Fossilised micrometeorites revealed

Novel technique to identify abundant cosmic dust in ancient sediments

The study of micrometeorites—sub-millimetre pieces of debris from asteroids and comets that are found on Earth—can provide insights into the nature of their parent bodies and the Earth’s upper atmosphere. Modern ‘cosmic dust’ particles are relatively easy to capture and examine, but it is much more challenging to study the ancient flux of micrometeorites to Earth. Indeed, pristine micrometeorites are notoriously difficult to identify and separate from their host rocks. In work recently published in Earth and Planetary Science Letters, however, scientists have described and successfully tested a new micrometeorite identification and separation methodology, with which they recovered more than 70 cosmic dust particles from Cretaceous chalk. The distinctive morphologies and textures of the micrometeorite spherules were clearly identifiable from high-resolution electron microscope images obtained as part of the study. Furthermore, from Raman spectroscopy measurements conducted in the offline support Characterisation Laboratory (Lab 91) at Diamond Light Source, it was found that the mineralogy of the particles was distinct from that of modern cosmic dust. The results are thus evidence for replacement of the micrometeorite mineral phases during diagenesis (fossilisation). The authors of the study believe that these ‘pseudomorphic’ micrometeorites should be common within the geologic record and can be recognised through careful observations of particle textures.

     


Figure 1: External (left) and internal (right) scanning electron microscope images of recovered fossilised micrometeorites, showing dendritic textures that are characteristic of quench-cooled cosmic dust particles. Both particles have a diameter of less than a tenth of a millimetre.


Cosmic time travellers

A continuous rain of cosmic dust, through the atmosphere to the Earth’s surface, is thought to have persisted throughout much of the Solar System’s history. This body of material thus has the potential to provide key information about micrometeorite parent bodies (e.g. asteroid formation and collisional histories), as well as the evolution of the Earth’s upper atmosphere. The vast majority of micrometeorite research, however, is conducted on recently fallen particles (e.g. dust recovered from Antarctic ice sheets or from deep-sea sediments). The insights that can be gleaned from the collected debris therefore does not span the full 4.5-billion-year history of the Solar System.

Nevertheless, several groups of scientists have recently started to search sedimentary rocks, of different ages, from around the world for ‘fossil’ micrometeorites. Ten such collections have been reported to date and these provide constraints, for example, on the past cosmic dust flux1, the timing of asteroid family formation2,3, and the ancient composition of the Earth’s upper atmosphere4. In these previous studies, the micrometeorites were identified on the basis of their distinctive mineralogies, compositions, and textures. The rare occurrence of such pristine samples, however, means that many cosmic dust particles in the geologic record are likely overlooked.

Novel identification method

In the new work, led by PhD student Martin Suttle (Imperial College London and The Natural History Museum), a new approach for the identification of fossilised micrometeorites was tested on about 9 kg of Cretaceous (about 87-million-year-old) chalk, taken from the UK’s North Downs. A combination of acid digestion and mechanical disaggregation was first used to obtain the <500 μm particle-size fraction of the chalk. Seventy-six spherical cosmic dust grains were then successfully removed via magnetic separation and optical picking, before undergoing in-depth analyses.

Mr Suttle and his team used electron beam techniques (at the Natural History Museum) to obtain high-resolution images [see Fig. 1] and chemical analyses for the recovered micrometeorites. To determine the specific mineral species within each dust particle, the scientists also conducted Raman spectroscopy measurements in Diamond’s offline support Characterisation Laboratory (Lab 91). The Raman results indicate that the mineralogy of the micrometeorites consists mainly of iron oxide phases, including manganese-bearing magnetite, and wüstite. These mineral assemblages are substantially different from those of modern cosmic dust particles, i.e. that commonly consist of silicates and iron metal. “This is one of our most exciting results”, explains Mr Suttle, “it means we identified the micrometeorites on the basis of their texture alone, and that these are true fossils”. In other words, a fossilisation (diagenetic) process has converted the original micrometeorites to the recovered fossil particles, yet their distinctive textures and morphologies—characteristic of micrometeorites—have been preserved.

Implications

The results of this study demonstrate that cosmic dust is more common in the geologic record than previously thought. Moreover, the novel identification approach is robust and should be employed to identify and separate micrometeorites from sediments in future work.

This chemical and mineralogical investigation of fossil micrometeorites also sheds light on the diagenesis processes that transformed mud deposits to chalk during the Cretaceous. That is, the presence of cosmic dust—with a range of chemistries and mineralogies—in the localised chalk deposits indicates that both oxidation and reduction processes were occurring on small spatial scales during the chalk formation.

Studying micrometeorites may also have economic value, posits study co-author Dr Matthew Genge (Imperial College London and the Natural History Museum), “as cosmic dust—if dredged from the Earth’s oceans—could present a substantial reservoir of precious metals, such as nickel and cobalt”.
 

To find out more about Diamond’s offline spectroscopy support laboratory facilities, please contact Prof Fred Mosselmans fred.mosselmans@diamond.ac.uk

Related publication:

Suttle MD and Genge MJ. Diagenetically altered fossil meteorites suggest cosmic dust is common in the geological record. Earth and Planetary Science Letters 476, 132 – 142 (2017). DOI:10.1016/j.epsl.2017.07.052.

References:

 1. Onoue T et al. Composition and accretion rate of fossil micrometeorites recovered in Middle Triassic deep-sea deposits. Geology 39, 567–570 (2011).
2.  Schmitz B et al. A rain of ordinary chondritic meteorites in the early Ordivician. Earth Planet. Sci. Lett. 194, 1–15 (2001).
3.  Heck PR et al. Noble gases in fossil micrometeorites and meteorites from 470 Myr old sediments from southern Sweden, and new evidence for the L-chondrite parent body breakup event. Meteorit. Planet. Sci. 43, 517–528 (2008).
4. Tomkins AG et al. Ancient micrometeorites suggestive of an oxygen-rich Archaean upper atmosphere. Nature 533, 235–238 (2016).