Water abundance in the Tagish Lake meteorite from TGA and IR spectroscopy: Evaluation of aqueous alteration

• Published in: Meteoritics & Planetary Science
• Main Authors: Gilmour, Cosette M., Herd, Christopher D. K., Beck, Pierre
• Other Authors: Natural Sciences and Engineering Research Council of Canada
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2019
• Subjects: Tagish; Tagish Lake
• Online Access: http://dx.doi.org/10.1111/maps.13362; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fmaps.13362; https://onlinelibrary.wiley.com/doi/pdf/10.1111/maps.13362; https://onlinelibrary.wiley.com/doi/full-xml/10.1111/maps.13362

Abstract Here, we evaluate the extent of aqueous alteration among five pristine specimens of the ungrouped Tagish Lake carbonaceous chondrite ( TL 5b, TL 11h, TL 11i, TL 4, and TL 10a) using thermogravimetric analysis ( TGA ) and infrared ( IR ) transmission spectroscopy. Both TGA and IR spectroscopy have proven to be reliable methods for determining the extent of aqueous alteration among different carbonaceous chondrites, in particular the CM chondrites (e.g., Garenne et al. 2014), with which Tagish Lake shares some affinities. Using these two methods, our goal is to incorporate TL 4 and TL 10a into the known alteration sequence of TL 5b < TL 11h < TL 11i (Herd et al. 2011; Blinova et al. 2014a). This study highlights the compositional variability of the Tagish Lake specimens, which we ascribe to its brecciated nature. Our TGA and IR spectroscopy results are congruent with the reported alteration sequence, allowing us to introduce the TL 4 and TL 10a specimens in the following order: TL 4 < TL 5b ≤ TL 10a < TL 11h < TL 11i. Notably, these two specimens appear to be similar to the least altered lithologies previously reported, and the alteration of Tagish Lake is similar to that experienced by lesser altered members of the CM chondrites (> CM 1.6). Based on these findings, Tagish Lake could be considered a 1.6–2.0 ungrouped carbonaceous chondrite. Visible and near‐IR reflectance measurements of Tagish Lake were also acquired in this study to revisit the Tagish Lake parent body connection. While other studies have paired Tagish Lake with D‐ and T‐type asteroid parent bodies, the reflectance spectra acquired in this study are variable among the different Tagish Lake specimens in relation to their alteration sequences; results match with spectra characteristic of C‐, X‐, Xc‐, and D‐type asteroids. The heterogeneity of Tagish Lake coupled with its low albedo makes the parent body connection a challenge.