Ilmenite transformations in suevites from the Ries meteorite impact structure, Germany

Glass fragments (Flädle) in suevites from Zipplingen within the Ries (Germany) meteorite impact structure contain round aggregates of polycrystalline ilmenite with various amounts of rutile, ferropseudobrookite (FeTi₂O₅), armalcolite ((Fe,Mg)Ti₂O₅) and titanite (CaTi[OSiO₄]). The 10-100s µm sized aggregates often have a thin rim of µm-sized magnetite grains. The ilmenite grains are 5-10 µm in diameter and form an equilibrium fabric with 4-6-sided grains with smoothly curved grain boundaries and 120° angles at triple junctions, i.e. a so-called foam structure. The ilmenite grains have random crystallographic orientations and do not show any internal misorientations. Rutile, typically a few µm in diameter, is associated with similarly fine-grained ilmenite and a high amount of pores. Coarser polygonal ilmenite grains can also show a marked grain boundary porosity. Only rarely in the center of the aggregates, a deformed single ilmenite crystal occurs, indicating that the aggregates originated from shocked coarse ilmenite crystals from the target gneisses. Ferropseudobrookite is intergrown with remnants of original ilmenite grains or secondary ilmenite grains without foam structure. A vermicular intergrowth of ilmenite, rutile, and magnetite can be present at the rim, where armalcolite can be enriched in Mg.

We interpret that ferropseudobrookite formed at high temperatures (>1010°C) and reducing conditions from coarse ilmenite crystals originating from the target gneisses according to the following reaction: 2FeTiO₃ → FeO + FeTi₂O₅. Some FeO migrated towards the rim due to the low oxygen fugacity, resulting in the observed porosity. Upon cooling, FeO migration caused ferropseudobrookite to disintegrate resulting in the formation of rutile and ilmenite: FeTi₂O₅ → FeTiO₃ + TiO₂. Silicate melt at the contact of the FeTi-oxides provided magnesium to form armalcolite from ferropseudobrookite and calcium to form titanite within fractures. Rapid cooling resulted in a shift in redox-conditions with the formation of pure Fe magnetite from FeO at the rim of the aggregates. Quenching of the system can explain the local preservation of ferropseudobrookite and armalcolite, whereas the ilmenite foam structure formed during back reaction of ferropseudobrookite at relatively slower cooling rates. The different cooling rates in the aggregates can be explained by the locally varying amount of surrounding superheated melt forming the Flädle-structure.