Home – Home – Jornals – Russian Geology and Geophysics 2015 number 6

Based on the analysis of experimental data on the viscosity of mafic to ultramafic magmatic melts with the use of our structure-chemical model for the calculation and prediction of the viscosity of magmas, we have first predicted that diamond-carrying kimberlite magma must ascend from mantle to crust with considerable acceleration. The viscosity of kimberlite magma decreases by more than three times during its genesis, evolution, and ascent from mantle to crust despite the significant decrease in the temperature of the ascending kimberlite magma (~150 ºC) and its partial crystallization and degassing. In the case of partial melting (<1 wt.%) of carbonated peridotite in the mantle at depths of 250-350 km, high-viscosity (~35 Pa·s) kimberlite melts can be generated at ~8.5 GPa and ~1350 ºC, the water content in the melt being up to ~8 wt.%, C(OH^–) = 0–2 wt.%, and C(H₂O) = 0–6 wt.%. On the other hand, during the formation of kimberlite pipes, dikes, and sills, the viscosity of near-surface kimberlite melts is much lower (~10 Pa·s) at ~50 MPa and 1200 ºC, the volume contents of crystals (V_cr) and the fluid phase (bubbles) (V_fl) are 35 and 5 vol.%, respectively, and the water content in magma, C(OH^–), is 0.5 wt.%. On the contrary, the viscosity of basaltic magmas increases by more than two orders of magnitude during their ascent from mantle to crust. The basaltic magmas which can be generated in the asthenosphere at depths of ~100 km have the minimum viscosity (up to ~2.3 Pa·s) at ~4.0 GPa, 1350 ºC, C(OH^–) ≈ 3 wt.%, and C(H₂O) ≈ 5 wt.%. However, at the final stage of evolution (e.g., during volcanic eruptions), the viscosity of basaltic magma is considerably higher (600 Pa·s) at ~10 MPa, 1180 ºC, V_cr ≈ 30 vol.%, V_fl ≈ 15 vol.%, and C(OH^–) ≈ 0.5 wt.%.