Nature and degree of aqueous alteration in CM and CI carbonaceous chondrites

Abstract

Abstract-We investigated the petrologic, geochemical, and spectral parameters that relate to the type and degree of aqueous alteration in nine CM chondrites and one CI (Ivuna) carbonaceous chondrite. Our underlying hypothesis is that the position and shape of the 3 lm band is diagnostic of phyllosilicate mineralogy. We measured reflectance spectra of the chondrites under dry conditions (elevated temperatures) and vacuum (10 À8 to 10 À7 torr) to minimize adsorbed water and mimic the space environment, for subsequent comparison with reflectance spectra of asteroids. We have identified three spectral CM groups in addition to Ivuna. "Group 1," the least altered group as determined from various alteration indices, is characterized by 3 lm band centers at longer wavelengths, and is consistent with cronstedtite (Fe-serpentine). "Group 3," the most altered group, is characterized by 3 lm band centers at shorter wavelengths and is consistent with antigorite (serpentine). "Group 2" is an intermediate group between group 1 and 3. Ivuna exhibits a unique spectrum that is distinct from the CM meteorites and is consistent with lizardite and chrysotile (serpentine). The petrologic and geochemical parameters, which were determined using electron microprobe analyses and microscopic observations, are found to be consistent with the three spectral groups. These results indicate that the distinct parent body aqueous alteration environments experienced by these carbonaceous chondrites can be distinguished using reflectance spectroscopy. High-quality ground-based telescopic observations of Main Belt asteroids can be expected to reveal not just whether an asteroid is hydrated, but also details of the alteration state.

Astronomical Observations of Volatiles on Asteroids

“…These meteorites experienced varying degrees of fluid-assisted alteration on their parent bodies (McSween, 1979). Spectroscopic, mineralogic, oxygen and hydrogen isotopic, and textural analyses of CM and CI carbonaceous chondites have revealed that these meteorites contain aqueously altered materials (Takir et al, 2013;Hiroi et al, 1996;Beck et al, 2010;Zolensky et al, 1993;Clayton and Mayeda, 1999;Lee, 1993, and others). Laboratory analyses of carbonaceous chondrites combined with astronomical observations of hydrated asteroids can lead to important clues about abundance and distribution of H 2 O in the early Solar System.…”

“…Laboratory analyses of carbonaceous chondrites combined with astronomical observations of hydrated asteroids can lead to important clues about abundance and distribution of H 2 O in the early Solar System. Takir et al (2013) measured meteorite spectra under dry (elevated temperatures) and vacuum (10 -8 to 10 -7 torr) conditions to minimize the effect of adsorbed water and mimic the asteroid-like environment. Beck et al (2010) previously measured carbonaceous chondrite transmission spectra; however, absorption features in transmission do not always have the same shape or position as they do when measured in reflectance, due to the scattering of reflected illumination by granular particles.…”

“…However, in those cases where terrestrial adsorbed water is still present, the absorption band is deeper and wider. Studies have shown that the band minimum in the 2.7-2.8 µm region in carbonaceous chondrites is indicative of phyllosilicate composition and degree of aqueous alteration (Sato et al, 1997;Osawa et al, 2005;Beck et al, 2010;Takir et al, 2013).The benefits derived from observing absorptions due to water and OH per se rather than indirectly via associated absorptions are partially offset due to the difficulties associated with observing at these wavelengths. Transmission through the Earth's atmosphere is closely related to the spectrum of water vapor, and the wavelengths with the strongest absorptions in hydrated and hydroxylated minerals typically have low atmospheric transmission.…”

“…However, in those cases where terrestrial adsorbed water is still present, the absorption band is deeper and wider. Studies have shown that the band minimum in the 2.7-2.8 µm region in carbonaceous chondrites is indicative of phyllosilicate composition and degree of aqueous alteration (Sato et al, 1997;Osawa et al, 2005;Beck et al, 2010;Takir et al, 2013).…”

Catalysis in the Primordial World

“…Except on Earth, clay minerals were found on Mars, 71,72 in meteorites, especially liquid-water-altered CI1 carbonaceous chondrites, 73,74 as well as on asteroids. 75 It was hypothesized that clay minerals are present on comets, making them allegedly places for prebiotic synthesis and even for the origin of life (interstellar panspermia).…”