C2-ungrouped
Fell January 18, 2000
59° 42' 15.7" N., 134° 12' 4.9" E.
Several hundred thousand fragments of this unique carbonaceous chondrite fell at 8:43:43 A.M. onto ice-covered Tagish Lake, located between Atlin, British Columbia and Carcross, Yukon Territory. The luminous meteor (magnitude ~–22), estimated to have been ~4 m in diameter and to have a pre-atmospheric weight of 56,000 kg (Brown et al., 2002), approached at a velocity of ~16 kilometers per second at an angle of 17.8°. Fragmentation models indicate that the object first exploded at an altitude of 34.4 km with an energy equivalent to 1.7 kT of TNT, which caused 88% fragmentation of the object; a total of 33 fragmentation events are thought to have occurred during decent (Ceplecha, 2007). At a height of 29.2 km, the 2,660 kg main mass entered the dark phase of its flight, its velocity now at 13.1 km/second. From models based on a porosity of 37–58%, it is estimated that ~1,300 kg of appreciable fragments fell to the ground, corresponding to a total ablation loss of over 97%.
The multiple explosions and accompanying dust cloud were seen and heard by eyewitnesses, while a few others heard hissing associated with electrophonic sound phenomena, or noticed a metallic or sulfurous odor (Hildebrand et al., 2006). The fall was detected by seismic stations and U.S. Department of Defense satellites, providing data that has helped ascertain for the first time an accurate orbit for a carbonaceous chondrite. The meteoroid moved in a direction of 151.5° ±2° with an elevation above the horizon of 17.8°. After calculating an orbit for this meteoroid, the aphelion was found to lie in the outer asteroid belt, possibly associated with the Apollo asteroid group—in particular, the low-albedo D-type asteroids. There is a high probability that transfer to an Earth-crossing orbit occurred by means of the v6 resonance. Excellent pictures of the event taken within two minutes of the explosion and spanning an extended time afterwards can be seen at the Atlin Realty news website.
It was fortuitous that local resident and pilot, Jim L. Brook, had been previously briefed by meteoriticists from the University of Western Ontario on how best to collect meteoritic dust samples. At ~4:00 P.M. on January 25, 2000, while driving on the ice of the Taku Arm of Tagish Lake, Brook spotted black fragments on and within the snow and recognized them as meteorites. Over the next two days he collected several dozen fragments and fine particles totaling ~870 g, which were placed into clean Ziploc freezer baggies and kept frozen until they could be turned over to the scientists. Between April 20 and May 8, 2000, after the heavy snow cover had dissipated, search parties from the University of Calgary and The University of Western Ontario utilizing ATVs and snowmobiles identified and marked 412 additional fragment sites in situ, some situated deeply within the ice. Approximately 200 of these fragments were eventually collected before the ice lost its structural integrity. A few more pieces were recovered by local residents during the summer. The total recovered weight of this lightweight, friable meteorite is 5–10 kg. The Tagish Lake strewnfield covers an area of at least 16 × 4 km.
Tagish Lake is among the most chemically primitive meteorites known, with a significantly higher carbonate abundance than any other carbonaceous chondrite. It has undergone the most pervasive aqueous alteration of any C2 chondrite studied, and contains water and carbon structurally bound in hydrated minerals, adsorbed onto mineral surfaces, and absorbed within the layers of smectite-group phyllosilicate clays, likely zones of prebiotic organic synthesis (Garvie and Buseck, 2007). Bulk density and porosity measurements of numerous Tagish Lake fragments reveal that the meteoroid had both a lower bulk density and a higher porosity than any other meteoroid similarly analyzed, with values very close to those of IDPs (Hildebrand et al., 2006). The composition of Tagish Lake can be separated into two isotopically distinct lithologies—a dominant carbonate-poor lithology, and a subordinate carbonate-rich lithology (Zolensky et al., 2002).
CARBONATE-POOR LITHOLOGY
The carbonate-poor lithology is composed of coarse- to fine-grained, phyllosilicate-rich clasts, together with interspersed, aqueously altered, rimmed chondrules (mainly porphyritic and barred olivine types), enclosing small FeNi-metal inclusions. Other constituents present in Tagish Lake include anhydrous forsteritic olivine grains (mainly Fo99), lithic fragments of mainly enstatite composition, magnetite, and sulfides (primarily pyrrhotite), with minor phosphides, chromite, spinel-rich spherules, and other rare refractory inclusions. Many of these constituents are enclosed by fine-grained, low-porosity dust rims composed of an unequilibrated assemblage of phyllosilicates, FeNi-sulfides, FeNi-metal, magnetites, low-Ca pyroxenes, and forsteritic olivines. It has been demonstrated that these rims were accreted in the solar nebula (Greshake et al., 2005). Alternatively, an aqueous alteration process has been invoked as the formation mechanism of these rims by Takayama and Tomeoka (2008). On a nm- to µm-scale, hollow, membrane-like globules composed of pristine amorphous carbon are present, attesting to a persistently cold environment since their formation before or during the formation of the Solar System (Nakamura et al., 2003). All of these components are contained within a dense, fine-grained matrix of mostly Mg-rich saponite and serpentine, along with Fe–Ni sulfides. Only rare carbonates are present within this matrix in the form of polycrystalline calcium carbonate grains. Micropores are partially lined by these carbonates.
