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

Cosette M. Gilmour, Christopher D. K. Herd, Pierre Beck

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Cosette M. Gilmour, Christopher D. K. Herd, Pierre Beck. Water abundance in the Tagish Lake meteorite from TGA and IR spectroscopy: Evaluation of aqueous alteration. Meteoritics and Planetary Science, 2019, 54, pp.1951-1972. 10.1111/maps.13362. insu-03635483

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

Cosette M. GILMOUR ID<fnref n="1" />*, Christopher D. K. HERD ID<fnref n="1" />, and Pierre BECK<fnref n="2" />

**Abstract**—Here, we evaluate the extent of aqueous alteration among five pristine specimens of the ungrouped Tagish Lake carbonaceous chondrite (TL5b, TL11h, TL11i, TL4, and TL10a) 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 TL4 and TL10a into the known alteration sequence of TL5b < TL11h < TL11i (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 TL4 and TL10a specimens in the following order: TL4 < TL5b ≤ TL10a < TL11h < TL11i. 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 (>CM1.6). 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.

## INTRODUCTION

The Tagish Lake meteorite is one of the most well-studied carbonaceous chondrites due to its primitive nature and unique chemical and physical properties. Tagish Lake is a dark, organic-rich, ungrouped, type 2.0 carbonaceous chondrite breccia with similarities to CI and CM chondrites (Brown et al. 2000; Zolensky et al. 2002). The bulk mineralogy of Tagish Lake consists of olivine (forsterite-rich) and pyroxene (enstatite-rich) found as chondrules, aggregates, and isolated grains in the matrix, as well as magnetite, carbonates, sulfides,

and abundant phyllosilicates (saponite and serpentine) (Brown et al. 2000; Zolensky et al. 2002; Bland et al. 2004; Izawa et al. 2010a; Blinova et al. 2014a). Tagish Lake has the lowest bulk density recorded for any meteorite (1.6 g cm⁻³), and is also quite porous (~40% porosity) and extremely friable (Brown et al. 2002; Zolensky et al. 2002; Bland et al. 2004; Ralchenko et al. 2014).

Tagish Lake specimens are classified as either “pristine” or “degraded” depending on how soon they were collected after the fall on January 18, 2000. Specimens collected after Tagish Lake’s fall and kept in

<footnote n="1">¹Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, Alberta T6G 2E3, Canada</footnote>

<footnote n="2">²UJF-Grenoble 1, CNRS-INSU, Institut de Planétologie et d'Astrophysique de Grenoble (IPAG), UMR 5274, Grenoble F-38041, France</footnote>

<footnote>*Corresponding author. E-mail: cgilmour@ualberta.ca</footnote>

<footnote>*(Received 07 October 2017; revision accepted 20 June 2019)</footnote>
cold conditions are the pristine specimens, whereas those collected after the spring melt are referred to as degraded. Of the pristine samples collected, three specimens (TL5b, TL11h, and TL11i) have been studied in detail (Herd et al. 2011; Alexander et al. 2012; Blinova et al. 2014a, 2014b). Petrological studies of TL5b, TL11h, and TL11i have reported mineralogical variations among these three specimens that are indicative of varying degrees of aqueous alteration in the order TL5b (least altered) < TL11h < TL11i (most altered) (Herd et al. 2011; Blinova et al. 2014a). Variations in hydrogen abundances (water and bulk) have also been reported for these three specimens, but interestingly in the opposite order: TL5b > TL11h > TL11i (Alexander et al. 2012).

Aqueous alteration degrees of carbonaceous chondrites are largely determined by petrological investigations (e.g., McSween 1979; Zolensky et al. 1993, 1997; Browning et al. 1996; Rubin et al. 2007; Harju et al. 2014); however, recent contributions from hydration studies (Alexander et al. 2013; Howard et al. 2014) have added to our understanding of the aqueous alteration process(es). As phyllosilicates are aqueous alteration products of anhydrous materials (i.e., olivine, pyroxene, and chondrule mesostasis; Browning et al. 1996; Hanowski and Brearley 2001; Rubin et al. 2007), the amount of phyllosilicate water present in a given sample should reflect the degree of aqueous alteration. Recent studies by Garenne et al. (2014) and King et al. (2015) have revealed that by using thermogravimetric analysis (TGA), it is possible to determine the extent of aqueous alteration among different carbonaceous chondrites based on their phyllosilicate water abundances. TGA is a quantitative method where the mass loss of a given substance is measured as a function of temperature while the substance is subject to a controlled temperature. This study uses TGA to determine the amount of water in pristine Tagish Lake samples. Given that TL5b, TL11h, and TL11i have been exposed to variable degrees of aqueous alteration (Herd et al. 2011; Blinova et al. 2014a), it is expected that the TGA results will show a variance in water abundances between the samples correlated with the alteration degrees (e.g., Garenne et al. 2014; King et al. 2015). Infrared (IR) transmission spectroscopy can also be used to determine the relative amount of water in carbonaceous chondrites from the 3 µm hydration feature (a common aqueous alteration indicator) and is useful in determining the ratio of phyllosilicates to anhydrous silicates (i.e., olivine) (e.g., Beck et al. 2010, 2014). Comparison of phyllosilicate and olivine absorption bands among different carbonaceous chondrite spectra allows for determining the relative extent of aqueous alteration between specimens (i.e., the

more prominent the olivine bands, the less altered the sample). TGA and transmission IR spectroscopy were selected for this aqueous alteration investigation of Tagish Lake based on the study of Garenne et al. (2014) who implemented these two methods to characterize the degree of aqueous alteration among several different carbonaceous chondrites. Both methods are individually suited for determining the extent of aqueous alteration in carbonaceous chondrites; however, these two methods are complementary and are expected to be congruent with one another (Garenne et al. 2014; King et al. 2015).

TGA and IR spectroscopy are used in this study to help address a significant unknown: whether the total water abundance in Tagish Lake corresponds to the degree of aqueous alteration; secondary to this question is which alteration phases are the reservoirs for water. By quantifying the amount of water in Tagish Lake, we address whether the current petrologic classification (type 2.0) is sufficient for all the pristine specimens. In addition to the aqueous alteration investigation, reflectance spectra of the Tagish Lake specimens were acquired to gain further insight into the asteroid parent body connection, and to test whether the spectra are consistent with previous studies (i.e., Hiroi et al. 2001; Hiroi and Hasegawa 2003). Here, we look at how the pristine samples of Tagish Lake compare to these earlier findings and address whether the different lithologies show notable differences in their reflectance signatures.

## SAMPLES AND METHODS

### Samples

Five pristine Tagish Lake specimens (TL5b, TL11h, TL11i, TL4, and TL10a) were available for study from the University of Alberta Meteorite Collection (Fig. 1); these pristine specimens have been kept cold (−28 °C) and dry, and have not been warmed to temperatures above 0 °C. The pristine nature of these specimens is ideal for an aqueous alteration investigation as the material best represents the actual parent body composition due to the lack of terrestrial weathering and contamination. The petrological study of TL5b, TL11h, and TL11i by Blinova et al. (2014a) revealed that the matrix, composed of a fine-grained mixture of saponite and serpentine, varies in abundance between TL5b, TL11h, and TL11i with 20 vol%, 40 vol%, and 50 vol%, respectively, in conjunction with their alteration degrees. TL4 and TL10a have not been studied in petrologic detail prior to this study. Little is known about their mineralogy and degree of aqueous alteration, although TL10a looks macroscopically similar to TL5b. TL4 has unusual bulk physical
<figure><img src="image_2.png" /><figcaption>Fig. 1. Images of the five Tagish Lake specimens in this study: (a) TL5b, (b) TL11h, (c) TL11i, (d) TL4, and (e) TL10a. Note the dark color of the specimens. TL4 shows fusion crust as well as the very fine-grained, friable material which is characteristic of Tagish Lake. Black scale bars for TL5b, TL11h, TL11i, and TL4 are 1 cm. Included in the TL10a image is a 1 cm cube for scale. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

properties as it has a much higher grain density and porosity relative to other Tagish Lake specimens (Ralchenko et al. 2014). One goal of this study is to introduce TL4 and TL10a into the reported aqueous alteration sequence of TL5b, TL11h, and TL11i (Herd et al. 2011; Blinova et al. 2014a); this will be possible if the reported sequence can be replicated with TGA and IR spectroscopy.

