¹⁶O poor cosmic spherules from near-Earth CY chondrite asteroids.

Abstract

Approximately 10% of cosmic spherules-microscopic extraterrestrial particles that melt upon atmospheric entry and dominate the influx of astromaterials to Earth-exhibit anomalous oxygen isotopic compositions, suggesting an asteroidal source not represented in current meteorite collections. We introduce a a previously unidentified subset of micrometeorites, the sulfur-rich cumulate olivine (SCumPo) cosmic spherules, characterized by cumulate textures evidencing the settling of olivine crystals, and oxygen-16 (¹⁶O)-poor bulk signatures. The systematically nickel-poor olivine phenocrysts, frequent iron-nickel-sulfur droplets, unusually sulfur-rich mesostasis, and a virtual absence of magnetite all point to unusually highly reducing conditions during atmospheric entry, which may reflect unusual precursor mineralogy. Numerical modeling of olivine settling under deceleration speeds of ~14 to 17 kilometers per second suggests high-eccentricity precursor orbits (<i>e</i> > 0.2), incompatible with typical main-belt asteroid sources. These findings point to a previously unsampled, primitive, sulfide-rich CY-like near-Earth asteroid, which represents a "missing" meteorite parent body that contributes distinctive ¹⁶O-poor cosmic dust to Earth.