In situ low-degree melts in peridotite xenolith from Majuagaa kimberlite, West Greenland

Mantle xenoliths provide a clear evidence of interaction with low-degree mantle melts, however, this evidence is mostly geochemical, manifested by incompatible element enrichment, or mineralogical, manifested by already crystallised phases (e.g. amphibole, phlogopite) as a result of this interaction.

Despite decades of research, the composition of low-degree melts generated in lithospheric mantle are still not very well-known. In situ characterisation of such melts is hampered due to their modification during the ascent as well as rapid alteration and weathering at the surface, while experiments are hampered by difficulties to produce and analyse very low-degrees (<2-3%) melts.

In this study we report a rare sample of well-preserved low-degree melts within a peridotite xenolith GGU473178 from Majuagaa kimberlite in West Greenland. We report alkali-carbonatitic-chloride melt pools and veins that may represent primary low-degree partial melts and products of their in situ crystallisation.

Melt pools are largely composed of carbonate (predominantly dolomite) and contain spinel, apatite, phlogopite as well as minor amounts of Fe-Ni sulphides, barite and halite.

Euhedral crystals of spinel present in these melt pools contain large usually round aggregates of mineral inclusions, which we explain as former melt pools captures by spinel. Mineral assemblage found in these spinel inclusions is consistently composed of ferropericlase, dolomite, alkali-rich carbonate and apatite, which is indicative of a strongly silicate-undersaturated alkali-carbonatitic melt that contains chlorine and phosphorous. Due to the almost complete absence of SiO₂, ferropericlase (instead of olivine) crystallises in equilibrium with dolomite and alkali-rich carbonate, implying incredibly low degrees of melting, when essentially only carbonated component is melted, or carbonate-silicate liquid immiscibility, previously reported for spinel lherzolite and garnet wehrlite xenoliths (Frezzotti et al., 2002; Soltys et al., 2016).

References

Frezzotti, M. L., Touret, J. L. R., and Neumann, E. R., 2002, Ephemeral carbonate melts in the upper mantle: carbonate-silicate immiscibility in microveins and inclusions within spinel peridotite xenoliths, La Gomera, Canary Islands: European Journal of Mineralogy, v. 14, no. 5, p. 891-904.

Soltys, A., Giuliani, A., Phillips, D., Kamenetsky, V. S., Maas, R., Woodhead, J., and Rodemann, T., 2016, In-situ assimilation of mantle minerals by kimberlitic magmas — Direct evidence from a garnet wehrlite xenolith entrained in the Bultfontein kimberlite (Kimberley, South Africa): Lithos, v. 256-257, p. 182-196.