LREE rich perovskite in antiskarn reactions – REE transfer from pyroxenites to carbonatites?

There is still no consensus in the literature concerning the most critical processes for rare earth elements (REE) enrichment in carbonatite rocks. One-third of alkaline‑carbonatite complexes are associated with ultrabasic and basic rocks. Some basic rocks contain similarly high concentrations of LREE as carbonatites where perovskite [CaTiO3] is responsible for this enrichment. Carbonatites form later than ultrabasic and basic rocks, and carbonate melt penetrates into silicate rocks and reacts with them. The reaction was named “antiskarn reaction”. Until now there is no detailed study on whether perovskite from pyroxenites can become a source for REE enrichment of carbonatite melt during antiskarn reactions.

We studied three ultrabasic-alkaline carbonatite complexes from the Kola Alkaline Province (Russia) to focus on the comparison of REE contents from pyroxenites and carbonatites and the fate of perovskite. Ultrabasic/basic rocks and carbonatites show the same broad range of REE contents. In most analyzed pyroxenites, perovskite is the main REE carrier mineral. For carbonatites/phoscorites, apatite and calcite are the minerals that control rocks' REE enrichment. Several samples show considerable variations of REE concentrations related to carbonatite infiltration, sometimes even at a very low local scale (mm²). If perovskite was present it was replaced near the carbonatite melt by titanite that has much lower REE contents. Calcites and apatites from the infiltrated carbonatite are highly enriched in LREE but show large local element variations. Thus, we propose that LREE liberated by the perovskite replacement can be dissolved in the carbonatite melt and transferred to its crystallizing minerals (calcite, apatite).