Methodology

The analytical procedure for TGA was intended to replicate the conditions used by Beck et al. (2013) and Garenne et al. (2014); however, due to instrument differences, the conditions used in this study are slightly different. TGA was completed in the Department of Renewable Resources at the University of Alberta; two analytical sessions were completed on separate occasions. Each Tagish Lake sample was a loose, heterogeneous mixture of chondrules and matrix. Approximately 12–20 mg was collected for analysis. For

the first TGA session, TL5b, TL4, and TL10a were crushed and homogenized by a mortar and pestle. This was done to assess if homogenization influences the TGA results. All remaining analyses were completed with samples as collected (heterogeneous mixtures). Tagish Lake samples were analyzed using a Perkin Elmer STA 6000 instrument under an inert N₂ atmosphere of 20 mL min⁻¹. Samples were put into a 30-µL ceramic crucible and heated from 30 to 900 °C at a rate of 10 °C min⁻¹. Measurements in Garenne et al. (2014) were completed up to 1200 °C; however, the Perkin Elmer instrument used in this study is only capable of heating to 900 °C. This temperature limit is not a concern as the water/OH abundances of interest are associated with saponite and serpentine which release OH at temperatures <850 °C (Mielenz et al. 1953; Nozaki et al. 2006; Földvári 2011; Lafay et al. 2012; Garenne et al. 2014; King et al. 2015). Mineral references were also analyzed via TGA including saponite, greenalite, clinochlore, calcite, dolomite, and siderite. These reference materials were chosen based on
the petrological study of Tagish Lake by Blinova et al. (2014a). A sample of insoluble organic matter of the Murchison (CM) carbonaceous chondrite (courtesy of C. Alexander) was also analyzed with TGA for comparison to Tagish Lake.

Analysis of reference material by TGA has revealed that minerals have characteristic peak temperatures at which they break down, and in some cases, recrystallize (e.g., Földvári 2011). These peak temperatures can be determined by calculating the derivative of the TGA results, which produces a derivative thermogravimetric (DTG) curve. The DTG curve represents the rate of weight loss during TGA (i.e., wt% °C⁻¹). Peaks on the DTG curves were used to identify the temperatures where water/OH is released from the hydrous mineral phases in Tagish Lake. The breakdown of other carbonaceous chondrite constituents such as organics, carbonates, (oxy)-hydroxides, and even sulfides can also be recorded by TGA and DTG (e.g., Miyazawa et al. 2000; Földvári 2011; Garenne et al. 2014). By obtaining TGA and DTG results for mineral references, a general idea of the decomposition temperatures of the different constituents in Tagish Lake, notably the phyllosilicates, can be determined. It is important to note that compositional variations of minerals can shift the position of the DTG peak (Földvári 2011) making it difficult to identify the specific mineral phases in Tagish Lake from the DTG curves; this is due to differences in the compositions between the reference materials and minerals in Tagish Lake. Therefore, identification of the hydrous mineral phases via DTG was not attempted in this study.

Transmission IR spectra were measured at the Institut de Planétologie et d'Astrophysique de Grenoble using a Bruker Vertex 70 Fourier transform infrared (FTIR) spectrometer with an 8-mm collimated beam. Prior to analysis, 1.0 mg of homogenized Tagish Lake powder (via mortar and pestle) of each specimen was mixed with 300 mg of ultrapure KBr powder and mechanically ground. The mixture was compressed (400 bars) into a pellet 13 mm in diameter for transmission analysis. Spectra were measured over the range of 2.5–25 µm from 64 scans at 8 cm⁻¹ spectral resolution.

Reflectance spectra of TL5b, TL11h, TL4, and TL10a were measured at the University of Winnipeg Planetary Spectrophotometer Facility using an Analytical Spectral Devices (ASD) FieldSpec Pro HR spectrometer with a 5 mm field of view and a spectral resolution between 2 and 7 nm. Samples were homogenized into a fine-grained powder (<45 µm) via mortar and pestle prior to analysis. A spectrum for TL11i was not acquired due to limited availability of the sample. The spectra were measured in the visible and near IR (0.35–2.5 µm) at an incidence (i) angle of

30° and an emergence (e) angle of 0° relative to a Spectralon® reflectance standard. The spectra were corrected for minor irregularities in absolute reflectance of Spectralon and detector offsets. A lampblack-coated aluminum mask (5 mm diameter central hole) was placed over the samples and reflectance standard as the amount of sample available for analysis did not fill the sample cup. The use of a mask is to ensure that aluminum features from the sample cup are not reflected back during analysis.

Bulk mineralogy of specimens TL4 and TL10a was determined by X-ray diffraction (XRD) for comparison to TL5b, TL11h, and TL11i acquired prior by Blinova et al. (2014a). XRD analysis was completed under the same operating conditions as Blinova et al. (2014a). TL4 and TL10a were ground in an alumina mortar and pestle due to minimal amount of available material. The fine-grained powdered samples were analyzed at the University of Alberta using a Rigaku Ultima IV Powder X-ray diffractometer equipped with a Co tube and a high-speed Si strip detector. The Co Kα radiation was filtered postsample with an iron metal foil; a monochromator was not used. The Rigaku XRD operates using Bragg-Brentano geometry and contains two presample slits: divergence (0.67°) and height-limiting (10 mm); and three postsample slits: vertical (5°), scattering (13 mm), and receiving (13 mm). The TL4 and TL10a samples were top loaded onto a zero background plate (offcut single-crystal silicon) and spun at a rate of 0.5 Hz. Conditions for analysis included using a Co Kα radiation of λ = 1.78899 Å and a step size of 0.02° 2θ with 0.6 second count time per step. Results were acquired over the range of 5–80° 2θ. Rietveld refinement was used to identify the mineral phases in TL4 and TL10a and quantify their relative abundances. Rietveld refinement is a quantitative technique that fits measured diffraction patterns to calculated data using the nonlinear least-squares fit method (Rietveld 1967; Young et al. 1977). The JADE 9.0 software (Materials Database Inc.) was employed for Rietveld refinement utilizing whole pattern fitting (i.e., implementation of Rietveld refinement combined with phase determination from pattern simulation and estimation of amorphous content) over the range of 15–70° 2θ. JADE uses Pseudo-Voight peak shape functions for these refinements. The starting models for the individual mineral phases analyzed (forsterite, magnetite, enstatite, pyrrhotite, siderite, calcite, aragonite, and clinochlore) were taken from the International Centre for Diffraction Data Powder Diffraction File (ICDD PDF) database. Cell parameters and phase abundances were the only parameters permitted to vary in this analysis. The data collected were not adequate to refine the compositions, atomic
<table><caption>Table 1. Bulk mineralogy of Tagish Lake from Rietveld refinement (XRD).</caption><thead><tr><th rowspan="2">Mineral</th><th rowspan="2">Density (g cm<sup>-3</sup>)<sup>a</sup></th><th colspan="2">TL4</th><th colspan="2">TL10a</th><th colspan="2">TL5b<sup>b</sup></th><th colspan="2">TL11h<sup>b</sup></th><th colspan="2">TL11i<sup>b</sup></th></tr><tr><th>wt%</th><th>vol%</th><th>wt%</th><th>vol%</th><th>wt%</th><th>vol%</th><th>wt%</th><th>vol%</th><th>wt%</th><th>vol%</th></tr></thead><tbody><tr><td>Forsterite</td><td>3.28</td><td>37.1 (5.5)</td><td>40</td><td>29.2 (2.5)</td><td>32</td><td>39</td><td>44</td><td>15</td><td>15</td><td>19</td><td>20</td></tr><tr><td>Magnetite</td><td>5.18</td><td>22.1 (2.0)</td><td>15</td><td>26.2 (1.5)</td><td>18</td><td>30</td><td>21</td><td>18</td><td>12</td><td>19</td><td>12</td></tr><tr><td>Pyrrhotite</td><td>4.60</td><td>7.6 (2.0)</td><td>6</td><td>7.3 (1.0)</td><td>6</td><td>3</td><td>3</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/d</td></tr><tr><td>Enstatite</td><td>3.20</td><td>11.6 (4.5)</td><td>13</td><td>2.3 (2.0)</td><td>3</td><td>8</td><td>6</td><td>8</td><td>6</td><td>4</td><td>3</td></tr><tr><td>Dolomite</td><td>2.84</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/d</td><td>2</td><td>3</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/d</td></tr><tr><td>Siderite</td><td>3.96</td><td>2.2 (1.5)</td><td>2</td><td>11.2 (1.5)</td><td>10</td><td>10</td><td>9</td><td>10</td><td>9</td><td>7</td><td>6</td></tr><tr><td>Calcite</td><td>2.71</td><td>1.6 (1.5)</td><td>2</td><td>1.4 (1)</td><td>2</td><td>2</td><td>2</td><td>0</td><td>1</td><td>1</td><td>1</td></tr><tr><td>Aragonite</td><td>2.95</td><td>1.3 (1.5)</td><td>2</td><td>2.6 (1.5)</td><td>3</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/d</td></tr><tr><td>Clays + amorphous</td><td>2.65</td><td>16.4 (1.5)</td><td>22</td><td>19.7 (2.5)</td><td>27</td><td>7.6</td><td>10.7</td><td>50</td><td>65</td><td>51</td><td>65</td></tr></tbody></table>

Forsterite = Fo<sub>95</sub>–Fo<sub>99</sub> (Blinova et al. 2014a).

