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

Abstract

Terrestrially 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 (Fe 3 Si) and xifengite (Fe 5 Si 3 ) spherules from two continents. Despite departures from terrestrial norms ( 87 Rb/ 86 Sr-0.0174; 87 Sr/ 86 Sr-0.700181; 3 He/ 4 He-7.57 × 10 −6 ; 40 Ar/ 36 Ar-325.9), the compositions of 143 Nd/ 144 Nd (0.512034) and 147 Sm/ 144 Nd (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.

Routes to reduction of phosphate by high-energy events

“…The size of the silicide-bearing spherules is much larger than those found previously 25 by about a factor of 10×, which were previously the largest spherules reported in the literature from fulgurites. The size of these spherules (1 cm), however, is comparable to those reported as purported distal impact ejecta 41 .…”

“…The reported presence of much larger iron silicides 41 , 42 in geologic specimens suggests that large spherules are formed by lightning and may be the source of these silicides that have been reported in various locations. For these reasons, we argue the CaHPO 3 could be a natural material and therefore represent a member of a new mineral group, one predicted to occur in the environment from biology and biochemistry 15 .…”