So Mike Jollands and I wrote a paper looking at small scale diffusion in mantle garnet, which shows that the diffusion occurred between 25 days and 400 years before kimberlite ascent (a very short time frame geologically). So that means we have a very rare sample of a mantle metasomatism event that has preserved (possibly a failed kimberlite ascent). It’s in Earth and Planetary Science Letters, feel free to go and read it:
Jollands, M. C., Hanger, B. J, Yaxley, G. M., Kilburn, M. R. and Hermann, J. (2018). Timescales between mantle metasomatism and kimberlite ascent indicated by diffusion profiles in garnet crystals from peridotite xenoliths. Earth and Planetary Science Letters 481, 143-153.