The alkaline rocks olivine variety: a case study of Namuaive Pipe, Kola alkaline province, Russia

An alkaline magma undergoes significant modification during its ascent to the surface due to assimilation of mantle and crustal material, exsolution of volatiles, trapping of crystals from earlier crystallized melt batches, fractional differentiation and explosive processes, making a thorough understanding of the petrogenesis of the magma and the nature of its mantle source in large igneous provinces difficult. The dikes and pipes of ultramafic lamprophyres, picrites and nephelinites are characterised by rapid rise of melt batches that could provide a high chance of preserving unworked mantle and crustal material. A study of olivine from these rocks may help to resolve some issues of their mantle sources and melt evolution.

The Namuaive pipe located in the northern part of the nepheline syenite Khibina Massif, Kola alkaline province. The pipe is filled by pyroclastic alkaline picrite (melanephelenite). The rock is texturally heterogeneous, consisting of 30-40 vol % magmaclasts and lapilli, phenocrysts and macrocrysts (up to 40 vol %) of phlogopite, olivine and clinopyroxene, xenoliths of Khibina Massif rocks, and fine grained matrix composed of phlogopite, apatite, perovskite, spinel, Ti-magnetite, nepheline, sodalite, high-Ti garnet.

Olivine is one of the most abundant macrocrysts in alkaline picrite, ranging up to 30 vol %; it is fresh or slightly replaced by serpentine or clinopyroxene-phlogopite intergrowth. Three groups of olivine based on core-to-rim zonation were observed:

• Euhedral-to-subhedral olivine grains up to 5 mm are phenocrysts. Some grains have spinel inclusions. Phenocrysts show normal Mg#-zonation. They consist of a more magnesian core (Mg# = 0.90-0.89) with Ni content from 1595 to 3058 ppm and a thin marginal ferruginous zone (Mg# = 0.89-0.83) with Ni content from 363 to 2663 ppm.
• Antecrysts have rounded and often corroded edges. Their size ranges from the first few hundred µm to 1 mm. They are characterised by Fe-rich cores (Mg# = 0.84-0.87) with a wide range of Ni contents from 1200 to 3200 ppm, surrounded by a transitional zone with a gradual increase in magnesium (Mg# = 0.86-0.89) and Fe-rich rind (Mg# = 0.89-0.86) with Ni contents from 1500 to 605 ppm. They have a small clinopyroxene-phlogopite rim and spinel along cracks.
• The xenocrysts have been divided into two subgroups according to their Mg# and Ni contents. The cores of the first subgroup have Mg# 0.90-0.91 and Ni contents from 2800 to 3200 ppm, the cores of the second subgroup have Mg# 0.91-0.93 and Ni content about 2000 ppm. The rims of both subgroups have Mg# 0.83-0.85 and Ni contents from 1236 to 433 ppm. Some olivine grains are intergrown with high-Cr clinopyroxene and high Mg phlogopite.

The antecrysts reflect mixing of the evolved lamprophyric melts during previous pulses with the partial melt batch that formed the Namuaive pipe. Phenocrysts and other olivine rims formed during fractional crystallization. Antecrysts and phenocrysts equilibrated to melts from wehrlite sources.

The study was supported by the Russian Science Foundation under Grant No 23-77-01052