Raw XRD data and Rietveld refinement parameters are available in the supporting information.

Reported errors are 1-sigma determined from whole pattern fitting with the JADE 9.0 software. These errors are a derivation of a statistical fit between known patterns and the analyzed data (Locock et al. 2010).

Vol% values are calculated from wt% and mineral densities.

n/d = not detected.

<sup>a</sup>Density values from www.mindat.org. These densities are the same as those used in Blinova et al. (2014a). The density for the clays + amorphous material is equivalent to the density of silica (i.e., quartz).

<sup>b</sup>XRD results from Blinova et al. (2014a).

coordinates, or displacement parameters, so these were kept fixed.

## RESULTS

### XRD and Rietveld Refinement

The mineral phases present in Tagish Lake and their modal abundances determined by XRD and Rietveld refinement are provided in Table 1. The vol% of each phase was calculated from the XRD results using the reported density of each mineral phase obtained from mindat.org. Results show that the Tagish Lake specimens are comprised of forsterite (>Fo<sub>95</sub>), magnetite, pyrrhotite, enstatite, carbonates, clays, and amorphous material in varying abundances (Table 1). Calcite, one of the carbonate phases detected, has relatively low abundances. The errors (1-sigma) associated with calcite for TL4 and TL10a are close to the detected values, implying that calcite may or may not have been detected during XRD in the refinement process for these two specimens. The same can also be said about enstatite in TL10a. Aragonite was detected in TL10a and TL4, but not in TL5b, TL11h, and TL11i. The similarity between the abundance and associated error of aragonite in TL4 and TL10a again suggests that aragonite may not actually be present; however, aragonite has been detected via XRD in Tagish Lake by Nakamura et al. (2003). Siderite was also detected among the specimens, although Blinova et al. (2014a) states that siderite grains were not found in their analysis. However, the presence of siderite and

Fe-rich carbonates has been reported in other Tagish Lake studies (Noguchi et al. 2002; Zolensky et al. 2002; Izawa et al. 2010a, 2010b).

It is known from previous Tagish Lake studies that the matrix is dominated by a fine-grained mixture of saponite (smectite) and serpentine (e.g., Zolensky et al. 2002; Blinova et al. 2014a). However, it is difficult to positively identify this fine-grained material, notably clays, in Tagish Lake using the Rietveld refinement method. Blinova et al. (2014a) were able to model their saponite-serpentine clay matrix using clinochlore as a best fit. Unfortunately, the clay bands from the XRD scans in this study were not adequate to be fit with a model for Rietveld refinement. In the TL4 and TL10a XRD results, there is an abundance of material modeled as “clays + amorphous” (Table 1), which is most likely saponite and serpentine. It is important to note that the phase abundances acquired from Rietveld refinement in this study are only an estimate (i.e., the data are indicative of the phases in Tagish Lake). The fact that the samples were handground, and the amorphous content was determined by peak fitting, gives these results large uncertainties (at least an order of magnitude greater than statistical fitting errors). For comparison purposes, the abundance of clinochlore (saponite-serpentine) for TL5b, TL11h, and TL11i from Blinova et al. (2014a) was added together with their amorphous abundances and reported as “clays + amorphous” in Table 1. TL11h and TL11i have relatively similar abundances of this material (50 wt%), while TL10a, TL4, and TL5b have significantly less (TL10a > TL4 > TL5b).
<figure><img src="image_3.png" /><figcaption>Fig. 2. TGA (blue) and DTG (orange) results for Tagish Lake between 30 and 900 °C from the first analytical session. (a) TL5b. (b) Homogenized (homo.) TL5b. (c) TL11h. (d) TL11i. (e) TL4 (homo.). (f) TL10a (homo.). Data available in the supporting information. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

TGA of Tagish Lake

The TGA and DTG curves for the Tagish Lake specimens from the two analytical sessions are shown in Figs. 2 and 3. To address the different DTG peaks, the results have been divided into temperature ranges corresponding to the decomposition of the mineral phases present in Tagish Lake (Table 2; Fig. 4). Dehydration of the Tagish Lake samples occurs between 30 and 200 °C. All the Tagish Lake samples have a DTG peak that occurs at ~75 °C (Figs. 2 and 3), which corresponds to the release of adsorbed H₂O (Yoldi-Martinez et al. 2011; Garenne et al. 2014); this result is consistent with the TGA analysis of Tagish Lake by Yoldi-Martinez et al. (2011). The water

released in this temperature range is not pertinent to aqueous alteration studies given that most of the water is terrestrial (Garenne et al. 2014). The next temperature range, 200–400 °C, is characteristic of the dehydroxylation of (oxy)-hydroxide minerals (Földvári 2011; Garenne et al. 2014; King et al. 2015). However, for Tagish Lake, the DTG peaks that occur between 200 and 400 °C (Figs. 2 and 3) are not characteristic of (oxy)-hydroxides given that such minerals are not present in the pristine samples of Tagish Lake, as indicated by the XRD results in this study and Blinova et al. (2014a). The DTG curve from TGA of Murchison insoluble organic matter reveals a decomposition occurring between 200 and 420 °C (Fig. 5), which is consistent with other organic studies of Murchison
<figure><img src="image_4.png" /><figcaption>Fig. 3. TGA (blue) and DTG (orange) results for Tagish Lake between 30 and 900 °C from the second analytical session. (a) TL5b. (b) TL11h. (c) TL11i. (d) TL4. (e) TL10a. Data available in the supporting information. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

(Kerridge et al. 1987; Kebukawa et al. 2010). Figure 5 shows the DTG curve for saponite where a peak between 200 and 300 °C occurs in addition to the dehydration of interlayer water around 60 °C. A peak between 200 and 300 °C was not found in the saponite analysis by Földvári (2011); therefore, interlayer dehydration is unlikely. Alternatively, interlayer dehydration occurred between 200 and 400 °C, but below the level of detection; indeed, the TGA curves (Figs. 2 and 3) indicate that loss of saponite interlayer water in this temperature range was minimal. Given that the organic material in Murchison is similar to Tagish Lake, we believe that the decomposition occurring in the Tagish Lake samples between 200 and

400 °C is attributable to the decomposition of organic matter. It is possible that some decomposition of pyrrhotite in this temperature region may occur (e.g., Garenne et al. 2014), but its contribution is expected to be minor, given the low relative abundance of pyrrhotite in these samples (Table 1).

Dehydroxylation of phyllosilicates (saponite and serpentine) in Tagish Lake has been designated to the temperature range of 400–700 °C. The majority of weight lost during TGA occurs in this temperature range (Table 2; Fig. 4). Other carbonaceous chondrite TGA studies have specified phyllosilicate dehydroxylation to occur over the temperature ranges of 400–770 °C (Garenne et al. 2014) and 300–800 °C
<table><caption>Table 2. TGA weight loss (wt%) as a function of temperature for Tagish Lake.</caption><thead><tr><th rowspan="2">Specimen</th><th colspan="2">30-200 °C<br>(adsorbed H<sub>2</sub>O)</th><th colspan="2">200-400 °C<br>(organics)</th><th colspan="2">400-700 °C<br>(phyllosilicates)</th><th colspan="2">700-900 °C<br>(carbonates)</th><th colspan="2">Total weight<br>loss</th><th colspan="2">200-900 °C<br>weight loss</th></tr><tr><th>First<br>run</th><th>Second<br>run</th><th>First<br>run</th><th>Second<br>run</th><th>First<br>run</th><th>Second<br>run</th><th>First<br>run</th><th>Second<br>run</th><th>First<br>run</th><th>Second<br>run</th><th>First<br>run</th><th>Second<br>run</th></tr></thead><tbody><tr><td>TL5b</td><td>4.2<br>6.3*</td><td>4.1</td><td>2.0<br>3.3*</td><td>1.6</td><td>8.6<br>8.4*</td><td>9.0</td><td>3.2<br>3.8*</td><td>3.0</td><td>18.0<br>21.8*</td><td>17.7</td><td>13.8<br>15.5*</td><td>13.6</td></tr><tr><td>TL11h</td><td>4.9</td><td>2.9</td><td>1.5</td><td>0.8</td><td>10.1</td><td>6.7</td><td>3.7</td><td>2.9</td><td>20.2</td><td>13.3</td><td>15.3</td><td>10.4</td></tr><tr><td>TL11i</td><td>4.5</td><td>3.7</td><td>1.8</td><td>0.7</td><td>8.3</td><td>9.8</td><td>3.3</td><td>3.6</td><td>17.9</td><td>17.8</td><td>13.4</td><td>14.1</td></tr><tr><td>TL4</td><td>4.0*</td><td>3.3</td><td>1.6*</td><td>0.6</td><td>5.7*</td><td>6.8</td><td>3.8*</td><td>3.1</td><td>15.1*</td><td>13.8</td><td>11.1*</td><td>10.5</td></tr><tr><td>TL10a</td><td>4.8*</td><td>3.9</td><td>1.5*</td><td>1.1</td><td>9.1*</td><td>9.6</td><td>3.6*</td><td>3.7</td><td>19.0*</td><td>18.3</td><td>14.2*</td><td>14.4</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>Fig. 4. TGA results divided into temperature ranges corresponding to the weight loss of different mineral phases. The results are stacked based on the weight loss from the phyllosilicates (purple) between 400 and 700 °C from least to greatest. (Top) First TGA session. Specimens marked with an asterisk have been homogenized. (Bottom) Second TGA session. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

