52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548)

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SPECTROSCOPIC CHARACTERISATION OF PRIMITIVE CO3 CARBONACEOUS CHONDRITES
E. Bonato1, A. J. King2, H. C. Bates2,3, P. F. Schofield2, C. Stangarone1, A. Maturilli1, J. Helbert1, M. R. Lee4 and S.
S. Russell2. 1Department of Planetary Laboratories, Institute for Planetary Research, DLR, Berlin, 12489, Germany,(e-mail: enrica.bonato@dlr.de), 2 Department of Earth Science, Natural History Museum, London, SW7 5BD, UK,
3Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford OX1 3PU, UK, 4School of Geographicaland Earth Sciences, University of Glasgow, Glasgow, G12 8QQ, UK.
Introduction: Primitive CO3 carbonaceouschondrite meteorites provide a detailed record of thegeological processes and events that have shaped oursolar system over the last 4.5 billion years. They containa fine-grained (≤ 1 µm) matrix (> 50 vol%) ofamorphous and crystalline silicates, oxides, sulphides
and metals that have remained largely unaltered since
the time they accreted into an asteroid [1].
The reflectance spectra of CO carbonaceouschondrites have linked them to the rare K-type asteroids[2, 3]. However, their low albedos also suggest apossible relationship to the dark C-complex asteroids,
which may have played a role in delivering volatile andorganic species to the terrestrial planet-forming regionof the protoplanetary disk [4]. The Dawn mission
provided detailed characterization of the C-type dwarf
planet Ceres, and in the last few months OSIRIS-REx
successfully sampled the B-type asteroid Bennu, while
Hayabusa2 returned ~5.4 g of material from the Cg-type
asteroid Ryugu back to Earth. Many C-complex
asteroids closely resemble the CI and CM carbonaceouschondrites, which contain abundant (>70 vol%)hydrated silicates. Nevertheless, there is evidence thatthe surfaces of Ryugu and Bennu are heterogeneous [5,6] and may contain regions of anhydrous silicatessimilar to those found in the CO chondrites. A possible
genetic link between the CO and CM chondrites ishinted at by similarities in isotopic and elemental
compositions [1]. It has therefore become important tocharacterise the spectral properties of CO chondrites so
we can identify features associated with theirmineralogical composition in the remote observationsof primitive C-complex asteroids.
Cloutis et al. [3] reviewed visible and near-infrared(IR) reflectance spectra for CO chondrites of petrologic
subtype from 3.0 to 3.6. They reported a ubiquitous
olivine feature at ~1 μm, which is related to the Fe2+
content in olivine. They also found that thermal
metamorphism led to an overall increase in thereflectance and a bluing of the spectra (i.e. decrease inreflectance with increasing wavelength). McAdam et al.
[7] presented visible, near- and mid-IR spectra of wellcharacterised CO chondrite samples but largely focused
their efforts on the primitive subtype 3.0’s. They foundthat the least processed COs have characteristic featuresat 1.4 μm and 21 μm related to the amorphous Febearing silicates in the matrix. Here, we present visible,
near- and mid-IR reflectance spectra for CO chondrite

powders across the entire petrologic range (3.0 ‒ 3.8).
The bulk modal mineralogy and Fe-oxidation states ofthe powders have previously been characterized [8],allowing us to directly link spectral features tocompositions.
Methodology: Spectra from a suite of 12 CO3chondrite powders (~50 mg and grainsize ≤ 25 μm) frompetrologic subtype 3.0 to 3.8 were collected at thePlanetary Emissivity Laboratory at the Institute ofPlanetary Research, German Aerospace Center (DLR).
We collected visible, near- and mid-IR reflectancespectra on powders that have been previously
characterised using X-ray diffraction (XRD) andMössbauer spectroscopy. Spectra of each CO wereacquired from 0.4–25 μm at different phase angles (15°,30°, 45° and 60°) using a bi-directional reflectancesetup with a Bruker Vertex 80V FTIR instrument. The
powders were not heated prior to analysis but spectra
were collected under vacuum.

Figure 1. Visible and near-IR spectra of the CO chondrites
showing the variations in slope and a broad feature near ~1.05 μm. Instrument artifacts are highlighted in grey.

