Rheology and Structure of Fe‐Mg‐Ca Enriched Silicate Melt: Benchmarking Viscosity Models for an Exotic Planetary Composition

Abstract

Silicate melt viscosity plays a pivotal role in the evolution of rocky bodies within the Solar System, exerting first‐order control on mantle differentiation and stratification, while dictating the metal‐silicate separation and the subsequent volcanic activity. Thus, predicting the viscosity of planetary compositions is essential to model and better understand their thermomechanical evolution. Notably, the chemical signatures of primordial planetary magmas, marked by extreme Fe, Mg, and Ca enrichment, drive a highly depolymerized and fragile rheological regimes that frequently fall beyond the calibration data sets used to model the viscosity. In this study, we characterize the effect on viscosity by doping a basalt with iron, magnesium, and calcium to resemble an exotic planetary composition. By integrating high‐ and low‐temperature viscometry with Raman spectroscopy and ultrasonic data, we show that this chemical enrichment significantly impacts the rheology, elasticity and the structural organization of the doped melt. In particular, viscosity decreases ∼2 times at high temperatures compared to the original basalt. This behavior is driven by the extreme depolymerization of the melt, and it is reflected in a shift toward Q ² and Q ¹ structural units. Vibrational analysis via the Boson Peak confirms a highly fragile state characterized by noticeably small correlation lengths. We tested several widely used semi‐empirical models and found that while traditional empirical formulations struggle to accurately predict the viscosity of these exotic compositions, spectroscopy‐based frameworks provide significantly better accuracy. This performance highlights the fundamental link between atomic‐scale vibrational properties and melt‐scale dynamics.