Temperature changes in ascending kimberlite magma

摘要

We present a thermodynamical model of kimberlite magma ascent from 200 km depth and from 400 km depth. We model four different magma compositions, a basalt and a carbonatite to bound kimberlite behaviour and two intermediate cases that represent Kimberlite 1 has properties intermediate to carbonatite and basalt, and kimberlite 2 has properties that are closer to basalt. The results show that adiabatic expansion of the melt phase can be a large cooling effect during kimberlite melt ascent, accounting for 140 degrees C cooling of kimberlite 1 and similar to 90 degrees C cooling of kimberlite 2. Melts with high volatile contents are more corrosive during magma ascent, enriching the melts in MgO as olivine xenocrysts are assimilated. Kimberlite magma temperatures decrease during ascent up to the onset of rapid pressure-induced olivine crystallization. The models show that little olivine assimilation occurs during kimberlite ascent (<1%), and this implies the magma composition is set at depth and is not acquired via olivine dissolution. Latent heat release counteracts contemporaneous cooling mechanisms such as gas exsolution and lithospheric entrainment, and in this regime the magma temperature increases as the pressure decreases. At shallow levels gas exsolution and expansion become dominant processes and the magma temperature cools during the final stages of ascent. Our models suggest that shallow magma temperatures consistent with estimates from geothermetric studies (1030-1170 degrees C) occur when the volatile content of the ascending kimberlite magma is less than 10 wt.% H2O. Models where 5 wt % H2O + >= 5 wt % CO2 is exsolved are consistent with observations of approximately 25% phenocrysts and 25% xenocrysts in many kimberlites.