Results: Fig. 1 shows that the visible and near-IR
spectra of the CO chondrites are generally featureless,although there are clear variations in the overall slope(0.65 / 0.45 μm reflectance ratio), with Colony (CO3.0)
and NWA 7892 (CO3.05) having the steepest slopes.
The “flattest” spectra are observed for the more
thermally metamorphosed falls Ornans (CO3.4), Moss(CO3.6), and Warrenton (CO3.7). Moreover, a featureat ~ 1.05 μm is observed in the spectra of all samples(Fig. 1). We also observe differences in the intensity of
the 3 μm band depth. The deepest 3 µm bands are seenfor Colony, NWA 7892, and ALHA77003 (CO3.6).
DOM 08006, MIL 090010 (CO3.1), Kainsaz (CO3.2),

52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548)

Lancé (CO3.5), and Isna (3.8) have intermediate band
depths, whereas the feature is weakest for Felix(CO3.3), Ornans, Moss, and Warrenton (Fig. 2).

Figure 2. 3 μm band for CO chondrites of petrologic types 3.0
to 3.8.

Figure 2. Mid-IR spectral reflectance features of CO
chondrites. Highlighted are the Christiansen Feature (CF),the Si-O stretching features and the Transparency Feature(TF).

In the mid-IR there are strong features from silicates(Fig. 3). The Christiansen Feature (CF) is located at ~8.7
μm for CO3.0 to CO3.3 chondrites and shifts to ~9.2 μmfor the CO3.4‒3.8 meteorites. Si – O stretching features
are recognisable in the regions 9.27 – 9.82 μm and 10.1
– 11.1 μm, while the Transparency Feature (TF) is in therange 11.9 – 13.9 μm (Fig. 3).
Discussion: Variations in the spectral slope
observed in the visible and near-IR regions of the CO
chondrites can be related to a combination of parent
body thermal metamorphism and terrestrial weathering.
The steepest slopes are observed for Colony and NWA
7892, which were both recovered from hot desert
environments. The spectrum of the Antarctic find
ALHA77003 also has a steep slope, and these threemeteorites have the deepest 3 μm bands, likely due to2364.pdf

the presence of terrestrial –OH/H2O-bearing phases. For
example, our XRD analyses indicated that Colony
contains ~5 vol% in Fe-(oxy)-hydroxides.
Felix, Ornans, Moss and Warrenton have theshallowest 3 μm features and some of the flattest spectrain the visible (Fig. 1). These four meteorites are allobserved falls, and therefore their shallow 3 μm feature
is likely reflecting OH/H2O related features.They alsoshow increasing overall reflectance with increasing
thermal metamorphism, as it was also reported by
Cloutis et al [3].
DOM 08006 and MIL 090010 are characterised by thelowest reflectance, flat slopes (Fig. 1), and 3 μm featureswith intermediate depth (Fig. 2), which is likely related
to the oxidised nature of the amorphous silicatescharacterising the matrix of primitive these samples[10]. Moreover, the olivine feature at ~1.05 μm
described also by Cloutis et al [3] show some variationin relation to thermal metamorphism.
In the mid-IR (Fig. 3), the effects of weathering are
likely less significant, with the features related to thesilicate mineralogy, which are influenced by parent
body metamorphism [3, 7]. The spectral contrast ofvibrational bands increases with increasing petrologic
subtype (Fig. 3). These changes are related to variations
in sample mineralogy with increasing petrologic
subtype as olivine and pyroxene abundances increase
from 30 to 50 vol% and from 25 to 35 vol%,respectively. Increase in the olivine and pyroxene
abundance is consequence of the crystallization of theFe-bearing amorphous silicates thermal metamorphism.
Olivine features become more defined as was seen by
McAdam et al. [7], and the CF shifts as the olivinechanges its composition towards richer Fe2+ members[11].
Spectra from Bennu are dominated by vibrationalfeatures associated with phyllosilicates [11] (similar towhat is observed in CM chondrite spectra) but thespectra in this study should help identify regions where
anhydrous silicates are dominant on the asteroid
surface.
Acknowledgements: Europlanet 2020 RI has
received funding from the European Union's Horizon
2020 research and innovation program under grantagreement No 654208. NHM Meteorite Collection andANSMET for the samples.
References: [1] Weisberg, M. K. et al. (2006) MESS II 19‒
52. [2] Burbine et al. (2001) M&PS 36:245 [3] Cloutis et al.
(2012) Icarus 220:466. [4] Alexander et al. (2012) Science337:721 [5] Tatsumi (2020) Nat.Astron. [6] DellaGiustina etal. (2020) Nat.Astron.).[7] McAdam et al. (2018) Icarus
306:32. [8] Bonato et al. (2016) M&PS 51:A172. [9] Bates etal. (2020) M&PS 55:77. [10] Bonato et al. (2018) 49th LPSC
#1917. [11] Hamilton et al. (2019) Nat Astron. 3:332.