The Viscosity of Albitic Melt at High Pressures and Implications for the Mobility of Crust‐Forming Magmas

Abstract The earliest form of continental crust was produced by tonalite‐trondhjemite‐granodiorite (TTG) magmas. Molten albite (NaAlSi 3 O 8 ) is representative of TTGs and also a major component of modern crust‐forming magma. The viscosity of the melt controls the magma ascent rate and hence influences the production of new continental crust. It is well known that the viscosity ( η ) of albitic melt exhibits an anomalous pressure ( P ) dependence. However, prior results on the melt η at high‐ P differ significantly which limits our ability to predict the movement of crust‐forming magma at depth. In this study, we more tightly constrained the P ‐effect on η in anhydrous albitic melt via high‐ P and high‐temperature ( T ) falling sphere experiments. We limited undesirable drag effects by using small sphere‐to‐capsule diameter ratios ( d / D ) such that d / D ≤ 0.12, and evaluated uncertainties due to such drag using a Monte Carlo approach. Our results show that melt η first decreases with P (i.e., ∂ η /∂ P < 0) and then increases with continued compression (∂ η /∂ P > 0) with a well‐defined η minimum ( η min ) at ∼6 GPa along a ∼2,000 K isotherm. We find that the viscosity of the melt can be described by an Arrhenius formalism with an activation volume that varies with P and T . The results indicate that η of aluminosilicate magmas decrease with depth and temperature in the crust, thereby mobilizing the magmas to promote rapid volcanic eruptions. The results also suggest that TTG magmas relevant for the early Earth could pond during ascent due to the anomalous P ‐effect on η .