Fossil micrometeorites from Monte dei Corvi: Searching for dust from the Veritas asteroid family and the utility of micrometeorites as a palaeoclimate proxy

We searched late Miocene sedimentary rocks in an attempt to recover fossil micrometeorites derived from the Veritas asteroid family. This study was motivated by the previous identification of a pronounced 3He peak (4-5x above background) within marine sediments with ages between ∼8.5–6.9 Ma ago (Montanari et al., 2017. GSA Bulletin, 129:1357–1376). We processed 118.9 kg of sediment from the Monte dei Corvi beach section (Italy), the global type-section for the Tortonian epoch (11.6–7.2 Ma). Samples were collected both before and within the 3He peak. Although a small number of iron-rich (I-type) fossil micrometeorites were recovered from each horizon studied (Ntotal = 20), there is no clear difference between the pre- and intra- 3He peak samples. All micrometeorites are compositionally similar, and three out of five horizons yielded similar abundances and particle sizes. Micrometeorites extracted from sediments at the base of the 3He peak were exclusively small (ø 3He values were relatively large (ø 3He signature combined with the absence of fossil micrometeorites or extraterrestrial spinels (Boschi et al., 2019, Spec. Pap. Geol. Soc. Am. 542:383–391) unambiguously related to the Veritas event suggests that the Veritas family is composed of highly friable materials that rarely survive on the sea floor to become preserved in the geological record. Our data supports the existing hypothesis that the Veritas asteroid family is an aqueously altered carbonaceous chondrite parent body, one that contains minimal native metal grains or refractory Cr-spinels. The low yield of fossil micrometeorites at Monte dei Corvi is attributed to loss of particles by dissolution whilst they resided on the sea floor but also due to high sedimentation rates leading to dilution of the extraterrestrial dust flux at this site. As with other fossil micrometeorite collections (e.g. Cretaceous chalk [Suttle and Genge, EPSL, 476:132–142]) the I-type spherules have been altered since deposition. In most particles, both magnetite and wüstite remain intact but have been affected by solid state geochemical exchange, characterised by partial leaching of Ni, Co and Cr and implantation of Mn, Mg, Si and Al. In some particles Mn concentrations reach up to 16.6 wt%. Conversely, in some micrometeorites wüstite has been partially dissolved, or even replaced by calcite or ankerite. Finally, we observe evidence for wüstite recrystallisation, forming a second generation of magnetite. This process is suggested to occur by oxidation during residence on the seafloor and has implications for the use of fossil I-type micrometeorites as a potential proxy for probing Earth’s upper atmospheric composition (oxidative capacity) in the geological past. However, solutions to the limitations of post-depositional recrystallisation are suggested. Fossil I-type spherules remain a potential tool for palaeo-climatic studies

Origin of radiogenic 129Xe variations in carbonaceous chondrites

International audienceCarbonaceous chondrites are pristine witnesses of the formation of the solar system. Among them, the carbon-rich Tarda and Tagish Lake meteorites are thought to have sampled very distant regions of the outer circumsolar disk (Hiroi et al., 2001 ). Here, we show that their noble gas isotopic compositions (especially 129Xe excesses) are similar, implying their formation in comparable environments. Combined with literature data, we show that the radiogenic excesses of 129Xe relative to solar wind in carbonaceous chondrites define anti-correlations with their respective iodine and carbon contents. These trends do not result from the heterogeneous distribution of 129I in the disk but rather evidence a xenon dilution effect; the radiogenic 129Xe excesses being dominated by trapped xenon in the most carbon-rich carbonaceous chondrites. Our data also suggest that both Tarda and Tagish Lake accreted beyond 10 astronomical units, in regions of the disk that were cold enough for CO2 to condense

A Consortium status report: the characterisation of the asteroid Itokawa regolith - a correlated study by X-ray tomography, micro-raman spectroscopy, and high-sensitivity noble gas analysis

Precious samples from S-type as-teroid 25143 Itokawa have been sampled by the JAXA (Japanese Space Agency) Hayabusa mission in 2005 and returned to Earth in 2010. Itokawa is, succeeding the Moon and comet Wild 2, the third planetary body successfully probed by a sample return mission. The initial studies revealed that Itokawa consists mostly of type LL5-6 material. It experienced severe surface alteration due to space weathering, as docu-mented by surficial, nanosize S-and Fe-bearing phases in some grains. Noble gas studies indicate that Itokawa experiences a surprisingly intense surface loss at a rate of tens of cm/Ma, implying that Itokawa (largest dimension ~540 m) will be destroyed quickly. We received material through JAXA in Sept. 2012 and aim to analyze noble gases in Itokawa samples with high sensitivity, including Kr and Xe, which could not be studied previously, because of the low concentrations. We will combine the noble gas studies with scanning microRaman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), and synchro-tron radiation X-ray tomographic microscopy (SRXTM). These provide non-destructive characteri-zations of grain density, mineralogy, structure, and potential space weathering, which are essential to as-sess gas concentrations of potentially present cosmogenic, solar, trapped and radiogenic compo-nents. Here, we summarize the work of our consortium performed so far. Further studies will be presented at the meeting

Raman spectroscopy of HAYABUSA particle RA-QD02-0051

Raman microscopy measurements are performed on Hayabusa sample RA-QD02-0051, provided by JAXA to our consortium in the scope of the 1st International Announcement of Opportunity. Raman microscopy is applied to identify the mineral phases of the particle. The interpretation of the spectra shows that the sample consists of olivine, pyroxene and feldspar

A Record of Early Precompaction Exposure of Hibonites to Energetic Particles: Evidence from Spallogenic Helium-3 and Neon-21

We find evidence for pre-exposure (excess 21Ne and 3He) exclusively in PLACs. This suggests that they were irradiated before incorporation into their parent body

High early solar activity inferred from helium and neon excesses in the oldest meteorite inclusions

Astronomical observations show that early in their evolution, stars experience stages of high activity associated with enhanced energetic particle fluxes. The Sun’s early activity is often inferred from the spallogenic isotope record (for example, 10Be) in the Solar System’s oldest materials, calcium–aluminium-rich inclusions (CAIs) in meteorites. However, the 10Be record could be affected by processes other than in situ irradiation by solar particles5. Noble gases can give less ambiguous insights because they are inert volatiles and hence not incorporated into CAIs during their formation. Here we show that hibonite-rich CAIs, considered to have formed before 26Al-rich CAIs, contain helium and neon excesses that can be unambiguously attributed to in situ irradiation by energetic particles. Given their volatile nature, we infer that the noble gases were produced by irradiation in a relatively cold region at a considerable distance from the Sun (not at the inner disk edge), requiring high particle fluxes and thus high early solar activity. Because more evolved CAIs lack comparable noble gas irradiation records, we conclude that the oldest Solar System materials experienced a phase of intense irradiation not recorded by materials that formed later. Consequently, disk properties or energetic particle fluxes changed significantly during the very early phases of Solar System evolution

Raman microscopy of Hayabusa particle RA-QD02-0051

Raman microscopy measurements are performed on Hayabusa sample RA-QD02-0051, provided by JAXA to our consortium in the scope of the 1st International Announcement of Opportunity. Raman microscopy is applied to identify the mineral phases of the particle. The interpretation of the spectra shows that the sample consists of olivine, pyroxene and feldspar