Diversity among fossil micrometeorites in the late Devonian

A total of 1222 Micrometeorites (MMs) from the late Devonian period were extracted from 26 kg of carbonates host rock fragments from the Chanxhe section in Belgium, from the Latest Famennian around 360 Myr, through magnetic separation and optical picking following dissolution with mild HCl, making it one of the largest fossil MMs collection, the largest from the late Devonian. The collection shows a wide diversity of texture, comparable to modern day collection but with different distribution. The majority of the MMs were I-type (90 %), with G-type particles constituting 6 % and S-type particles at 1 %. Some of the S-types spherules are amongst the first silicate-type spherules, and amongst the most well-preserved in terms of texture and composition, to be described in fossil MMs collections. Additionally, intermediate type G/I representing <1 % of the sample are introduced for future fossil MMs classification.

Distinguishing extraterrestrial (ET) MMs from terrestrial spherules is challenging due to weathering effects that modify both texture and composition during long residency time on Earth. The Na₂O + K₂O versus Fe/Si ratio plot is used for distinguishing ET from terrestrial spherules. Using textural and compositional data in combination creates a reliable ET spherule identification.

I-type spherules show significant terrestrial alteration with notable loss of Ni and Cr, also observed in S-type spherules, with their silicate phases recrystallized in palagonite. G-type spherules display a mix of characteristics from I-type and S-type MMs. The study also highlights the presence of smaller spherules (<125 µm) compared to modern micrometeorites (210–330 µm), attributed to the predominance of I- and G-type spherules and long-term dissolution effects.

Despite some alteration for some spherules, due diagenesis of the sedimentary host rocks, the collection shows extremely well-preserved spherules, with even some oxygen isotopes signature being preserved. Indeed, triple oxygen isotope analysis reveals that 5.8 % of the particles are related to ordinary chondrites (OC) and 33 % to carbonaceous chondrites (CCs), yielding a CC/OC ratio of approximately 5.6, with comparable distribution for all major types. Also, 9 % of I- and G/I-types are OC-related. Most I-type spherules likely originate from CM, CR, or H chondrites, with some possibly from iron meteorites. The findings suggest that the source materials of the ET flux have remained relatively consistent over the past 360 Myr, providing insights into historical Solar System events and Earth’s environmental changes and extends the study of ET flux to Earth to CC compared to meteorites. In addition, combined with chemical and isotopic proxies and chrome spinel, the fossil MMs could assess the complete flux of cosmic dust to Earth. Finally, the use of fossil MMs could represent potential proxies for paleo-atmospheric oxygen levels and CO₂ contents.