Constraints on the origins of iron silicide spherules in ultrahigh‐temperature distal impact ejecta

Abstract

AbstractTerrestrially occurring iron silicide spherules, described in the geological literature for 160 years as cosmogenic and approved as “extraterrestrial” minerals by IMA CNMMN in 1984, so far have escaped any serious examination by meteoriticists. Our isotopic and REE data, obtained for silicide spherules for the first time, disagree with the meteoritic origin of gupeiite (Fe3Si) and xifengite (Fe5Si3) spherules from two continents. Despite departures from terrestrial norms (87Rb/86Sr—0.0174; 87Sr/86Sr—0.700181; 3He/4He—7.57 × 10−6; 40Ar/36Ar—325.9), the compositions of 143Nd/144Nd (0.512034) and 147Sm/144Nd (0.06357), as well as REE abundances, clarify provenance from upper crust sediments for samples with U/Pb age of 121–314 ka from the Ala‐Tau range in the Urals. However, the morphology of flanged button shapes, ring waves, and eccentro‐radiating ridges reliably constrains the origin of silicide spherules to distal meteoritic impact ejecta. Arc jet ablation experiments have previously demonstrated that similar morphologies, observed on australite tektites, reflect aerodynamic ablation rates corresponding to flight velocities well into orbital range. These features are generally accepted as conclusive evidence for hypervelocity atmospheric entry from space. Internal structure, consistent with accretion through the coalescence of 3–5 µm droplets, and composition, closely corresponding to 1893–1154 K span of C‐type condensation sequences, indicate a high probability of processing through recondensation of ejecta vapor.