Epigenetic aragonite in a sheared lherzolite xenolith from the Udachnaya kimberlite pipe

Here we report the first finding of the high-pressure polymorph of calcium carbonate (aragonite) in the interstitial space of a sheared lherzolite xenolith from kimberlites of the Udachanaya diamond deposit (Siberian craton, Russia). Xenoliths with a sheared texture are the deepest mantle rocks sampled by kimberlite magma from 180-230 km depth. According to experimental data, aragonite is the high-pressure polymorph of calcium carbonate, which is stable at upper mantle pressure and temperature. Thereby aragonite is used as a reliable geobarometer in studies of magmatic and ultrahigh-pressure metamorphic rocks.
Aragonite was determined by Raman spectroscopy study. The Raman bands at 208 cm^-1, 702-706 cm^-1 and 1462 cm^-1 are the identification features of aragonite. Chemical analyses of aragonite were obtained by scanning electron microscope with an energy dispersive system. Some analyses were verified by electron microprobe as well. The concentration of SrO in aragonite ranges from 0.5 to 8.8 wt.%. Aragonite has a Na₂O concentration of 0.1-1.1 wt.%.
Aragonite (up to 100 µm) is the most common subordinate mineral from the interstitial space of this xenolith. It occupies on average 70 vol.% of the interstitial space. Aragonite grains consist of three chemically distinct zones. The first zone (core) is characterized by a low content of SrO (<1.5 wt.%) and low Mg# (~15). The second zone has roughly the same SrO but noticeably higher Mg# (~50). The third zone (rim) contains much higher concentration of SrO (up to 8.81 wt.%) and high Mg# (~50).
Sheared peridotite are located in the lithospheric mantle significantly below the aragonite-calcite equilibrium line. In particular, the investigated peridotite equilibrated at 1350°С and 69 kbar (~215 km). The presence of zoned aragonite from this peridotite means that this rock has been infiltrated by metasomatic agent. Numerical calculations reveals that such zoning can be preserved for 1 year at 1300°С (~equilibrium temperature of sheared peridotites) and for 10 years at 1000°С (~temperature of kimberlite magma at subsurface conditions). The short preservation time of zoning in aragonite (1-10 years) proves that aragonite could be formed immediately prior to kimberlite magmatism or after the capturing of the xenolith by kimberlite magma. Using adiabats of kimberlite magma and P-T parameters of aragonite stability in the upper mantle, aragonite in the studied sample was formed at the depth range of 80-215 km.
As the preservation time of zoning in aragonite is noticeably short (taking into account high temperatures), the best candidate for the role of an agent, which infiltrated the xenolith, is a primitive kimberlite melt of the Udachnaya pipe. The high percentage (70%) of aragonite in the interstitial space of the studied sheared lherzolite xenolith proves that such primitive kimberlite melt had carbonatitic composition. Our results show that not only different silicate-rich melts, but also carbonate or cabonated silicate melts might play a key role in mantle modifications. Carbonate melts are very suitable diamond-forming media and may support the idea of a genetic link between some diamonds and kimberlite magmatism.
This study was supported by the Russian Science Foundation (grant No 18-77-10062).