(King et al. 2015). The TGA of saponite and serpentine mineral references completed in this study (Fig. 5), along with the findings of previous phyllosilicate dehydroxylation studies (Mielenz et al. 1953; Nozaki et al. 2006; Földvári 2011; Lafay et al. 2012; Garenne et al. 2014; King et al. 2015), reveals that both saponite and serpentine break down anywhere between 400 and

850 °C depending on their compositions. The 400-700 °C range for the dehydroxylation of the phyllosilicates in this study was constrained based on the findings of Nozaki et al. (2006), in which the observations of bulk mineralogical changes in Tagish Lake via XRD between 400 and 900 °C revealed that saponite and serpentine were not present in Tagish Lake

<footnote>*Results acquired from samples that were crushed and homogenized by grinding in a mortar and pestle prior to analysis.</footnote>
<figure><img src="image_6.png" /><figcaption>Fig. 5. DTG curves between 30 and 900 °C of mineral references: (a) saponite; (b) greenalite and chlorite; (c) carbonates; (d) Murchison insoluble organic matter (IOM). Data available in the supporting information. (Color figure can be viewed at wile yonlinelibrary.com.)</figcaption></figure>

beyond 700 °C. While the Tagish Lake samples analyzed by Nozaki et al. (2006) are not the same samples analyzed in this study, the cutoff at 700 °C is justified based on analysis of the same meteorite. Sulfides also break down between 400 and 700 °C (Garenne et al. 2014), which may possibly contribute to the weight loss in this temperature range. Comparing the DTG curves between 400 and 700 °C reveals similarities and differences between the results of the five Tagish Lake specimens (Figs. 2 and 3). Results from both the TGA sessions show a DTG peak at ~525 °C present in all samples. The next DTG peak in the phyllosilicate temperature range occurs at ~580 °C and is present in all the second TGA session samples (Fig. 3) as well as the first TGA session heterogeneous samples (TL5b, TL11h, and TL11i), but is absent in the homogenized samples (TL5b, TL4, and TL10a) (Fig. 2). The final peak occurring at ~675 °C is found in all the specimens for

both analytical sessions, except TL11i. The ~675 °C peak is absent in the first analytical session TL11i DTG curve (Fig. 2), and this corresponding peak temperature occurs at ~690 °C in the second analytical session TL11i DTG curve (Fig. 3). The DTG temperature peaks between 400 and 700 °C for the two TGA sessions vary in intensity among the five specimens.

The release of CO₂ from the decomposition of carbonates is also recorded by TGA and contributes to the total weight loss (Table 2; Fig. 4). Based on their composition, carbonates can decompose anywhere between 400 and 1000 °C (Fig. 5) (Nozaki et al. 2006; Rodriguez-Navarro et al. 2009; Földvári 2011; Garenne et al. 2014; King et al. 2015). Carbonate peak temperatures are affected by the composition, crystallinity, particle size, and amount of a given carbonate mineral (Földvári 2011; Archer et al. 2013). The weight loss corresponding to the release of CO₂ in
<table><caption>Table 3. Calculated hydrogen abundances from phyllosilicate OH.</caption><thead><tr><th rowspan="2">Specimen</th><th colspan="2">TGA weight loss 400–700 °C</th><th colspan="3">H abundance (wt%)</th></tr><tr><th>First run</th><th>Second run</th><th>First TGA run</th><th>Second TGA run</th><th>Alexander et al. (2012)</th></tr></thead><tbody><tr><td>TL5b</td><td>8.6<br>8.4*</td><td>9.0</td><td>0.51<br>0.50*</td><td>0.53</td><td>0.68–0.69</td></tr><tr><td>TL11h</td><td>10.1</td><td>6.7</td><td>0.60</td><td>0.40</td><td>0.62–0.67</td></tr><tr><td>TL11i</td><td>8.3</td><td>9.8</td><td>0.49</td><td>0.58</td><td>0.50–0.57</td></tr><tr><td>TL4</td><td>5.7*</td><td>6.8</td><td>0.34*</td><td>0.40</td><td>—</td></tr><tr><td>TL10a</td><td>9.1*</td><td>9.6</td><td>0.54*</td><td>0.57</td><td>—</td></tr></tbody></table>

Tagish Lake has been designated to temperatures between 700 and 900 °C similar to the temperatures specified by Garenne et al. (2014) (770–900 °C). One peak occurs in this temperature range at ~750 °C for all Tagish Lake specimens (Figs. 2 and 3). However, it is important to note that a Mg-Fe carbonate has been identified in the carbonate-rich lithology of Tagish Lake (Noguchi et al. 2002; Nakamura et al. 2003; Nozaki et al. 2006), including TL5b (Blinova et al. 2014a), which has been found to decompose at 600 °C (Nozaki et al. 2006). Siderite, which is found to be relatively abundant among the Tagish Lake specimens (Table 1), also has low decarbonation temperatures (~450 to 550 °C; Fig. 5) (Földvári 2011). Therefore, it is possible that the weight loss recorded between 400 and 700 °C (Table 2) includes some weight loss due to decarbonation.

Under the assumption that the weight loss between 400 and 700 °C is solely from the dehydroxylation of phyllosilicates, hydrogen abundances were calculated from the OH weight loss in this temperature range (Table 3). Water/OH hydrogen abundances from Alexander et al. (2012) for comparison are included in Table 3. The calculated values are generally in agreement with the accepted values. The H abundances calculated for TL11i are similar to the range reported by Alexander et al. (2012). TL11h shows the greatest variability in the calculated abundances, where only the result from the first TGA session is comparable to the accepted values. Similar calculated H abundances are observed between TL5b and TL10a; however, the calculated TL5b H abundances are significantly lower than the TL5b results from Alexander et al. (2012). Despite the inconsistency between the TL5b H abundances in this study and Alexander et al. (2012), the calculated H for TL5b is within the range of the reported H abundances for Tagish Lake (0.50–0.69 wt%; Table 3). Specimen TL4 has the lowest calculated H abundances relative to the other specimens. The overall agreement between the H abundances from this study and Alexander et al. (2012)

suggests that weight loss from mineral phases other than the phyllosilicates (i.e., carbonates, sulfides) between 400 and 700 °C is minimal.

## IR Transmission Spectroscopy

The transmission spectra of Tagish Lake are shown in Fig. 6 between 5 and 25 µm. Spectra of reference minerals (olivine, saponite, serpentines, and carbonates) as well as the Orgueil (CI1), Murchison (CM2), and MET 01070 (CM1) carbonaceous chondrites are included in Fig. 6. Phyllosilicate and olivine absorption features are of interest as the relative intensities of these features are indicative to the extent of aqueous alteration (e.g., Beck et al. 2014; Garenne et al. 2014); the greater the phyllosilicate/olivine ratio, the greater the degree of alteration. It is clear from the IR spectra that there is diversity among the Tagish Lake specimens, which is consistent with the XRD (Table 1) and TGA (Table 2; Fig. 4) results.

Olivine (forsterite) in carbonaceous chondrites generally has characteristic absorption bands occurring at 11.2, 16.5, and 19.5 µm (Fig. 6) (Beck et al. 2014). In Fig. 6, the Tagish Lake specimens are stacked such that the olivine bands decrease in intensity from top to bottom. Conversely, from top to bottom, there is an increase in the phyllosilicate band occurring at 22 µm. The Tagish Lake spectra also reveal a phyllosilicate Si-O stretching mode at ~10 µm; this feature bears similarities to the saponite spectrum, which has a doublet feature at higher wavelengths between 21 and 23 µm (Fig. 6). The 22 µm absorption band best seen in the TL11h and TL11i spectra has a subtle flat top (Fig. 6), suggesting that the spectra may be representative of saponite with a weak doublet feature at this wavelength. Further inspection of the Tagish Lake spectra confirmed the presence of a weak doublet feature in the 22 µm region; however, the wavelengths of the Tagish Lake doublet peaks are closer together (21.9 and 22.2 µm) than doublet peaks in both the saponite and Orgueil spectra (21.6 and 22.5 µm; Fig. 6) (Beck et al. 2014).

