ingashi lamproites deeper than 120 km in depth. Based on the range of typical cratonic geotherms and the presence of diamonds, the Ingashi lamproite magma originated at a depth greater than 155 km.

Abstract Petrographic, geochemical and mineralogical characteristics of diamond deposits from the Upper Muna field have been investigated. Geochemically, diamondiferous kimberlites from Upper Muna belong to the most widespread Fe–Mg-rich rocks in the Yakutian kimberlite province (average FeO total = 8.4 wt%, MgO = 32.36 wt%, TiO 2 = 1.6 wt%). Striking mineralogical features of Upper Muna kimberlites are: (1) abundance of monticellite and perovskite in the groundmass; (2) rare occurrence of Mg-ilmenite; (3) abundance of phlogopite megacrysts (up to 8 cm across); and (4) coexistence of low-Cr (0.1–4 wt% Cr 2 O 3 , with 0.8–1.2 wt% TiO 2 ) and high-Cr (3–8 wt% Cr 2 O 3 , with 0.1–0.6 wt% TiO 2 ) garnet megacrysts with contrasting rare earth element patterns. The compositional features of groundmass minerals, the relatively low CaO and CO 2 contents in kimberlites and few deuteric alteration in Upper Muna kimberlites suggest high-temperature melt crystallization during pipe emplacement. Based on the compositional data of garnet and Cr-diopside from megacrysts and peridotites, we suggest a poor Cr dunite–harzburgitic and lherzolitic mantle source beneath the Upper Muna field where Cr-diopside crystallized within a wide pressure and temperature range (40–65 kbar and 900–1350°С). The mineral geochemistry, trace element distribution and Sr–Nd isotope variations of Upper Muna kimberlites are typical for group I kimberlites and reflect a deep-seated asthenospheric (convective mantle) source for the kimberlites.