In the proposed alteration sequence, which occurred on the parent body at very low temperatures (<100°C), olivine and metal were replaced by phyllosilicates and pyrrhotites. Phyllosilicates also replaced Ca-rich phases in the rimmed objects, leading to the transport of Ca out of these objects by aqueous fluids and into the matrix component where carbonates were then precipitated (Greshake et al., 2005). Subsequent oxidation conditions resulted in the replacement of some pyrrhotite by magnetite. This was followed by the replacement of magnetite by the precipitation of fine-grained sulfides and the growth of carbonates. Thereafter, sulfur was incorporated into organic phases.
CARBONATE-RICH LITHOLOGY
The carbonate-rich lithology is composed of very fine-grained phyllosilicates of mostly saponite, with only a very limited amount of fine-grained clasts, CAIs, and magnetite present. Whitish Fe–Mg–Ca–Mn carbonate grains are very abundant, and include calcite, dolomite, and breunnerite (formed in that sequence), while very little calcium carbonate occurs independently. The Fe–Mg–Mn carbonates, called siderite, likely replaced existing Ca carbonate grains through the percolation of fluids following impact fracturing. Micropores are completely lined by these carbonates.
In the proposed alteration sequence, which occurred on the parent body at higher temperatures (>300°C) than for the carbonate poor lithology, olivine and metal were replaced by phyllosilicates and magnetite, which were subsequently replaced by Ca carbonate. Following impact fracturing, siderite replaced some Ca carbonates, and coarse-grained sulfides were deposited. The estimated water/rock ratio during this phase is ~2. It may be assumed that the carbonate-rich lithology succeeded the carbonate-poor lithology.
X-ray diffraction techniques and Mössbauer spectroscopy have been used by Bland et al. (2004) to determine the modal mineralogy of several carbonaceous chondrites, including Tagish Lake, and to quantify the compositional range of the olivine phases (here its pure forsterite). In addition, the grain density can be readily estimated from the mode data, and therefore, in combination with the calculated bulk density, the porosity can be determined. The modal mineralogy (vol%) of Tagish Lake is as follows:
Olivine (Fo100) ------------- 7.0
Fe-Mg carbonate ------------ 8.0
Pyrrhotite -------------------- 5.3
Pentlandite ------------------- 0.3
Magnetite --------------------- 4.5
Saponite–serpentine ----- 71.2
TOTAL --------------------- 100.0
grain density = 2.84 g per cubic cm; porosity = 41%!
Tagish Lake has a high bulk carbon content of ~5.8 wt%, higher than CI chondrites and much higher than CM chondrites, with ~2.5 wt% of this carbon incorporated in organic components. The water-soluble organic component, which comprises only 0.01 vol% (much less abundant than in CM chondrites), consists of mostly monocarboxylic and dicarboxylic acids, the former including the straight chain compound formic acid as has been found in the C2 chondrite EET 96029; all of these compounds could have formed by parent body or nebular processes. The remainder of the organic component (>99%) is present as insoluble C, which is predominantly composed of two high-molecular weight PAHs having an aromatic structure. This composition is in contrast to the mostly aliphatic-structured compounds of higher molecular weight found in CM chondrites. The insoluble organic matter in Tagish Lake contains a high proportion of diradicaloids among the aromatic moieties, which is considered to be a distinguishing characteristic of an extraterrestrial organic source (Binet et al., 2004). Since the most highly altered CI and CM group members also contain these same diradicaloids, this organic component must have been synthesized in the presolar nebula, and further discussions presented by Alexander et al. (2007) argue for an origin in the interstellar medium.
