Viscosity of mafic magmas at high pressures

While it is accepted that silica‐rich melts behave anomalously with a decrease of their viscosity at increased pressures (P), the viscosity of silica‐poor melts is much less constrained. However, modeling of mantle melts dynamics throughout Earth's history, including the magma ocean era, requires precise knowledge of the viscous properties of silica‐poor magmas. We extend here our previous measurements on fayalite melt to natural end‐members pyroxenite melts (MgSiO3 and CaSiO3) using in situ X‐ray radiography up to 8 GPa. For all compositions, viscosity decreases with P, rapidly below 5 GPa and slowly above. The magnitude of the viscosity decrease is larger for pyroxene melts than for fayalite melt and larger for the Ca end‐member within pyroxene melts. The anomalous viscosity decrease appears to be a universal behavior for magmas up to 13 GPa, while the P dependence of viscosity beyond this remains to be measured. These results imply that mantle melts are very pervasive at depth. Key Points Viscosity of pyroxene and olivine melts decreases with increased pressure up to 8 GPa Lower values of melt viscosity at higher pressures suggests tetrahedral ring units control viscosity A compilation of data obtained on melts shows that their viscosity converge toward a value of 20–30 mPa s at the highest P investigated