THERMAL ALTERATION OF CI AND CM CHONDRITES:
LINKS TO PRIMITIVE C-TYPE ASTEROID SURACES
A. J. King1, A. Maturilli2*, P. F. Schofield1, J. Helbert2, and S. S. Russell1 1Department of Earth Science, NaturalHistory Museum, London, UK (a.king@nhm.ac.uk), 2Institute of Planetary Research, German Aerospace Center
DLR, Berlin, Germany (alessandro.maturilli@dlr.de).
Introduction: In the meteorite collection there are >30 CI and CM carbonaceous chondrites that experienced
low temperature (<150°C) aqueous alteration followed by short-lived thermal metamorphism (300 ‒ 1000°C)[1]. As the surfaces of many low albedo C-type asteroids are likely to be a mixture of hydrated and dehydrated
materials, and with C-type asteroids the focus of several current space missions (e.g. Dawn, Hayabusa-2 andOSIRIS-REx), it has become important to systematically characterize the spectral properties of heated CI andCM chondrites. We have previously investigated the bulk modal mineralogy and H 2O contents of 16 heated CIand CM chondrite powders (~50 mg) using X-ray diffraction (XRD), thermogravimetric analysis (TGA) andtransmission mid-infrared (IR) spectroscopy [2]. Thermal metamorphism results in distinct mineralogical andchemical changes that can be used to constrain the alteration history of these meteorites. Here, we present visible, near- and mid-IR reflectance spectra collected from the same meteorite powders. Our aim is to directly relate characteristic features in the spectra to the known properties and alteration history of the heated CI and CM
chondrites in order to support remote sensing observations and accurately interpret the surface mineralogy ofprimitive C-type asteroids.
Experimental: The spectroscopic measurements were made at the Planetary Spectroscopy Laboratory (PSL)of DLR in Berlin with two identical FTIR Bruker Vertex 80V evacuated to ~.1 mbar and equipped with twopairings of detector+beamsplitter to cover the spectral range from 0.45 to 16 µm. One of the spectrometers has
internal optics optimized for measurements from the UV to the VNIR spectral range; this instrument was used tocover the VIS spectral range (0.45 to 1.1 µm), mounting a Si Diode detector coupled with a CaF 2 beamsplitter.
The second spectrometer is best suited for covering from the VNIR to the FIR spectral range; the IR spectralrange (1 to 16 µm) was covered using an MCT (HgCdTe) detector coupled to a KBr broadband beamsplitter [3].
Each sample was packed into a black PFTE 10 mm diameter diffuse reflectance sampling cup and measured
for illumination angles i=e spanning from 13° to 70° (i=incidence, e=emerging angle) by using an automatic andcomputer controlled accessory optical unit (Bruker A513). To prevent the light source projection exceeding thesample surface dimension, an aperture of 1 mm was chosen as a standard for all the measurements. The reference targets (measured under the same instrumental and illumination conditions) were spectralon for the VIS
spectral range (0.45 to 1.1 µm) and gold sandpaper for the IR region (1 to 16 µm). The illumination source wasan external Wolfram 24V lamp for the VIS spectral range, and an internal Globar lamp for the IR spectral range.
Results & Discussion: One of the most diagnostic features in CI and CM chondrite IR spectra is the 3 μmband, which is attributed to the -OH/H2O in the phyllosilicates and Fe-(oxy)hydroxides. The measurements weremade under ambient conditions so the 3 μm band is observed in all spectra of the heated CI and CM chondrites.
Nevertheless, the shape and intensity of the 3 μm band varies with the extent of thermal metamorphism. For example, the depth of the 3 μm band is correlated with the H2O abundance of the meteorites, and both features decrease with increasing peak metamorphic temperature due to the dehydration of the phyllosilicates. Exceptions
are PCA 91008 and PCA 02010, which are both heavily weathered and have deep, broad 3 μm bands.
The 0.7 μm feature is often linked to Fe2+-Fe3+ charge transfer in phyllosilicates and oxides, and has been detected for several C-type asteroids [e.g. 4] and in CI and CM chondrites [5]. We only observe the 0.7 μm featurein the heated CM chondrites that experienced the lowest peak metamorphic temperatures (<300°C). This suggests that thermal metamorphism led to a reduction of Fe3+ in the phyllosilicates. Preliminary analyses also indicate that with increasing metamorphism up to ~500°C the spectral slope (2.4/0.56 μm reflectance ratio) becomes
greater before an overall flattening is observed in the meteorites heated >500°C.
We identify weak absorption bands in the 0.8 ‒ 1.3 μm region, which likely result from both phyllosilicatesand silicates. More prominent features from these phases occur in the mid-IR. In the least heated CM chondrites(<300°C) we observe a broad feature at ~9 μm from Mg-rich phyllosilicates, whereas in CM chondrites heated to>500°C this becomes a doublet due to presence of olivine. This reflects dehydration of the phyllosilicates backinto anhydrous silicates and is consistent with XRD data showing that these meteorites contain high abundances(>20 vol%) of fine-grained and/or poorly crystalline olivine.
Our initial findings demonstrate that reflectance spectra of well characterized meteorite samples can be used
to understand remote sensing observations and assess the degree of aqueous alteration and thermal metamorphism on the surfaces of C-type asteroids.
References: [1] Nakamura T. (2005) J. Min. Pet. Sci., 100, 260‒272. [2] King A. J. et al. (2015) Meteorit.
Planet. Sci., 50, A5212. [3] Maturilli A. et al. (2016) EPS, 68, 113. [4] Vilas F. (2008) Astronom. J., 135, 1101‒
1105. [5] McAdam M. M. et al. (2015) Icarus, 245, 320‒332.
Acknowledgements: This research was supported by the European Union's Horizon 2020 research and innovation programme under grant agreement No 654208.