INTRODUCTION
Spherules of iron silicide, an exceedingly rare mineral, alien to terrestrial conditions and native to carbon stars, have puzzled scientists since their first discovery in 1859. These enigmatic spherules with aerodynamic ablation features, found in sedimentary rocks of various geological ages, seemed out of place on Earth. Over a century and a half went by before previously unavailable isotopic, chemical, and morphological data solved the mystery.
A groundbreaking study published in the peer-reviewed Meteoritics and Planetary Science journal in August 2021 has revealed conclusive evidence that iron silicide spherules belong to a previously unknown family of ultrahigh-temperature distal impact ejecta objects, formed beyond the atmosphere. The study showed that samples from the Ala-Tau range in the Southern Urals are a product of superheated vapor ejected with cosmic velocity thousands of miles into space as the result of an asteroid impact of catastrophic scale over 300,000 years ago. In the cold of space, the ejected gas condensed into droplets producing an aggregate of extremely rare cosmogenic minerals in the shape of spherules. These spherules ablated on hypervelocity atmospheric re-entry.
Unlike meteorites, composed of mundane minerals common on Earth, or tektites not much different from volcanic glass, Ala-Tau silicide spherules are composed of exotic mineral species known exclusively from space. Similar cosmogenic silicides and carbides occur in some rare meteorites only as micro-inclusions. Ata-Tau spherules represent a variety of iron silicide minerals. Gupeiite, Xifengite, Hapkeite, Linzhiite, and Naquite mineral phases are some of the known cosmogenic compounds present. Seven previously unknown minerals are represented by several variations of uranium-bearing silicides and silicon-rich carbides of titanium, aluminum, zirconium, or calcium composition. No wonder. The extreme conditions of condensation in space have turned the vapor of terrestrial impact origin into minerals alien to Earth. Even more. Silicide spherules are the only tangible samples of cosmogenic mineral species available to appreciate with the naked eye.
Spherules of iron silicide, an exceedingly rare mineral, alien to terrestrial conditions and native to carbon stars, have puzzled scientists since their first discovery in 1859. These enigmatic spherules with aerodynamic ablation features, found in sedimentary rocks of various geological ages, seemed out of place on Earth. Over a century and a half went by before previously unavailable isotopic, chemical, and morphological data solved the mystery.
A groundbreaking study published in the peer-reviewed Meteoritics and Planetary Science journal in August 2021 has revealed conclusive evidence that iron silicide spherules belong to a previously unknown family of ultrahigh-temperature distal impact ejecta objects, formed beyond the atmosphere. The study showed that samples from the Ala-Tau range in the Southern Urals are a product of superheated vapor ejected with cosmic velocity thousands of miles into space as the result of an asteroid impact of catastrophic scale over 300,000 years ago. In the cold of space, the ejected gas condensed into droplets producing an aggregate of extremely rare cosmogenic minerals in the shape of spherules. These spherules ablated on hypervelocity atmospheric re-entry.
Unlike meteorites, composed of mundane minerals common on Earth, or tektites not much different from volcanic glass, Ala-Tau silicide spherules are composed of exotic mineral species known exclusively from space. Similar cosmogenic silicides and carbides occur in some rare meteorites only as micro-inclusions. Ata-Tau spherules represent a variety of iron silicide minerals. Gupeiite, Xifengite, Hapkeite, Linzhiite, and Naquite mineral phases are some of the known cosmogenic compounds present. Seven previously unknown minerals are represented by several variations of uranium-bearing silicides and silicon-rich carbides of titanium, aluminum, zirconium, or calcium composition. No wonder. The extreme conditions of condensation in space have turned the vapor of terrestrial impact origin into minerals alien to Earth. Even more. Silicide spherules are the only tangible samples of cosmogenic mineral species available to appreciate with the naked eye.
Cosmogenic Silicide Spherules
SILICIDE SPHERULES FROM ALA-TAU
For description click on the image to enlarge.
For description click on the image to enlarge.
WHAT IS CSS?
Origin
Cosmogenic silicide spherules (CSS) are products of ultrahigh-temperature distal impact ejecta to space. The composition of CSS derives from a mixture of crustal and extraterrestrial materials vaporized in a hypervelocity asteroid impact. The spherules condensed in Earth’s orbit from the impact vapor plume ejected into space beyond the atmosphere. The condensed objects ablated on atmospheric re-entry at a cosmic velocity exceeding 14 km/s.
Scientific significance
CSS samples present physical evidence of large-scale catastrophic impacts. They offer a unique first-hand perspective into impact vapor ejecta to space – a previously neglected phenomenon in the science of impact studies. CSS samples recovered from the Ala-Tau range in the Southern Urals were dated by the U/Pb isotopic method to an impact event 121-314 thousand years ago.
Scientific implications
The occurrence of CSS in sedimentary rocks of various ages indicates that large-scale cosmic impacts of catastrophic magnitude are far more frequent in geological history than proposed by evidence of cratering discovered so far. The velocity of atmospheric re-entry of CSS suggests that the bulk of enormous kinetic energy, released in the large-scale impacts, is irrevocably disposed into space rather than the atmosphere. This material brings into question the ability of cosmic collisions to disrupt global ecosystems or cause mass extinctions.