<footnote>*Results acquired from samples that were crushed and homogenized by grinding in a mortar and pestle prior to analysis.</footnote>
<figure><img src="image_7.png" /><figcaption>Fig. 6. IR spectra of the five pristine Tagish Lake specimens compared with Orgueil, MET 01070, Murchison, and mineral references (olivine, phyllosilicates, and carbonates) from Beck et al. (2014). The Tagish Lake specimens are stacked in order corresponding to the decrease in olivine features (11.2, 16.5, and 19.5 µm) from top to bottom. Data available in the supporting information. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

Carbonates were detected in the Tagish Lake spectra from the presence of a feature at 7 µm (Fig. 6). Carbonates have four C-O vibrations that occur at 7, 9.4, 11.4, and 14.7 µm (White 1974; Izawa et al. 2010a; Bonal et al. 2013). Based on the position of the 7 µm feature relative to the carbonate mineral references, the carbonate feature is indicative of dolomite, except for

TL11i where the position of the 7 µm feature is indicative of calcite (Fig. 6). This is an interesting result given that dolomite and calcite are not abundant carbonates in Tagish Lake (Table 1). TL11i also stands out from the other specimens given that its spectrum has a feature occurring at 11.4 µm consistent with a carbonate rather than the 11.2 µm olivine feature (Fig. 6).
<figure><img src="image_8.png" /></figure>

Fig. 7. Comparison of the 3 µm hydration feature and the 11.2 µm olivine feature of the five Tagish Lake specimens along with CM chondrites from Beck et al. (2014); CM chondrites with italicized names have been subject to heating. Note that our CM data plot differently compared to the data in Beck et al. (2014) due to band parameter differences used in calculating the band depth at 3 µm and that the 11.2 µm band depth for TL11i is likely overestimated due to the contribution of carbonates. The petrologic types associated with ALH 83100, ALH 84029, DOM 08003, LEW 87022, LEW 90500, MCY 05230, MET 01070, Murchison, and QUE 97990 are from Alexander et al. (2013). ALH = Alan Hills; DOM = Dominion Range; LEW = Lewis Cliff; LON = Lonewolf Nunataks; MCY = MacKay Glacier; MET = Meteorite Hills; MIL = Miller Range; PCA = Pecora Escarpment; QUE = Queen Alexandra Range; WIS = Wisconsin Range. (Color figure can be viewed at wileyonlinelibrary.com.)

In addition to the relative intensities of the olivine and phyllosilicate bands, the 3 µm hydration band (water/OH) is also an indicative feature of aqueous alteration. It is believed that the 3 µm hydration feature is associated with the presence of phyllosilicates in carbonaceous chondrites (Beck et al. 2010; Takir et al. 2013). As seen with the ~22 µm phyllosilicate feature in Fig. 6, the intensity of the 3 µm band also increases with the extent of aqueous alteration. To illustrate this, the relationship between the 3 µm hydration feature and 11.2 µm olivine feature for the Tagish Lake specimens and some CM chondrites is illustrated in Fig. 7; the CM results are from Beck et al. (2014). The inverse relationship between these two features is obvious—the more intense the 3 µm band, the less intense the olivine band—and is congruent with the varying degrees of aqueous alteration. The extent of aqueous alteration in CM chondrites is quite variable and spans a large range. It is important to note that heating and dehydration of phyllosilicates can occur in aqueously altered meteorites (i.e., Akai 1992; Ikeda 1992; Tonui et al. 2002; Nakamura 2005; Nakato et al. 2008)

resulting in a decrease of the 3 µm band and recrystallization of olivine. Tagish Lake is an unheated, primitive meteorite based on the following evidence: (1) basal lattice spacing of matrix sulfide rims (known as poorly graphitized carbon) suggests that Tagish Lake was not heated above 200–375 °C (Zolensky et al. 2002); (2) the presence of primitive amorphous material (Blinova et al. 2014a; see Insights from XRD section below); (3) bulk chemical concentrations of Cs to Cd (the most thermally mobile elements) in Tagish Lake are indicative of primary nebular condensation, not postsecondary processes (Wang and Lipschutz 1998; Blinova et al. 2014b); (4) an enrichment of Cr in olivine contained in amoeboid olivine aggregates suggests that no significant heating event has occurred since their formation (Grossman and Brearley 2005; Izawa et al. 2010b); and (5) bulk isotopic and chemical compositions suggest that Tagish Lake is more primitive than a CI chondrite (Brown et al. 2000).

While the spectra plotted in Fig. 6 are useful for qualitatively determining the relative degrees of aqueous alteration among the Tagish Lake specimens, Fig. 7 quantitatively illustrates the range of alteration. Compared to the alteration degrees of CM chondrites after Alexander et al. (2013), we observe that the alteration sequence of the Tagish Lake specimens is similar to the least altered CM chondrites (>1.6). Notably, TL11h and TL11i are most similar to type 1 alteration of CMs, while TL 5b, TL10a, and TL4 are more similar to CM type 2 carbonaceous chondrites. The comparison between the 3 µm hydration and 11.2 µm olivine features (Fig. 7) clearly illustrates that TL5b and TL10a are similar, TL4 is the least altered, and TL11h and TL11i were subject to more extensive aqueous alteration.

## Reflectance Spectroscopy

Reflectance spectra for TL5b, TL11h, TL4, and TL10a between 0.35 and 2.5 µm are shown in Fig. 8. The spectral slopes among the four specimens vary in redness, which is an interesting observation given that spectral slopes are one of the characteristics used to distinguish between different asteroid types (e.g., DeMeo et al. 2009). It is apparent that the most altered specimen of the four (TL11h) has the lowest spectral slope. Between the absolute and normalized reflectance spectra, we see some overlap in spectra between the lesser altered specimens (TL5b, TL4, and TL10a). Overall, the spectra of Tagish Lake are featureless despite the presence of an absorption band centered at 1.5 µm. This 1.5 µm feature is a detector artifact, caused by a dip in sensitivity from an inadequate ratio of digital numbers between the standard and sample;
<figure><img src="image_9.png" /><figcaption>Fig. 8. Reflectance spectra of four pristine Tagish Lake specimens: (a) absolute reflectance; (b) normalized (scaled) to 1 at 0.55 µm. The spectral features seen around 0.62 µm and at 1.5 µm are spectral artifacts and are not significant; otherwise, the spectra are noticeably featureless. The dark nature of Tagish Lake is exemplified in (a) with relatively low reflectance values (1.75–3.5%). Although the different Tagish Lake specimens are believed to originate from the same parent body, they display very different spectral slopes, as highlighted in (b). There appears to be a systematic variation in the spectral slopes related to different degrees of aqueous alteration (see text). Data available in supporting information. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

such a feature is only seen in dark spectra. There also appear to be small features around 0.62 µm; however, these are likely also artifacts from the order sorting filter in the ASD spectrometer (Cloutis et al. 2011a). The spectra acquired have a low signal-to-noise ratio, which is attributable to the fact that Tagish Lake is not a very reflective material; it has a very low albedo, between 2 and 4% (Hiroi et al. 2001). The known asteroid types that are characteristic of featureless reflectance spectra and low albedos are D-, T-, and C-type asteroids (DeMeo et al. 2009; Ryan and Woodward 2010).

To match the Tagish Lake spectra to an asteroid type, the Bus–DeMeo classification system was used (DeMeo et al. 2009). The Bus–DeMeo classification system is an extension of the Bus (1999) classification system and classifies asteroid spectra measured between 0.45 and 2.45 µm using principal component analysis (PCA). PCA is a technique used to find correlations in a data set that represent the largest variance by transforming the data set into a coordinate system of principal components (i.e., PC1, PC2, PC3, etc.). In the DeMeo et al. (2009) study, 40 wavelength channels were transformed by PCA into 40 principal components where most of the variable data are contained in the first principal component (PC1) and the remaining variability is contained in PC2, PC3, PC4, and PC5. The reflectance data of the Tagish Lake specimens were input into the online Bus–DeMeo taxonomy classification tool created by MIT (http://smass.mit.edu/busdemeoclass.html) where classification is achieved by matching spectral slope and principal component values to reference data. By uploading the Tagish Lake reflectance data into the online classification tool, the slope and principal component values for each spectrum were obtained. Considering that spectral slope and principal component values overlap for some asteroid types (DeMeo et al. 2009), multiple asteroid types were output by the classification tool as possible matches. To illustrate the overlap, the slope and principal component values (PC1 and PC2) of the four Tagish Lake specimens are plotted with the 371 asteroids classified by DeMeo et al. (2009) in Fig. 9. To distinguish which asteroid type is the best match to the reflectance data, a visual inspection of the spectra is required. DeMeo et al. (2009) developed specific criteria to follow during the visual inspection based on the presence (or absence) of characteristic absorption features.

