Iron-based eutectics, a valuable geological record

Abstract

Metal iron-based eutectics are common in iron and stony‑iron meteorites, where their presence traces space melting of parent celestial bodies or accompanies atmospheric ablation processes. It is less known that the same structures and compositions occur in situ within telluric iron in terrestrial crustal rocks. We have studied iron-based eutectics confined to native iron assemblages in volcanic rocks of Disko Island, Greenland, and combustion metamorphic lithologies of the Hatrurim Complex, Israel. In addition, Fe-P alloys were synthesized, enabling to reconstruct the composition and cooling rates of the initial melt. Despite the different geological settings of the two occurrences, the highly reducing (several units below iron – wüstite buffer) and high-temperature (∼1200 °C) environment results in the appearance of the same P-, C-bearing iron melts, with subsequent rapid crystallization (crystallization speed up to 5 °C/min) into eutectic structures (iron - cohenite (Fe₃C), iron - schreibersite (Fe₃P), cohenite - schreibersite and iron - barringerite (Fe₂P)). The discovered eutectic structures clearly indicate the possibility of forming iron melt under conditions of low pressure and moderately high temperatures (typical for basalts), which may also be applicable to a number of celestial bodies.