Reassessing the Pionerskaya pipe of NW Russia: effect of crustal xenolith-kimberlite reactions on host rock classification

Abstract

Correctly classifying primary diamondiferous volcanic rocks as kimberlites or lamproites is important because correct identification of a melt provides insight into the lithospheric thickness and thermal conditions during melt formation and because the economic potential of each rock type may be different. This study investigates the impact of crustal xenolith assimilation on host rock classification using samples from the Late Devonian Pionerskaya pipe located in the mined Lomonosov diamond deposit, part of the Zolotitsa cluster in NW Russia’s Arkhangelsk Diamond Province. Historically, the pipes in the Zolotitsa cluster have been classified as transitional or Group II kimberlites (now called Carbonate-rich Olivine Lamproites) due to the presence of groundmass clinopyroxene, high modes of phlogopite, and transitional Sr–Nd isotope systematics. We conducted detailed petrographic and mineralogical studies of the Pionerskaya pipe-fill samples using microbeam electron techniques, X-ray diffraction, X-ray fluorescence, and thermodynamic modelling with Perple_X. The pipe-fill at Pionerskaya consists of two units of pyroclastic kimberlite (PK-1 and PK-2), hypabyssal kimberlite (HK), and transitional hypabyssal kimberlite (HKt). Volcanologically, we classify Pionerskaya as a Kimberley-type pyroclastic kimberlite (KPK) due to the presence of HKt and the mineralogy and texture of PK-2, which contains distinct oval magmaclasts with thin melt selvages and microlitic fringes of clinopyroxene. The Pionerskaya pipe contains granitoid, diorite, and metabasite xenoliths that reacted with the kimberlite. An archetypal kimberlitic affinity for the Pionerskaya pipe is supported by: 1) the metasomatic origin of groundmass clinopyroxene and phlogopite, both modelled as subsolidus, low-temperature phases; 2) variation in phlogopite size, morphology, inclusion content, and compositional zoning trends near igneous and metamorphic crustal xenoliths; 3) the presence of monticellite and 4) modelling of Sr–Nd systematics of the Pionerskaya pipe showing contamination from mafic and felsic crustal xenoliths. The high degree of contamination by crustal xenoliths through metasomatic reactions between the xenoliths and the kimberlite resulted in the previous erroneous classification of the Pionerskaya pipe.