The results of the Bus–DeMeo classification for the Tagish Lake spectra are found in Table 4. The variability in these results is not a surprise given the diversity in spectral slopes (Fig. 8). Izawa et al. (2015) found similar Bus–DeMeo classifications (D-type and X-type) for degraded Tagish Lake specimens. Distinguishing between a Xc-type and C-type from the Bus–DeMeo classification system for TL11h was difficult given how noisy its spectrum is (Fig. 8). In general, it is difficult to distinguish between the X- and C-type complexes because they overlap in their principal component values (Fig. 9) (Bus 1999). DeMeo et al. (2009) distinguishes C-type asteroids from Xc-type asteroids by a broad and shallow absorption feature that is centered between 1.0 and 1.3 µm (Fig. 10). The TL11h spectrum in Fig. 8 appears to dip around 1.0 µm, which could be an indication of the absorption
<figure><img src="image_10.png" /></figure>

feature characteristic of C-type asteroids. The spectrum of TL5b appears to have the broad 1.0–1.3 µm feature, and thus was classified as being a match to a C-type asteroid. TL4 does not appear to have the characteristic absorption feature of a C-type asteroid, or any other characteristic spectral features, and thus was classified as a X-type asteroid based on its spectral slope (Table 4; Fig. 9). A D-type asteroid was matched with

Fig. 9. (a) Plot of the first two principal components with slope removed (PC1' and PC2') from the reflectance data of 371 asteroids by DeMeo et al. (2009). Line α represents the “grand divide” which splits the objects between those with and without the 2 µm absorption feature. Objects to the left of the line do not contain the 2 µm feature. Four of the Tagish Lake specimens have been included on this diagram and plot to the left of line α in the X- and C-complex cluster; TL4 and TL11h plot in the same location. The Tagish Lake principal component values were obtained from inputting the reflectance data into the online Bus–DeMeo taxonomy classification tool (MIT). (b) The same plot as the top diagram but to a different scale to better see the distribution of asteroids to the left of line α. (c) A plot of PC1' versus slope of the objects that plot left of line α. TL5b and TL11h plot in the C-complex cluster; TL4 plots near the X-type; and TL10a plots in the D-type field. Tagish Lake slope values were obtained from the online Bus–DeMeo Taxonomy classification tool (MIT). Adapted from DeMeo et al. (2009). (Color figure can be viewed at wile

the reflectance data of TL10a based on its steep spectrum which is characteristic of D-type asteroids (Figs. 9 and 10). Although one possible interpretation of the reflectance data is that the Tagish Lake specimens represent lithologies that originated from multiple sources that were accreted to the same parent body, it is generally accepted that most of the Tagish Lake specimens are from the same parent body that was subject to different degrees of aqueous alteration (e.g., Herd et al. 2011) followed by a brecciation event that redistributed the different altered components together at the surface. Although there are indications of material from other sources (e.g., CM1 inclusion noted by Zolensky et al. 2002), such lithologies have not been documented in the majority of the Tagish Lake specimens.

It should be noted that the Bus–DeMeo classification is designed specifically for asteroid spectral data, so caution is taken when assessing the validity of the asteroid spectral matches assigned to the Tagish Lake specimens. Furthermore, it is difficult to gather accurate reflectance measurements in the lab for dark material, which can lead to difficulties in finding an asteroid match to the meteorite spectra. The results presented in this study were gathered in an attempt to see how well the spectra compare to previous meteorite–asteroid connection studies which is addressed in the discussion.

DISCUSSION

Aqueous Alteration via TGA and IR Spectroscopy

TGA and IR spectroscopy have proven to both be reliable for aqueous alteration investigations in this
<table><caption>Table 4. Bus–DeMeo classification of Tagish Lake reflectance data.</caption><thead><tr><td>Specimen</td><td>Classification</td></tr></thead><tbody><tr><td>TL5b</td><td>C-type</td></tr><tr><td>TL11h</td><td>Xc-type or C-type</td></tr><tr><td>TL4</td><td>X-type</td></tr><tr><td>TL10a</td><td>D-type</td></tr></tbody></table>

Bus–DeMeo classification determined from MIT online tool: http://smass.mit.edu/busdemeoclass.html

<figure><img src="image_11.png" /><figcaption>Fig. 10. Prototype asteroid spectra of the Tagish Lake spectral matches listed in Table 4. 1143 Odysseus (D-type), 153 Hilda (X-type), 10 Hygiea (C-type), and 21 Lutetia (Xc-type). Adapted from DeMeo et al. (2009).</figcaption></figure>

## Tagish Lake study.
The results from these methods are in agreement with each other, as well as with the reported aqueous alteration sequence of TL5b < TL11h < TL11i (Herd et al. 2011; Blinova et al. 2014a).

Comparing the phyllosilicate water abundances from TGA reveals that the results between TL5b and TL10a are similar (Table 2; Fig. 4), consistent with their macroscopic similarities. The TGA results for TL11h and TL11i are the most variable in terms of their phyllosilicate water abundances between the two TGA sessions (Table 2; Fig. 4). The significance of these variabilities is great given that the order of TL11h and TL11i relative to the other Tagish Lake specimens differs for each TGA session (Fig. 4); the order of TL4, TL5b, and TL10a remains constant between the two TGA sessions. Variability in phyllosilicate water abundances is also observed for TL4, but to a lesser extent. Mass differences of a given sample can result in weight loss discrepancies (Földvári 2011); however, similar amounts of TL11h (12.1 and 12.9 mg) and TL11i (13.6 and 14.2 mg) were used in each TGA session. It is clear in this case that differences in sample amounts cannot explain the variable TGA results, especially considering that TL4 had a greater difference in sample amounts between the two runs (12.1 and 18.4 mg), yet has less variable phyllosilicate water

abundances (Table 2). The discrepancy in TGA results for TL11h and TL11i suggests that these two specimens are lithologically heterogenous at the scale of sampling (e.g., Garenne et al. 2014); at least 1 g of sample may be required to properly represent the overall lithology of TL11h and TL11i. This lithological heterogeneity is largely evident from the two TL11i DTG curves where the ~675 °C peak seen in the other specimens is absent in the first session (Fig. 2) and occurs at ~690 °C in the second session (Fig. 3). TL4 is an unusual specimen given its low phyllosilicate water (Table 2; Fig. 4) and hydrogen (Table 3) abundances relative to the other Tagish Lake specimens. These observed results for TL4 are likely in relation to its above-average grain density and porosity (Ralchenko et al. 2014). The greater porosity likely accounts for less water or alteration phases, which would coincide with a greater abundance of anhydrous minerals present in TL4. Such a case for TL4 provides evidence for it being the least altered specimen. Based on these observations, it is likely that TL4 represents a new lithology; a petrological study is needed to confirm this. Overall, it is apparent that the minimal differences between the TGA phyllosilicate water abundances for the homogenized and heterogenous samples of TL5b, TL4, and TL10a reveal that homogenization prior to TGA does not have a significant effect on the TGA results (Table 2; Fig. 4). It is important to note, however, the absence of the ~580 °C DTG peak for the homogenized samples (Fig. 2). It is possible that this peak is simply more prominent in heterogeneous samples, or that the homogenization process slightly altered the samples. Regardless, we do not believe that the absence of this DTG peak in the homogenized samples has hindered our results.

IR spectroscopy of Tagish Lake provides insight into the relative extent of aqueous alteration between the specimens in addition to identifying the main alteration phase. The Tagish Lake IR transmission spectra show a decrease in the intensity of the olivine features and increase in the 22 µm phyllosilicate feature from top to bottom (Fig. 6); these observations correlate with an increase in aqueous alteration (top to bottom) due to the replacement of olivine with phyllosilicates. It is evident from the spectra that the olivine present in Tagish Lake is forsterite-rich, consistent with XRD results (Table 1), based on absorption features at 11.2, 16.5, and 19.5 µm. The presence of an 11.4 µm carbonate feature in the TL11i spectra instead of the 11.2 µm olivine feature is consistent with there being less olivine in TL11i than the other Tagish Lake specimens (Table 1). Classification of the phyllosilicate(s) causing the Si-O vibrations at 10 and 22 µm in carbonaceous chondrites,
notably CI and CM, has proven to be difficult given that saponite and serpentine reference spectra are distinctly different from each other at these wavelengths, as well as from carbonaceous chondrite spectra in general (Beck et al. 2010, 2014). The serpentine spectra have a complex band with subfeatures occurring in the 10 µm region and a simple band at wavelengths between 20 and 25 µm, whereas saponite has a simple band at ~10 µm and a doublet feature around 22 µm (Fig. 6). The inconsistency between CM and CI phyllosilicate bands and phyllosilicate reference spectra (e.g. Fig. 6) has been attributed to the presence of a disordered, poorly crystalline phyllosilicate in the carbonaceous material; such phyllosilicates are known to have an unstructured absorption band around 10 µm (Beck et al. 2010, 2014). Alternatively, cation substitutions in serpentine (i.e., Al-Si and Mg-Al) can cause a broadening of the Si-O feature (Farmer 1974) causing a saponite-like appearance. Such a discrepancy between phyllosilicate bands is not apparent in Tagish Lake. The matrix of Tagish Lake is dominated by a fine-grained mixture of saponite and serpentine (Zolensky et al. 2002; Blinova et al. 2014a); however, the phyllosilicate bands (10 and 22 µm) in Tagish Lake appear to both be saponite-like, indicating that saponite is the prominent hydrous phase in Tagish Lake.