Reference
Peer-reviewed publication – Constraints on the origins of iron silicide spherules in ultrahigh-temperature distal impact ejecta. Sergei Batovrin, Boris Lipovsky, Yury Gulbin, Yury Pushkarev, Yury A. Shukolyukov. 2021. Meteoritics and Planetary Science, vol. 56, issue 7, pp.1369-1405.
Open access article
BRIEF NOTES ON CSS SAMPLES FROM ALA-TAU LOCATION
Extreme rarity
Naturally occurring silicides are inter-metallic compounds that belong to the rarest mineral group in the Solar System. Oxygen is excluded from the composition of these highly reduced mineral phases impervious to oxidation. The formation of silicide minerals requires extreme temperatures absent in terrestrial geological processes. Cosmogenic silicide spherules (CSS) are the only known crystalline material that formed in Earth’s orbit by condensation of impact ejecta vapor.
Cosmogenic mineral assemblage
While meteorites are composed of minerals common on Earth, Ala-Tau silicide spherules represent exotic mineral species known exclusively from space. Similar cosmogenic silicides occur as micro-inclusions on micrometre scale in some ureilite and aubrite meteorites, or in the lunar regolith. CSS samples from the Ata-Tau location are composed of a variety of iron silicide mineral phases. Gupeiite, Xifengite, Hapkeite, Linzhiite, and Naquite are among the known cosmogenic mineral species present. Seven previously unknown minerals are represented by several versions of uranium-bearing silicides and silicon-rich carbides of titanium, aluminum, zirconium, or calcium composition.
Unusual isotopic composition
The formation process of cosmogenic silicide spherules from Ala-Tau is reflected in unusual isotopic composition. While the isotopic ratio of uranium and lead in the examined silicide spherules matches the meteoritic standard, and the ratio of rubidium and strontium is comparable to values for achondrites, the origin of these exotic objects is evident from the complexity of the composition. Departures from terrestrial norms in isotopic compositions of argon, and hydrogen are one side of the coin. On the flip side, the isotopic ratios of samarium and neodymium point to upper-crust sediments as precursor material. Such dichotomy in isotopes is a testament to impact origin.
Atmospheric entry record
The morphology of silicide spherules closely reproduces aerodynamic features considered unique for australite tektites. Melt-flow surface textures forming radiating ridges, ring-waves, and equatorial flanges, which make silicide spherules look like buttons, can unambiguously be ascribed to atmospheric entry only. Arc-jet ablation experiments have previously demonstrated that similar surface features, observed on australite tektites and meteorite models, reflect aerodynamic ablation rates corresponding to flight velocities well into the orbital range. These features are universally accepted as conclusive evidence for hypervelocity atmospheric entry from space.
Ablation features
Ala-Tau samples are represented as spherules (0.05 mm – 10 mm) and as irregular-shaped particles (maximum 2.7cm). Spherules display aerodynamic features similar to typical shapes and surface textures of tektites. These objects demonstrate sculptured anterior surfaces, melt-flow ridges, radiating flow striations, and other characteristics associated with aerodynamic ablation during a hypervelocity atmospheric entry. Often samples have flight-oriented shapes. They also display a thin layer of dark fusion crust as an affirmation of aerodynamic origin. Due to the common occurrence of moderately dark rims, the strong metallic luster of iron silicide is visible primarily on fragments of broken spherules.
Aerodynamic restriction
Cosmogenic silicide spherules are aerodynamically restricted to 10 mm in diameter. Samples sized 10 mm-25 mm are fragments of large spherules that exploded in hypervelocity atmospheric passage. These irregularly shaped particles with the brilliant metallic luster of iron silicide constitute 75% of CSS samples from the Ala-Tau location. 14% of Ala-Tau samples demonstrate the articulated shape of spherule fragments with partial fusion crust and aerodynamic features. Only 11% of CSS samples from Ala-Tau are complete spherules with fusion crust and aerodynamic ablation features on the surface.
Chemical stability
Iron silicide is impervious to weathering. It is chemically inert to aqueous solutions of any kind. Iron silicide is insoluble in oxidizing and non-oxidizing acids with the exception of concentrated heated hydrofluoric acid. Unlike meteorites, vulnerable to weathering and rust, cosmogenic silicide spherules do not require any measures of conservation.
Availability of CSS samples
Over 50 terrestrial occurrences of cosmogenic silicide spherules with aerodynamic ablation features were reported in scientific literature since the first discovery in 1859 by C.U. Shepard in North Carolina. However, the reported silicide samples are predominantly sub-mm or difficult to recover by drilling at significant burial depths in hard sediments. CSS samples of the large size (5-25 mm) are exceedingly rare. The combined weight of all CSS recovered worldwide since 1859 is under 7 kg. Among these, 4.8 kg was collected at the Ala-Tau site in the Southern Urals. They are held in scientific institutions and are designated for studies.
The provenance of CSS samples in the Radzinsky collection
Two decades ago Oleg Radzinsky, a British banker, privately supported scientific expeditions to the Ala-Tau range – a CSS strewn field in the Ural mountains. In 2003, as a token of appreciation for this contribution to silicide research, his longtime friend Sergei Batovrin, the lead author of the published study, presented 250 g of Ala-Tau silicide samples to the Radzinsky Collection. These 250 g are the only CSS in private hands.
We would like to thank the Radzinsky collection for providing photographs of silicide spherules for illustration purposes.