5:15 PM - 6:30 PM
[PPS12-P06] Reflectance spectra, mineralogy, and chemistry of experimentally heated Tagish Lake carbonaceous chondrite
Keywords:Tagish Lake, dehydration
The Tagish Lake carbonaceous chondrite fragments were experimentally heated in a vacuum at temperatures of 400, 600, and 900°C for 50 hours. Unheated and heated samples were analyzed using the powder and synchrotron X-ray diffraction (XRD) methods for bulk mineralogy, field-emission scanning electron micro scope (FE-SEM) for texture, elemental analyzer for C, N, S contents, mass spectrometer for carbon isotope ratios, and FT-IR for reflectance spectra.
XRD patterns and FE-SEM observation showed that the unheated Tagish Lake sample consists of saponite, Mg-Fe carbonate, pyrrhotite, olivine and magnetite. Saponite was replaced by olivine, and recrystallization of secondary olivine was proceeded from 600°C to 900°C. Similarly, by heating at 600°C, Mg-Fe carbonate was decomposed into magnesiowüstite, which was reduced into taenite by heating at 900°C.
Carbon content and isotope ratio (δ13C values) start decreasing from the temperature of 600°C. Decomposition of carbonate, which has δ13C value of ~+67‰ (Grady et al., 2002), is responsible for the decrease of carbon content and δ13C value up to 600°C. Meanwhile, from 600 to 900°C, carbon content and δ13C value decreased due to gasification of organic carbons (δ13C of ~-9‰; Grady et al., 2002) by thermal cracking and oxidation taken place during heating at 900°C.
The 3.0-μm absorption band in a reflectance spectrum is responsible for interlayer and structural water of saponite and absorbed water. The 3.0-μm band of the unheated sample was deep and broad, which becomes shallower and shaper with increasing heating temperatures. Interlayer water was dehydrated up to 600°C, and structural water was dehydrated up to 900°C. In contrast, structural water of serpentine in the Murchison carbonaceous chondrite, which is derived from a C-type asteroid, was dehydrated at lower temperature of 600°C (Yamashita et al., 2015). Therefore, it may be possible that C- and D-type asteroids have different water content when they are heated at the same temperature.
Mineralogical, chemical, and spectral changes of Tagish Lake proceed with increasing heating temperatures; (1) saponite is dehydrated and recrystallized into olivine, (2) interlayer and structural water of saponite are dehydrated step by step, (3) Mg-Fe carbonate is replaced by magnesiowüstite and finally reduced into taenite, and (4) volatile elements including carbon, nitrogen, and sulfur are gasified and C isotope ratios are changed accordingly.
Comparison to Phobos and Deimos spectra suggests that 400°C and 600°C heated samples of Tagish Lake matches better. But 0.65-μm absorption band that is characteristic absorption of the Phobos red areas was not reproduced. Therefore, the reproduction of 0.65-μm absorption requires additional conditions, if Phobos was originally D-type asteroids.