A qualitative assessment of the Tagish Lake spectra shows that the relative order of aqueous alteration occurs as $TL4 < TL5b = TL10a < TL11h < TL11i$ (Fig. 6). When assessing the relative extent of aqueous alteration among the Tagish Lake specimens compared to CM chondrites (Fig. 7), the degree to which $TL11i$ is altered compared to the other Tagish Lake specimens is surprising. It was originally anticipated that the range in the Tagish Lake alteration degrees would be quite small compared to CM chondrites given that the specimens are genetically linked (Herd et al. 2011), whereas the CM chondrites may not be and thus span a wide range of TGA phyllosilicate water abundances (Garenne et al. 2014) and phyllosilicate/olivine spectral ratios (Fig. 7) (Beck et al. 2014). Even though the aqueous alteration degrees of Tagish Lake in general are relatively low (>1.6), it is evident that the five specimens were subject to quite different degrees of aqueous alteration prior to brecciation, notably $TL11h$ and $TL11i$. From the IR spectra assessment, the relative intensity of the olivine and hydration/phyllosilicate bands are fundamentally the same for $TL5b$ and $TL10a$ (Figs. 6 and 7). $TL4$ has the most intense olivine features, which implies that it is the least altered among the specimens, consistent with the TGA results (Table 2; Fig. 4).

The agreement between the TGA and IR spectroscopy results provides sufficient evidence to

incorporate $TL4$ and $TL10a$ into the previously reported alteration sequence. Although the $TL11h$ and $TL11i$ TGA results are variable (Table 2; Fig. 4), there is no concern regarding the validity of the TGA method or the $TL4$ and $TL10a$ alteration degrees given that (1) the order of $TL4$, $TL5b$, and $TL10a$ is consistent among the TGA and IR spectroscopy results and (2) the calculated H abundances from the TGA phyllosilicate water abundances are in general agreement with the literature (Table 3). On this basis, the proposed alteration sequence from least to most altered is $TL4 < TL5b \le TL10a < TL11h < TL11i$.

## Insights from XRD

The Tagish Lake results from XRD are in agreement with TGA and IR spectroscopy. Comparing the modal abundances of minerals present in Tagish Lake derived from XRD, the relative proportions of forsterite to clays + amorphous material suggest that $TL4$ and $TL10a$ are less altered than $TL11h$ and $TL11i$ (Table 1). $TL10a$ has more clay + amorphous material than $TL5b$, which is consistent with its slightly higher TGA phyllosilicate water abundances (Table 2). The relative proportions of forsterite between $TL4$ and $TL5b$ are similar; however, $TL4$ has much more clays + amorphous material (Table 1). This observation is inconsistent with the TGA results and IR spectra considering that both methods indicate that $TL4$ is the least altered. In this case, it is likely that $TL4$ contains more amorphous material than clays. The presence of amorphous material has been proposed to be indicative of more primitive components, such as nebular condensates (e.g., Brearley 1993; Chizmadia and Brearley 2008; Abreu and Brearley 2010); indeed, Blinova et al. (2014a) argue—on the basis of Rietveld refinement results and detailed transmission electron microscope studies of the Tagish Lake matrix—that the presence of more amorphous material in $TL5b$ is likely because it contains a relatively higher proportion of primitive, relatively unaltered material.

## Insights from Reflectance Spectroscopy

Reflectance spectra of Tagish Lake appear to display some systematic variations associated with alteration. While these variations are not as defined as TGA and IR spectroscopy, there is an obvious divide between the spectral slopes of $TL5b$, $TL4$, and $TL10b$ and the more altered $TL11h$ (Fig. 8). This may be due to mineralogical controls on reflectance spectra. One such control could be the differences in sulfide (i.e., pyrrhotite) abundances. The XRD results from this study (Table 1) and Blinova et al. (2014a) reveal that
pyrrhotite is present in TL5b, TL4, and TL10b, but is not detected in TL11h. Pyrrhotite has a red spectral slope (Clark et al. 2007; Baldridge et al. 2009), which may help to explain why the slopes of TL5b, TL4, and TL10a are redder than TL11h. Consequently, a lower abundance of pyrrhotite corresponds to a higher abundance of saponite and serpentine in TL11h. Saponite generally has a blue reflectance spectrum while serpentine tends to have a relatively flat spectrum (Clark et al. 2007; Baldridge et al. 2009). Therefore, the increase of serpentine and saponite in TL11h relative to the other three specimens likely results in a bluer slope for TL11h.

Although phyllosilicates are abundant in Tagish Lake, it is important to note that the pristine spectra lack a typical metal-OH feature around 2.3 µm (Fig. 8). This can be explained by the spectral effects of opaques (i.e., magnetite, pyrrhotite) and organic material in Tagish Lake. These constituents are found to be a major controlling factor (notably the organics) on the absorption features in carbonaceous chondrites as they tend to reduce the optical path length and suppress features associated with mafic-silicates (olivine and pyroxene) and phyllosilicates (Johnson and Fanale 1973; Singer 1981; Clark 1983; Cloutis et al. 1990, 2011a, 2011b; Milliken and Mustard 2007). Despite the control organics have on absorption features, their presence does not impact the spectral slopes. Moreover, although the Tagish Lake specimens have variable organic abundances, there is little difference in the nature of their organic fractions (Quirico et al. 2018). Absolute reflectance spectra of TL5b and TL10a appear to have similar slopes at wavelengths greater than 1.5 µm (Fig. 8a); however, when normalized, the spectra of TL5b and TL4 have very similar slopes (Fig. 8b). TL4 and TL5b have similar forsterite and enstatite abundances greater than what is found in TL10a (Table 1), which may explain why these two have similar spectral slopes. We acknowledge the possibility that the scale of sampling is too small for the extent of heterogeneity among the samples, as mentioned above with TGA, which likely can explain why the spectra of TL5b, TL4, and TL10a do not follow the alteration sequence. However, this is of little concern as these three specimens do not have a large variation in their degree of alteration (e.g., Fig. 7).

## Parent Body Connection

Connecting Tagish Lake to an asteroid parent body is an important endeavor as such a connection can provide additional insight into Tagish Lake's extraterrestrial history including formation, organic chemistry, and parent body alteration processes.

Although the Tagish Lake specimens are known to be genetically linked (Herd et al. 2011), matching Tagish Lake to a parent body is not an easy task given its dark nature and heterogenous composition evident by the diversity in reflectance spectra (Fig. 8; Table 4).

Previous attempts to match Tagish Lake to a parent body were completed by Hiroi et al. (2001) and Hiroi and Hasegawa (2003). Initially, Hiroi et al. (2001) proposed that Tagish Lake's parent body asteroid was the D-type asteroid 368 Haidea based on a spectral match in the visible-near-infrared (VNIR) and similar albedos. Later, Hiroi and Hasegawa (2003) revisited the Tagish Lake parent body investigation to address the possibility of a T-type asteroid as a parent body and found that Tagish Lake also spectrally matched well in the VNIR with the T/D-type asteroid 308 Polyxo (Fig. 11). This possible parent body connection was subsequently dismissed by Dotto et al. (2004) who found spectral discrepancies between Tagish Lake and 308 Polyxo in the mid-IR (7–26 µm). Vernazza et al. (2013) came to the same original conclusion as Hiroi et al. (2001) that Tagish Lake is best spectrally matched to the D-type asteroid 368 Haidea; however, this was based on mid-IR thermal measurements which were converted to reflectance using Kirchoff's law (emissivity = 1–reflectance). Given that thermal emission of asteroids largely depends on their surface temperature (e.g., Harris and Lagerros 2002), converting laboratory spectra from thermal to reflectance can result in deviations from what is expected, and thus, caution must be taken when interpreting such results. While these previous studies focused their searches among Main Belt asteroids and Jupiter Trojans, Izawa et al. (2015) were the first to consider the possibility of a near-Earth asteroid as the Tagish Lake parent body. From spectral analysis of five degraded Tagish Lake specimens, they discovered three potential parent asteroids: (3554) Amun, (17274) 2000 LC16, and (326732) 2003 HB6; Amun and 2000 LC16 have been identified as D-type asteroids (Thomas et al. 2014).

