Out-of-equilibrium growth recorded by mineral inclusions with implications for their suitability as melt proxies: Evidence from chromite

Silicate inclusions in chromite crystals often occur at high population densities and sometimes even share a common shape preferred orientation. These inclusions are arranged in a two-dimensional grid, herein referred to as “waffle-iron patterns” (WIPs). WIPs have been observed in the different geological contexts where chromite crystals nucleate. Their ubiquity and exotic mineralogy raise questions about their origin. Crystallographic data acquired on WIP-bearing chromite crystals demonstrate a systematic host:inclusion crystallographic orientation relationship. It is shown that WIPs result from syngrowth epitaxial nucleation of silicates onto initially skeletal chromite crystals; thus, they are markers of cryptic and transient out-of-equilibrium magmatic systems. The implication is that chromite-hosted silicate inclusions may not be reliable proxies for the early stages of magmatic systems.