Substitution of Fe ³⁺ into anorthite in oxidized, Al‐deficient ferrobasaltic systems with implications for the petrogenesis of angrite meteorites

Abstract Angrite meteorites are critically silica‐undersaturated igneous rocks with high Ca/Al and Fe/Mg, along with depletion in volatile lithophile elements Na and K such that they crystallize anorthite along with olivine and calcic pyroxene. The anorthite in angrites contains substantial Fe, and in NWA 1670 and NWA 1296, it is present at major element levels correlated with deficiency in Al, suggesting abundant Fe 2 O 3 ‐Al 2 O 3 exchange. One possible explanation is that these angrites had higher Fe 3+ contents and their final interstitial melts crystallized under higher oxygen fugacities ( f O 2 ) than those of other angrites. Here, we present melting experiments on angritic compositions over a range of f O 2 conditions. These experiments crystallized anorthite and Ca‐rich pyroxene among other phases, showing that large substitutions of Fe 3+ into Al‐deficient anorthite (a “ferri‐anorthite” component) are natural consequences of angritic melt crystallization at high f O 2 . At face value, these experiments suggest substantial anorthite Fe 2 O 3 ‐Al 2 O 3 exchange in NWA 1670 and NWA 1296 due to fractionated magmas crystallizing near the surface of the angrite parent body under oxidizing conditions. However, the inferred f O 2 is several log units higher than the iron‐wüstite redox buffer and even higher than the fayalite–magnetite–quartz redox buffer. While extensive fractional crystallization might raise f O 2 by ~1 log unit by removal of ferrosilicates, it seems unlikely to explain the occurrence of this phase. The presence of Fe 3+ ‐rich anorthite might be attributed to rapid crystallization driving kinetic incorporation of excess Fe and disequilibrium degassing of SO 2 from a small dissolved sulfate component, leading to metastable preservation of this phase.