Garnet dissolution and the emplacement of kimberlites

摘要

The dissolution of mantle-derived garnet in H_2O-bearing kimberlite melt was investigated experimentally to provide some constraint on the survival of garnet xenocrysts commonly found in kimberlites. Garnet dissolution was determined by measuring the rate of radius change f garnet s[heres in synthetic kimberlite melt above its liquidus as a function of both temperature and pressure. To apply the results, a population of 80 garnet xenocrysts in a hypabyssal facies, macrocrystic kimberlite in the Slave Province, Canada, were examined in detail petrographically. The time scale required for kimberlite assent in order to preserve these garnets is dependent on the temperature path of the magma adopted during ascent. Assuming a minimum temperature of the kimberlite solidus, some estimate of the minimum ascent rate of kimberlites can be derived. Garnet xenocrysts would dissolve in one to ten hours at 1000-1200 deg C in kimberlite magmas rising from the mantle. The dissolution data also require that kimberlite magmas could only have existed near their liquids temperature in the mantle for time periods of minutes. Newly-formed fritted dissolution rims on cracked garnet xenocrysts, possibly related to explosive emplacement, indicate that the very final stage of kimberlite ascent in the crust to the root zone of the diatreme possibly occurred on a time scale of only minutes to seconds.