Dr. Vadim KAMENETSKY ("New Star" Professor in Earth Sciences, School of Earth Sciences, CODES ARC Centre of Excellence in Ore Deposits, University of Tasmania, Austràlia) SEMINARIS DE LA FACULTAT DE GEOLOGIA I L'INSTITUT DE CIÈNCIES DE LA TERRA 'JAUME ALMERA'
Existing reconstructions of the kimberlite melt emphasise carbonate-bearing ultramafic compositions with significant amounts of dissolved volatiles CO2 and H2O (10-20 wt%). These volatiles are considered to be a major factor in reducing viscosity of the kimberlite melt, governing fast ascent from mantle depths. The exsolution of these volatiles from the melt during ascent and emplacement is viewed as being responsible for violent eruption of the magma and related brecciation of country rocks and the kimberlite itself. Magmatic volatiles and groundwaters have an unequivocal role in present models of kimberlite emplacement (fluidisation and phreatomagmatism, respectively). Our study of the diamondiferous Udachnaya-East pipe (Siberia) kimberlites provides additional constraints on rheological properties of the kimberlite melt and shows that neither of major magmatic volatiles in the form of fluids was responsible for well-known “explosivity” of kimberlites.

Kimberlites from deep levels of the Udachnaya-East pipe are exceptionally fresh, with low H2O (<0.5 wt%), but high CO2 (up to 14 wt%), Cl (up to 6 wt%), and alkalies (up to 6 wt% Na2O and 2.0 wt% K2O). The carbonatite-chloride composition of the parental melt of the Udachnaya -East kimberlite and similar compositions recorded in olivine-hosted melt inclusions in other kimberlites strongly support the previously assumed low viscosity and density of kimberlite magmas. Massive degassing of H2O and CO2 is unlikely in the case of the Udachnaya-East kimberlite, because the melt is poor in H2O, and CO2 is bonded in the carbonatitic melt. After crystallization of olivine the kimberlite melt evolves towards essentially dry carbonate-chloride compositions. The gravitational separation of silicate solids within the kimberlite pipes drives light, low viscosity carbonate-chloride melt to the top. The hydrogen species, such as H2 and CH4 that are recorded in significant quantities from boreholes.

Dia: 23-05-2012
Hora: 12:00
Lloc:Sala de Juntes de la Facultat de Geologia