New Insights into Carbonaceous Chondrites: Windows into Early Solar System Evolution

According to a recent study published in Icarus, a group of researchers from China discovered new information about the early solar system by studying the infrared (IR) spectra of carbonaceous chondrites (CCs) (1).

CCs are primitive meteorites that serve as crucial records of the early Solar System and the geological history of their parent bodies (1,2). Understanding their composition and the alterations they have undergone is vital for reconstructing the conditions and processes that shaped our solar system. The study by Zhao and his team examined the infrared (IR) spectral signatures of 17 CC samples, spanning wavelengths from 1 to 25 micrometers, to unveil the mineralogical and chemical changes induced by secondary processing (1).

A swarm of asteroids, boulders, or stone meteorites. Generated with AI. | Image Credit: © cherif - stock.adobe.com

IR spectroscopy is a powerful tool in planetary science, allowing researchers to identify and quantify minerals and molecules within meteorites (1–3). Zhao's team meticulously analyzed the spectral data of 17 CCs with varying petrological characteristics and degrees of alteration. Their findings revealed distinct spectral patterns that correlate with the intensity of aqueous alteration and thermal metamorphism (1).

The researchers pointed out two spectral features in particular. First was the 3 μm region absorption feature associated with OH-bearing minerals and water; the second was the 6 μm band indicative of water molecules (1). As aqueous alteration intensifies, these features grow in intensity and their band centers shift towards shorter wavelengths. This shift indicates an increase in hydrous alteration minerals, such as magnesium-rich phyllosilicates (1). A new absorption feature near 2.72 μm, resembling the OH absorption in serpentine and saponite minerals, also emerged with increasing alteration (1).

The study also explored the effects of thermal metamorphism on CCs by comparing laboratory-heated samples. It was observed that the 3-μm region OH/H2O absorption feature varies significantly between CCs heated below and above approximately 300 °C (1). This variation reflects the dehydration and recrystallization of hydrated minerals, marking a critical temperature threshold in the thermal evolution of CCs.

The researchers also examined the vibrational features of silicate materials. In the 10–20 μm regions, it was observed that as alteration progresses, the 12.4 μm/11.4 μm reflectance ratio diminishes, and the reflectance peak in the 9–14 μm range shifts towards shorter wavelengths (1). Meanwhile, in the 15–25 μm region, the impact of thermal metamorphism becomes even more pronounced. The single reflectance peak at 22.1 μm transforms into two distinct peaks at 19 μm and 25 μm. This transformation is primarily because of the increased presence of anhydrous silicates and the recrystallization of olivine, a common mineral in many meteorites (1).

As a result, this study provides insights into the volatile-rich compositions of C-complex asteroids and the thermal evolution histories of their parent bodies. The findings suggest that the spectral features in the 9–14 μm and 15–25 μm regions can serve as proxies for determining the degree of alteration and the content of phyllosilicates in CCs (1).

The research also highlights the complexity of reflectance spectra in CCs and underscores the influence of several factors on the spectral features of aqueous alteration. For instance, the 3 μm band center and depth are correlated, indicating an increase in hydrous alteration minerals with increasing alteration (1). During thermal metamorphism, the depth of the 3 μm band decreases and the absorption center shifts to longer wavelengths, indicating the dehydration of minerals (1).

Moreover, the 6 μm band depth increases with aqueous alteration, reflecting an overall increase in the water content of CCs. This band disappears at temperatures above 600 °C, marking the dehydration of interlayer and mineral-bound water (1). By unraveling the spectral variations associated with aqueous alteration and thermal metamorphism, this research offers a deeper comprehension of the processes that have shaped these ancient celestial objects.

References

(1) Yu, J.; Zhao, H.; Cloutis, E. A.; et al. Near-Mid Infrared Spectroscopy of Carbonaceous Chondrites: Insights into Spectral Variation Due to Aqueous Alteration and Thermal Metamorphism in Asteroids. Icarus 2024, 411, 115951. DOI: 10.1016/j.icarus.2024.115951

(2) Kallemeyn, G. W.; Wasson, J. T. The Compositional Classification of Chondrites—I. The Carbonaceous Chondrite Groups. Geochimica et Cosmochimica Acta. 1981, 45 (7), 1217–1230. DOI: 10.1016/0016-7037(81)90145-9

(3) Clave, E.; Beyssac, O.; Bernard, S.; et al. Radiation-Induced Alteration of Apatite on the Surface of Mars: First In Situ Observations with SuperCam Raman Onboard Perseverance. Sci. Rep. 2024, 14, 11284. DOI: 10.1038/s41598-024-61494-5