Characterising the CI and CI-like carbonaceous chondrites using thermogravimetric analysis and infrared spectroscopy
Ashley King, Jake Solomon, Paul Schofield, Sara Russell
Earth, Planets and Space
2015, 67:198 (9 December 2015)
“The CI and CI-like chondrites provide a record of aqueous alteration in the early solar system. However, the CI-like chondrites differ in having also experienced a late stage period of thermal metamorphism. In order to constrain the nature and extent of the aqueous and thermal alteration, we have investigated the bulk mineralogy and abundance of H 2 O in the CI and CI-like chondrites using thermogravimetric analysis and infrared spectroscopy.
The CI chondrites Ivuna and Orgueil show significant mass loss (28.5–31.8 wt.%) upon heating to 1000 °C due to dehydration and dehydroxylation of abundant phyllosilicates and Fe-(oxy)hydroxides and the decomposition of Fe-sulphides, carbonates and organics. Infrared spectra for Ivuna and Orgueil have a prominent 3-μm feature due to bound −OH/H 2 O in phyllosilicates and Fe-(oxy)hydroxides and only a minor 11-μm feature from anhydrous silicates. These characteristics are consistent with previous studies indicating that the CI chondrites underwent near-complete aqueous alteration. Similarities in the total abundance of H 2 O and 3 μm/11 μm ratio suggest that there is no difference in the relative degree of hydration experienced by Ivuna and Orgueil.
In contrast, the CI-like chondrites Y-82162 and Y-980115 show lower mass loss (13.8–18.8 wt.%) and contain >50 % less H 2 O than the CI chondrites. The 3-μm feature is almost absent from spectra of Y-82162 and Y-980115 but the 11-μm feature is intense. The CI-like chondrites experienced thermal metamorphism at temperatures >500 °C that initially caused dehydration and dehydroxylation of phyllosilicates before partial recrystallization back into anhydrous silicates. The surfaces of many C-type asteroids were probably heated through impact metamorphism and/or solar radiation, so thermally altered carbonaceous chondrites are likely good analogues for samples that will be returned by the Hayabusa-2 and OSIRIS-REx missions.”
We have used TGA and IR spectroscopy to characterise bulk samples of CI and CI-like chondrites for which modal mineralogy was previously determined by King et al. (). TGA and IR spectroscopy are useful tools for quantifying hydrous and anhydrous species in these meteorites, providing new insights into the nature and extent of aqueous and thermal alteration. In summary we find the following:
(1) The CI chondrites Ivuna and Orgueil contain the same abundance of H 2 O, whilst the intensity of their 3- and 11-μm features—attributed to bound –OH in phyllosilicates and anhydrous silicates, respectively—is very similar. This is consistent with almost identical total phyllosilicate abundances and suggests that at the bulk scale, the CI chondrites experienced equal degrees of aqueous alteration.
(2) The CI-like chondrites Y-82162 and Y-980115 contain >50 % less H 2 O than the CI chondrites. The 3-μm feature is almost absent from these meteorites, but the 11-μm feature is intense. This is consistent with thermal metamorphism of the CI-like chondrites that caused dehydration and dehydroxylation of the phyllosilicates and subsequent recrystallization back into olivine.
(3) Peak metamorphic temperatures experienced by the CI-like chondrites exceeded 500 °C and were possibly as high as 700–800 °C. Y-980115 contains more H 2 O and has an increased 3 μm/11 μm ratio relative to Y-82162. It was either heated to a lower metamorphic temperature than Y-82162, or the duration of heating was shorter. The cause of the thermal metamorphism was probably impacts and/or solar radiation.
(4) The H 2 O abundances of CI chondrites are in reasonable agreement with those estimated for the surfaces of the most water-rich C-type asteroids. However, many C-type asteroids appear to contain significantly less H 2 O and are more consistent with the CI-like chondrites. Thermally altered carbonaceous chondrites are likely good analogues for samples that will be returned from surfaces of C-type asteroids by the Hayabusa-2 and OSIRIS-REx missions.”