It should be noted that all Tagish Lake reflectance spectra in Hiroi et al. (2001) and Hiroi and Hasegawa (2003) were acquired from powdered samples, including this study, whereas the reflectance spectra for the asteroids were obtained by telescopic measurements. Because asteroid surfaces are exposed to space weather, the regolith has been altered to a different lithology than the subsurface material (i.e., Chapman 2004; Gaffey 2010; Pieters et al. 2012). As meteorites are typically excavated from below an asteroid's surface due to impact, they are considered more representative of an asteroid's true composition. However, telescopic reflectance measurements are only capable of analyzing asteroid surfaces, and therefore, care must be taken
<figure><img src="image_12.png" /><figcaption>Fig. 11. Reflectance spectra of Tagish Lake compared to the T/D- asteroid 308 Polyxo from Hiroi and Hasegawa (2003). The original plot is from Hiroi and Hasegawa (2003). The Tagish Lake spectrum labeled as “old” is from the Hiroi et al. (2001) study which was collected at viewing angles of i = 30° and e = 0°. The Tagish Lake “new” spectra are from Hiroi and Hasegawa (2003) obtained at incidence angles of 14° and 30° and an emergence angle of 0°. The Tagish Lake reflectance spectra (i = 30°, e = 0°) of TL5b, TL11h, TL4, and TL10a from this study are overlain for comparison. The TL10a spectrum (D-type; Table 4) is the only spectrum from this study that matches with the Hiroi et al. (2001) and Hiroi and Hasegawa (2003) results. All Tagish Lake spectra were acquired from powdered samples whereas the spectra for 308 Polyxo were acquired by telescopic measurements. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

when comparing meteorite and asteroid spectra. Comparisons are typically done with meteorites that are fresh (i.e., falls; not terrestrially altered); however, some studies have simulated space weathering on meteorites via laser and ion bombardment in attempts to reproduce similar lithology as asteroid regolith (i.e., Strazzulla et al. 2005; Fulvio et al. 2012; Vernazza et al. 2013; Matsuoka et al. 2015). Due to the rarity of the Tagish Lake specimens in this study, simulating the effects of space weathering was not an option. Space weathering processes also pulverize the surfaces of asteroids into fine-grained material (i.e., 30–300 µm; Dollfus et al. 1989; Cellino et al. 2016). Because different particle sizes can alter the intensity of spectral bands and slopes (i.e., Clark et al. 1992; Hiroi et al. 1994; Cloutis et al. 2011a, 2011b), it is important to collect spectral measurements of powdered samples with similar grain sizes to asteroid regolith (e.g., Gaffey 1976).

Space- and ground-based telescopic observations are typically bidirectional reflectance measurements (i.e., Hapke 1981). As asteroids are irregular in shape and generally have rough surfaces, scattering of light from

the surface occurs. Hapke (1981) derived a number of corrective calculations, known as the Hapke Parameters, to account for this radiative transfer effect. It is common practice in the laboratory to measure meteorite reflectance at an incident angle of 30° and emission angle of 0° relative to a known standard (i.e., Spectralon; Cloutis et al. 2018); this ensures that laboratory and telescopic measurements are comparable. The findings from Hiroi et al. (2001) and Hiroi and Hasegawa (2003) have emphasized that different geometric viewing angles used for acquiring reflectance data can produce different results. Hiroi et al. (2001) found that Tagish Lake best matched the albedo of 368 Haidea when measured at viewing angles of i = 6° and e = -6°. These viewing angles differ in comparison from the angles used in this study which were i = 30° and e = 0°. Additionally, Hiroi and Hasegawa (2003) found that the reflectance of Tagish Lake increased in brightness with smaller incident angles as also shown in Beck et al. (2012). Heterogeneity of Tagish Lake was also a factor for reflectance measurements collected by Hiroi and Hasegawa (2003) as they found significant differences in the spectra of the same Tagish Lake sample measured twice (“old” and “new”) with the same viewing geometry of i = 30° and e = 0° (Fig. 11). However, Hiroi and Hasegawa (2003) also found that the “new” sample measured with a viewing geometry of i = 14° and e = 0° compared reasonably well with the “old” (30, 0) sample (Fig. 11), which led them to conclude that variations in spectra due to changes in incident angles is minimal compared to the effects of sample heterogeneity.

The spectra of the TL5b, TL11h, TL4, and TL10a overlain on the Hiroi and Hasegawa results (Fig. 11) support the claim that heterogeneity has a strong influence on the reflectance measurements. Both Hiroi et al. (2001) and Hiroi and Hasegawa (2003) utilized the 3 µm hydration feature in their parent body investigations; however, acquisition of reflectance measurements for Tagish Lake beyond 2.5 µm was not possible in this study due to the darkness of the specimens and limited amounts available for analysis. TL10a is the only spectrum that matches the spectra measured by Hiroi et al. (2001) and Hiroi and Hasegawa (2003); it is comparable to the (14,0) spectrum shortward of 1.4 µm, but is more similar to the old (30,0) spectrum between 1.6 and 2.2 µm (Fig. 11). If such spectral heterogeneity (Figs. 8 and 11) can occur under the same viewing geometries, it is most likely that Tagish Lake is too heterogeneous on the scale of sampling required for reflectance spectroscopy, which clearly makes the meteorite parent body connection difficult.
CONCLUSIONS

The analysis of Tagish Lake using TGA and IR spectroscopy has provided promising results in determining the extent of aqueous alteration among the five pristine specimens. The diversity of the Tagish Lake specimens is reflected in both TGA and IR spectroscopy, which supports the usefulness of these analytical techniques for aqueous alteration studies. However, these methods are limited when it comes to identifying the mineral hosts of water in carbonaceous chondrites given that (1) it is difficult to match DTG peaks to reference minerals as compositional variations of a given mineral can shift the DTG peak position (Földvári 2011) and (2) discrepancies occur between the IR spectra of CM and CI chondrites and terrestrial phyllosilicates which are believed to be due to either the presence of disordered phyllosilicates or serpentine cation substitutions in carbonaceous chondrites (Farmer 1974; Beck et al. 2010, 2014). Tagish Lake is an exception to the IR spectra phyllosilicate discrepancy because its spectra resemble saponite at ~10 and 22 µm, suggesting that saponite is the main hydrous phase in Tagish Lake.

Comparison of the TGA results and IR spectra not only reveals similarities in the relative order of the Tagish Lake specimens (i.e., TL5b, TL4, and TL10a) but also emphasizes variability at the scale of sampling relative to the scale of lithological variation (e.g., TL11h and TL11i). Consistencies between the XRD, TGA, and IR spectroscopy have provided reliable information to incorporate TL4 and TL10a into the original proposed alteration sequence of TL5b, TL11h, and TL11i (Herd et al. 2011; Blinova et al. 2014a). The revised alteration sequence for the pristine Tagish Lake specimens is TL4 < TL5b ≤ TL10a < TL11h < TL11i.

The diversity of the Tagish Lake specimens is also apparent in their reflectance spectra, which have different spectral slopes (Fig. 8). Comparing the reflectance data to the Bus–DeMeo asteroid taxonomic system has provided several different possible asteroid matches to the Tagish Lake specimens (D-, C-, X-, and Xc-types). Previous studies have suggested that Tagish Lake’s parent body is possibly a D- or T-type asteroid (Hiroi et al. 2001; Hiroi and Hasegawa 2003; Vernazza et al. 2013; Izawa et al. 2015), but these studies have also encountered difficulty constraining Tagish Lake’s parent body. These difficulties stem from the compositional heterogeneities and very low albedo of Tagish Lake which create challenges in matching its spectra to an asteroid parent body.

Acknowledgments—This study was carried out as part of a master’s thesis by C.M.G. We thank the following

people for the analysis collected in this study: M Derek MacKenzie and Nduka Ikpo (University of Alberta Department of Renewable Resources) for TGA; Andrew Locock (University of Alberta Department of Earth and Atmospheric Sciences) for XRD and Rietveld refinement; and Ed Cloutis, Paul Mann, Matthew Cuddy, and Daniel Applin (University of Winnipeg Planetary Spectrophotometer Facility) for acquisition of the reflectance spectra. We thank Laurène Flandinet for her help in the preparation of pellets and Conel Alexander (Carnegie Institution) for the sample of Murchison for TGA. We also thank Benoit Rivard (M.Sc. thesis examiner), Michael Zolensky, Matthew Izawa, and an anonymous reviewer for constructive feedback on earlier versions of the manuscript, which resulted in significant improvements. This research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) discovery grant 261740 to C.D.K.H.

*Editorial Handling*—Dr. Michael Zolensky

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## SUPPORTING INFORMATION

Additional supporting information may be found in the online version of this article.

**Data S1.** Data sets for TGA and DTG curves, transmission FTIR and reflectance spectra, and XRD analysis.

**Data S2.** Processing of powder XRD data using JADE for whole-pattern fitting/Rietveld refinement.