Formation, cooling history and age of impact events on the IIE iron parent body: evidence from the Miles meteorite

Rachel S. Kirby, Penelope L. King, Marc D. Norman, Trevor R. Ireland, Margaret Forster, Arthur D. Pelton, Ulrike Troitzsch, Nobumichi Tamura

Geochimica et Cosmochimica Acta
In Press, Journal Pre-proof, Available online 28 October 2022


“Most iron meteorites formed in planetary cores during differentiation, but the IIE iron meteorites have chemical and physical features that are inconsistent with this origin. By combining mineral chemistry, mineral modes and three-dimensional petrography, we reconstruct the bulk chemistry of the felsic silicate-bearing Miles IIE iron meteorite and demonstrate that the silicate inclusion compositions are similar to partial melts produced experimentally from an H chondrite composition. We use the reconstructed bulk composition, mineralogy and thermodynamic modelling to show that melting above ∼1200 °C under reducing conditions formed metal (Fe-Ni alloy) and felsic silicate partial melts. Upon cooling, the melts crystallized Mg-rich pyroxenes, Na- and K-rich feldspars, and tridymite. Importantly, this mechanism enriches cosmochemically volatile elements (i.e., those with a 50% condensation temperature of ∼430-830 °C, like Na and K) to the level found in the felsic silicate inclusions.

The presence of crystallographically disordered srilankite (only stable above 1160 °C) and an absence of Widmanstätten texture require both high peak temperatures and rapid cooling, which cannot be explained by core formation Instead they point to small melt volumes, a transient heat pulse, and small thermal mass, and imply efficient physical segregation of silicate and metallic melts through buoyancy separation followed by rapid cooling that arrested the separation of metal and silicate liquid phases. In situ 207Pb/206Pb ages of 4542 ± 4.0 Ma in Zr-oxide minerals determined here date the melting event that formed the silicate inclusions. This age aligns with the earliest ages found in other IIE iron meteorite silicates and requires a heating event ∼25 million years after the solar system formed. We found 39Ar/40Ar ages of 3495 ± 52 Ma (low-T) and 4303 ± 7 Ma (high-T) in a K-feldspar grain, with the 3495 Ma age aligning with later thermal events recorded in other IIE iron meteorites. Dating reveals the complex petrogenetic and thermal history of Miles and the IIE iron meteorites. This is the first IIE iron meteorite found to record evidence of impact bombardment at 4.5 and 3.5 Ga.

High-velocity impact(s) into an iron-rich, porous chondritic parent body at ∼4.54 Ga produced immiscible metal and silicate melts that cooled rapidly and trapped low density silicate inclusions within high density metal. Other IIE irons that formed at lower peak temperatures (900-1000 °C) contain chondritic silicate inclusions and relict chondrules, supporting this conceptual model. Our model is consistent with thermodynamic modelling, experimental data and the wide range of peak temperatures and cooling rates observed in the IIE iron meteorites.”