Ascent rate of the Udachnaya-East kimberlite melts from olivine diffusion chronometry

Abstract

Kimberlite melts are low volume, volatile-rich melts formed in the asthenosphere at depth >150-250 km. Due to their low viscosity and the exsolution of CO2 triggered by their interaction with the sub-cratonic mantle, these melts are highly buoyant and rise rapidly through the lithosphere, ultimately reaching the surface and erupting at sonic to super-sonic speed. Studies based on fluid/melt dynamics, fracturing and disaggregation of the transported mantle-derived cargo, as well as on H diffusion in olivine and Ar diffusion in magmatic phlogopite showed that the ascent rate of kimberlite melts can vary between 0.1 and 30 m/s. However, these rates are extrapolated from processes starting at different depths, and thus may not be representative for the whole ascent path of kimberlite melts through the sub-cratonic lithosphere. To shed light on this issue, we applied diffusion chronometry to the deepest feature ascribable to the liquid line of descent of the melts, i.e. early liquidus olivine formed around mantle-derived xenocrystic cores from the Udachnaya-East pipe (Siberian craton). Our results showed that the time elapsed between the formation of the Internal Zone II (early olivine on the liquidus) or rims (typical magmatic olivine) around xenocrystic cores and final cooling is between 5 and 74 days. Considering travelled distances of 80-110 km in accordance with thermobarometric calculations, this implies overall average ascent rates of kimberlite melts of 0.02 to 0.23 m/s, which are comparable to those of typical alkali basalts, albeit starting at much greater depth. This indicates that kimberlite melts start with moderate ascent rates and only later increase their speed, likely due to massive CO2 exsolution triggered by decarbonation reactions during melt/fluid-rock interactions in the lithospheric mantle. The overall timescales of 5 to 74 days obtained in our study overlap with the lowest values reported in the literature from chemical zoning of mantle-derived minerals (25 days to >100 years), confirming that the time elapsed between kimberlite-related mantle metasomatism and the ascent of kimberlite melts was short. Importantly, our data show that Fe-Mg interdiffusion in olivine can be used as a trustable chronometer in kimberlite systems, opening new perspectives on the study of ascent dynamics in kimberlites worldwide.