All of the organic compounds present in Tagish Lake, including nitriles with low H content, are consistent with a low degree of aqueous processing following accretion of large interstellar molecules. The lower abundance of heavier organics in Tagish Lake compared to those in Murchison is evidence for lower alteration temperatures and lower degrees of chemical evolution on this asteroid. Consistent with this is the discovery of sub-µm-sized, hollow globules of amorphous carbon, containing isotopically anomalous N and H, first identified within the phyllosilicate matrix of Tagish Lake. The presence of these globules infers that they formed in a cold molecular cloud or in the outer parts of the protosolar disk, or alternatively, they accreted onto the Tagish Lake asteroid and were stored in a very cold environment (–253 to –263°C) over its entire history (see the PSRD article below). Tagish Lake has a bulk density of 1.67 g/cc, much lower than CI or CM, as well as all other meteorites and most IDPs. The carbonates display a wide range of compositions, some unknown from any other CI and CM chondrites, and show heavy oxygen isotope enrichment consistent with a high water/rock ratio (~2) more similar to CI than to CM (~0.6). Still, unlike both CI and CM, different components in Tagish Lake demonstrate a lack of isotopic equilibrium.
The anhydrous component of Tagish Lake was studied and compared to that of the CM and CI chondrites (Simon and Grossman, 2003). Tagish Lake contains the silicate andradite, which is absent from CM, but it contains no tochilinite, which is abundant in CM. Tagish Lake contains primitive refractory forsterite grains, rare Fo-rich chondrules, and isolated olivine grains, the latter probably reflecting crystallization within and subsequent fragmentation of host chondrules (Russell et al., 2010). This anhydrous component in Tagish Lake (~13 wt%) is much lower than that in CM (ave. 48 wt%), but higher than that in CI. Other features that were identified in both Tagish Lake and CM but not in CI include hibonite-bearing and spinel-rich refractory inclusions, CAIs, and Cr- and Al-rich spinel. Furthermore, thick, fine-grained, accretionary dust mantles surrounding clasts and inclusions are found in Tagish Lake and CM, but not in CI.
Phyllosilicates in Tagish Lake matrix and rims are predominantly Mg-rich saponite in contrast to the Fe-rich serpentines found in CM. This Mg-rich composition is likely due to the transfer of phyllosilicate Fe into magnetite. Notably, the unique chondrites MET 00432 (CM2), WIS 91600 (C2), and Bells (C2) have similar abundances of saponite and magnetite to those of Tagish Lake, and it has been conjectured that these four carbonaceous chondrites may constitute a grouplet (Nakamura et al., 2009). On the other hand, both WIS 91600 (C2) and Tagish Lake have spectra that match the blue spectra of the bright Stickney crater on Mars' moon Phobos, but not the remaining redder, space-weathered regions of the moon (Vernazza et al., 2010). They raise the hypothesis that WIS 91600 (C2) and/or Tagish Lake may originate from the deeper, fresher material on the captured moon Phobos.
Sulfides are more abundant in Tagish Lake than in either CI or CM, although the range in sulfide phases is more similar to that of CM than CI. Still, a rosette morphology is found exclusively in Tagish Lake. The lower ratio of pyrrhotite to intermediate sulfides in Tagish Lake reflects a higher degree of aqueous alteration than that present in CM. In further contrast, no sulfate salts occur in Tagish Lake as they commonly do in CI. The magnetite abundance in Tagish Lake is similar to that in CI, but much higher than it is in CM. The nitrogen content of Tagish Lake was determined to be 0.12 wt%.
In contradiction to the many CM-like primary features, Tagish Lake exhibits evidence of an alteration history more similar to the CI group (Russell et al., 2010). Both the bulk rock and the matrix O-isotopic composition of Tagish Lake are 16O-poor, plotting close to the CI field and following the Orgueil trend line above the TFL. The phyllosilicate O- and H-isotopic compositions of Tagish Lake are also similar to those of CI, indicating similarities in water composition and alteration temperature. Still, the anhydrous minerals O-isotopic composition range overlaps that for all carbonaceous chondrite groups. Moreover, the chondrule O-isotopic composition is most similar to CM and CV groups and shows a positive correlation between 16O and Fo contents. Beyond that, the bulk H-isotopic composition of Tagish Lake is more like that of CR, with a much higher deuterium content than that of CI or CM.
Neon and nitrogen isotopic studies of Tagish Lake indicate that interstellar grains (nanodiamonds, SiC, graphite) are present in a greater abundance than it is in other C2 chondrites. Isotopically anomalous osmium has been identified in the carrier phase SiC in Tagish Lake, the first such finding ever reported (Humayun et al., 2005). This osmium consists of a greater proportion of r-process over s-process osmium, and may reflect the preservation of presolar stardust that is typically heated and destroyed. In another case, primordial ratios of planetary noble gases such as helium and argon have been found encapsulated within the three-dimensional form of carbon known as fullerene.
