Basalt melts under pressure: Is there really a viscosity minimum?

Basalts are the most numerous and voluminous magmas on Earth, the moon and other terrestrial planets and moons. Melt viscosity plays a major role in modulating the rates and efficacy of many magmatic and volcanic processes (e.g., melt extraction, ascent rates, eruption styles). The pressure dependence of melt viscosity is particularly relevant to basalts because of the wide range of pressures they experience during ascent from their mantle sources to Earth's surface. Here, we review and critically analyse the published high pressure experimental data for the viscosity of basaltic melts. Our compilation of high-pressure measurements of basalt viscosity is relatively sparse comprising a total of 56 experiments. The experiments span a temperature range of 1275 to 2000 °C, pressures from 0.5 to 7 GPa, and anhydrous melt compositions ranging from MORB, to Hawaiian tholeiite, to alkali olivine basalt (AOB). We focussed our analysis on nineteen modern falling sphere experiments on MORB (Sakamaki et al., 2013) and AOB (Bonechi et al., 2022) melts that were imaged with real-time, x-ray radiography. Our analysis of these data suggests a monotonic positive pressure dependence for basaltic melt viscosity. On that basis, we present a predictive model for the Newtonian viscosity of AOB melts as a function of temperature (T), pressure (P):$\log \eta =-4.55+\frac{7845.4+295.4\left(P\left(\mathrm{GPa}\right)-0.001\right)}{T\left(K\right)}$

A similar model applied to the MORB dataset is consistent with a significantly lower (i.e. 13.3 vs. 295.4) positive pressure coefficient implying a less pressure-dependent viscosity. Our observations call into question the concept of a minimum in the pressure dependence of basaltic melt viscosity which has been argued to inhibit the ascent of basaltic magmas.