Signatures of the post-hydration heating of highly aqueously altered CM carbonaceous chondrites and implications for interpreting asteroid sample returns. (15th November 2020)

• Record Type: Journal Article
• Authors: Lindgren, Paula; Lee, Martin R.; Sparkes, Robert; Greenwood, Richard C.; Hanna, Romy D.; Franchi, Ian A.; King, Ashley J.; Floyd, Cameron; Martin, Pierre-Etienne; Hamilton, Victoria E.; Haberle, Chris
• Is Part Of: Geochimica et cosmochimica acta. Volume 289(2020)
• Page Start: 69
• Page End: 92
• Publication Date: 2020-11-15
• Subjects: CM carbonaceous chondrites -- Asteroid sample returns -- Post-hydration heating; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2020.08.021
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: The CM carbonaceous chondrites have all been aqueously altered, and some of them were subsequently heated in a parent body environment. Here we have sought to understand the impact of short duration heating on a highly aqueously altered CM through laboratory experiments on Allan Hills (ALH) 83100. Unheated ALH 83100 contains 83 volume per cent serpentine within the fine-grained matrix and altered chondrules. The matrix also hosts grains of calcite and dolomite, which are often intergrown with tochilinite, Fe(Ni) sulphides (pyrrhotite, pentlandite), magnetite and organic matter. Some of the magnetite formed by replacement of Fe(Ni) sulphides that were accreted from the nebula. Laboratory heating to 400 °C has caused partial dehydroxylation of serpentine and loss of isotopically light oxygen leading to an increase in bulk δ 18 O and fall in Δ 17 O. Tochilinite has decomposed to magnetite, whereas carbonates have remained unaltered. With regards to infrared spectroscopy (4000–400 cm −1 ; 2.5–25 µm), heating to 400 °C has resulted in decreased emissivity (increased reflectance), a sharper and more symmetric OH band at 3684 cm −1 (2.71 µm), a broadening of the SiO stretching band together with movement of its minimum to longer wavenumbers, and a decreasing depth of the MgOH band (625 cm −1 ; 16 µm). The SiO bending band is unmodified by mild heating. With heating to 800 °C the serpentine has fully dehydroxylated and recrystallized to ∼Fo60/70 olivine. Bulk δ 18 O has further increased and Δ 17 O decreased. Troilite and pyrrhotite have formed, and recrystallization of pentlandite has produced Fe, Ni metal. Calcite and dolomite were calcined at ∼700 °C and in their place is an un-named Ca-Fe oxysulphide. Heating changes the structural order of organic matter so that Raman spectroscopy of carbon in the 800 °C sample shows an increased (D1 + D4) proportional area parameter. The infrared spectrum of the 800 °C sample confirms the abundance of Fe-bearing olivine and is very similar to the spectrum of naturally heated stage IV CM Pecora Escarpment 02010. The temperature-related mineralogical, chemical, isotopic and spectroscopic signatures defined in ALH 83100 will help to track the post-hydration thermal histories of carbonaceous chondrite meteorites, and samples returned from the primitive asteroids Ryugu and Bennu.