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

Silicate melt viscosity plays a pivotal role in the evolution of rocky bodies within the SolarSystem, exerting first‐order control on mantle differentiation and stratification, while dictating the metal‐silicateseparation and the subsequent volcanic activity. Thus, predicting the viscosity of planetary compositions isessential to model and better understand their thermomechanical evolution. Notably, the chemical signatures ofprimordial planetary magmas, marked by extreme Fe, Mg, and Ca enrichment, drive a highly depolymerizedand 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 toresemble an exotic planetary composition. By integrating high‐ and low‐temperature viscometry with Ramanspectroscopy 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 hightemperatures compared to the original basalt. This behavior is driven by the extreme depolymerization of themelt, and it is reflected in a shift toward Q2 and Q1 structural units. Vibrational analysis via the Boson Peakconfirms a highly fragile state characterized by noticeably small correlation lengths. We tested several widelyused semi‐empirical models and found that while traditional empirical formulations struggle to accuratelypredict the viscosity of these exotic compositions, spectroscopy‐based frameworks provide significantly betteraccuracy. This performance highlights the fundamental link between atomic‐scale vibrational properties andmelt‐scale dynamics.