Trace element data, especially observed in a plot of Zn/Mn vs. Sc/Mn, indicate that Tagish Lake is a carbonaceous chondrite distinct from all others. Furthermore, both Raman and Fourier Transform Infrared Spectroscopic analyses of the carbonate-rich lithology have revealed that the carbonaceous component of Tagish Lake is unique from that of other carbonaceous chondrite groups (Djouadi et al., 2003). A cosmogenic nuclide analysis indicates that the Tagish Lake meteoroid had a relatively low CRE age of ~7.8 m.y. and the meteorite has a low shock stage of S1, which is consistent with other C chondrites.
Based on mineralogy, petrology, isotopic composition, bulk chemical composition, and organic chemistry, Tagish Lake shares some similarities with, but is unique from, both the CM and CI chondrite groups. Based on Re–Os systematics and highly siderophile element concentrations, a scenario has been proposed in which Tagish Lake could have originated from a region overlying CI chondrites and underlying CM chondrites on a common parent body, with aqueous alteration increasing towards the interior (Brandon et al., 2005). On the other hand, studies of the large diversity of clasts present in the Kaidun chondrite breccia led investigators (MacPherson et al., 2009) to the conclusion that a near-continuum of carbonaceous chondrite objects exists, encompassing CR, CM, CI, and other similar precursor objects that experienced unique alteration histories. Tagish Lake is thought to be the first representative of a new, evolutionarily distinct C2 carbonaceous chondrite object.
Tagish Lake is spectrally most similar to the T-class and D-class asteroids, the first such match ever made to either of these asteroid classes, although the Antarctic find WIS 91600 also shares these spectrographic characteristics. The T/D-class asteroids are mostly located in the outer belt region, consistent with the calculated orbital aphelion for Tagish Lake of ~3.3 AU. One asteroid in particular, 308 Polyxo, exhibits almost identical visible and near-infrared reflectance spectra to that of Tagish Lake, and it also has a distinct 3-µm absorption band like that in Tagish Lake (and in WIS 91600) indicative of the presence of hydrated minerals (T. Hiroi and S. Hasegawa, 2003). However, it was shown that there are other D asteroids besides 308 Polyxo located even closer to one of the Kirkwood Gaps, from which an efficient transfer to Earth's orbit could be executed. Nevertheless, the totality of the evidence raises the possibility that Tagish Lake could have been derived from D asteroid 308 Polyxo.
In a Cr-isotopic study of Tagish Lake, Luu et al. (2009) has found that it contains the highest 54Cr excess ever measured in a silicate fraction of a meteorite. This lead them to describe Tagish Lake as the most pristine meteorite yet studied, even more so than CI1 chondrites, likely derived from a comet or D-type asteroid in the cold outer asteroid belt.
Among D-class asteroids with similar reflectance spectra to Tagish Lake, 773 Irmintraud and 368 Haidea provide the best matches in the visible–near-infrared range. The fact that 773 Irmintraud is located closer to the 7:3 mean motion resonance of Jupiter than 368 Haidea is located to the 2:1 resonance, may have a bearing on their possible association. The T/D-class asteroid 308 Polyxo also has similar reflectance spectra to Tagish Lake in the visible-near-infrared range. Also noteworthy is the finding that 773 Irmintraud, 308 Polyxo, and the C-class asteroid 511 Davida all contain hydrated materials as are present in Tagish Lake. Data for this 3-µm hydration band indicates that the closest match to Tagish Lake is 511 Davida (Hiroi et al., 2003). Further studies will be necessary to differentiate between these possible asteroid candidates.
Notably, parts of the martian moons Phobos and Deimos (trailing edge and leading edge, respectively) are spectrally D-class objects. The low bulk densities calculated for these two moons are also similar to Tagish Lake. The low densities on the moons are thought to represent macroporosity effects consistent with the ~40% porosity calculated for Tagish Lake (Brown et al., 2001). In spite of this possibility, an origin for Tagish Lake in the outer asteroid belt remains a strong probability. Beyond that, a small "CI1" lithology previously found in Kaidun (a clast-rich carbonaceous chondrite) is reported to have an O-isotopic composition identical to that of Tagish Lake, and with a similar petrology as well. Besides those examples, an unusual clast found in Tagish Lake is similar to another clast in Kaidun, one that was identified as a CM1. Finally, a number of phyllosilicate-rich micrometeorites that were recovered in Antarctica have mineralogies similar to the Tagish Lake carbonate-poor lithology. All of these independent clasts and micrometeorites present a valuable opportunity to increase our understanding of this unique carbonaceous group. The specimen of Tagish Lake shown above is a 0.24 g specimen showing the thin fusion crust, and was exported from Canada under Cultural Property Export Permit #65152, August 3, 2000.
