Changes in Parent Body Sources of Extraterrestrial Material Through Geological Time: Insights from Late Devonian Fossil Micrometeorites

Introduction: Micrometeorites (MMs) represent the dominant influx of extraterrestrial (ET) material to Earth today, originating primarily from asteroid belt sources and to a lesser extent from comets. These sub-millimeter particles offer a unique and underexploited archive for reconstructing the compositional diversity and delivery mechanisms of Solar System material through time. The MM record highlights a broader diversity of source material than that observed from the meteorite record, allowing to assess a more complete parent body source than meteorites. While modern micrometeorite collections have documented the present-day relative contributions of various parent bodies, a robust record of how these contributions may have varied across geological times remains missing.

Most existing reconstructions of the past ET flux rely on indirect or bulk geochemical proxies, such as iridium enrichment, ³He content, osmium isotopes, and extraterrestrial spinel grains, that reflect only part of the general ET  flux [1–4], obscuring finer temporal or source-related variations. Direct recovery of fossil micrometeorites, preserved in ancient sediments, offers a promising alternative. These spherules retain petrologic, geochemical, and isotopic information that can be linked to specific parent body types (e.g., carbonaceous, ordinary, or enstatite chondrites and achondrites [5]). When systematically extracted and analyzed across stratigraphic sequences, fossil micrometeorites offer a time-resolved record of parent body source variations, shedding light on the evolving composition of the interplanetary dust complexand the processes governing extraterrestrial material delivery to Earth.

In this study, we present the first fossil micrometeorite dataset for the Late Devonian (~360 Ma). Our aim is to reconstruct the source diversity of extraterrestrial material reaching Earth during this period, evaluate possible changes in the parent body contributions through time, and explore the implications for asteroid belt dynamics and collisional evolution.

Methods: To assess reproducibility and rule out local terrestrial effects on micrometeorite preservation, we studied the micrometeorites flux variations in multiple contemporary locations. Two well-preserved carbonate section in Belgium (Chanxhe and Royseux) were studied and compared to a section in Drewer in Germany. We collected more than 50 kg of sediment spanning over an interval of <2 Myr. This interval corresponds to a time of known climatic and biotic turnover, yet the extraterrestrial dust flux during this period remains uncharacterized to date.

A novel fossil micrometeorite extraction protocol is applied, which is designed to reduce size and magnetic biases common in other methods. The procedure involves dissolution of carbonates using dilute HCl, magnetic separation, manual optical picking under a Zeiss Stereoscope Discovery V.20 binocular microscope to isolate spherules based on morphology, texture, and metallic luster. The recovered particles were characterized using a JEOL JSMIT300 SEM-EDS for petrographic classification and to observe potential weathering evidence. Next, electron microprobe (EMPA) analyses provided an accurate geochemical quantification, facilitating classification of spherules into textural and chemical types and enabling study of the terrestrial weathering effects. Finally, secondary ion mass spectrometry (SIMS) was employed for triple oxygen isotope analysis, enabling comparison to known meteorite groups and constraining the parent body origin. The dataset will be expanded to include trace element and non-traditional stable isotopic data, in order to better understand the effect of terrestrial residency as well as constrain the Solar System source.

Results: The present study provides the first fossil micrometeorite collection recovered from Devonian samples, representing one of the largest geochemical and isotopic compositions dataset of fossil micrometeorites to date. The Late Devonian fossil micrometeorites show smaller size than modern equivalents, with an overabundance of I- and G-type particles. Despite extensive weathering, S-type spherules are present, possibly reflecting a favorable preservation condition. Triple oxygen isotope analysis reveals a CC/OC (carbonaceous/ordinary chondrite) ratio of ~5.6, with 33% of particles linked to carbonaceous sources and 5.8% to ordinary chondrites (compared to ~3.6 CC/OC ratio, with 61% related to CC and 17% related to OC for modern collections [6]). Most I-types likely derive from CM, CR, or H chondrites, with a possible contribution from iron meteorites.

We also identify potential variations in the extraterrestrial flux across the Late Devonian. Additional conventional proxies are planned to be constrained (iridium concentrations, osmium isotope ratios, ³He content, and extraterrestrial spinels), which allows to discuss the differences between the various characterized proxies, proposing an effective multiparameter method for future extraterrestrial flux reconstructions.

Discussion: The recovered fossil micrometeorites dating back to more than 352 million years ago, show that parent body sources have remained broadly consistent over time for micrometeorites, dominated by CC parent bodies, in contrast with meteorite records that have been dominated by OC L-type since the mid-Ordovician large asteroid break-up event [7]. Tracing variations in the flux composition could offer clues to ancient Solar System dynamics and Earth’s environmental conditions, and provide direct evidence that the flux of extraterrestrial material to Earth has varied over geological time, even outside time intervals characterized by major impact events.

The variations in the abundance of micrometeorites linked to different chondritic groups may reflect changes in the collisional history or dynamical stability of specific asteroid families. Periods of increased carbonaceous input could correspond to disruption events in the outer main belt or comet-like activity from transitional bodies.

Conclusion: The use of fossil micrometeorites represents a novel tool for the study of the Solar System history. By establishing stratigraphically resolved fossil micrometeorite datasets from multiple ages and locations, we can begin to trace how the composition and structure of the asteroid belt, and possibly cometary reservoirs, have evolved through deep time.

 

 

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