Mineralogy of Tagish Lake: An ungrouped type 2 carbonaceous chondrite

Abstract

Abstract-In this paper we describe the recovery, handling and preliminary mineralogical investigation ofthe Tagish Lake meteorite. Tagish Lake is a type 2 carbonaceous chondrite which bears similarities to CIl and CM chondrite groups, but is distinct from both. Abundant phyllosilicates as well as chondrules (however sparse) and common olivine grains in the matrix preclude any other classification.The bulk density of Tagish Lake (1.67 glee) is far lower than CI or CM chondrites (2.2-2.3 and 2.6-2.9 glee, respectively), or any other meteorite for that matter. We have identified two lithologies: a dominant carbonate-poor lithology and a less-abundant carbonate-rich lithology. The meteorite is a breccia at all scales.We have noted similarities between Tagish Lake and some clasts within the enigmatic meteorite Kaidun; possibly there are genetic relationships here worth exploring. In the paper we describe a clast ofCMl material within Tagish Lake which is very similar to a major lithology in Kaidun.

Extended chronologies of aqueous alteration in the CM2 carbonaceous chondrites: Evidence from carbonates in Queen Alexandra Range 93005

“…In the carbonate-poor lithology of Tagish Lake Ca-carbonate is the most abundant carbonate followed by dolomite and rare siderite. By contrast the carbonate-rich lithology contains abundant Ca-carbonate grains that are rimmed and cross-cut by siderite (Zolensky et al, 2002). The crystallisation sequence of Ca-carbonate followed by Fe-Mg carbonate in Tagish Lake is therefore opposite to the sequence inferred from the QUE 93005 polymineralic grains.…”

“…This compares with the most Fe-rich breunnerite from QUE 93005 whose composition of Ca 5.7 Mg 47.7 Fe 43.4 Mn 3.0 lies close to the breunnerite-siderite boundary. Tagish Lake (ungrouped C2) contains Ca-carbonates (aragonite and calcite), calcian dolomite and siderite (Zolensky et al, 2002;Nakamura et al, 2003;Isawa et al, 2010) and so also has a similar carbonate mineralogical diversity as QUE 93005. In the carbonate-poor lithology of Tagish Lake Ca-carbonate is the most abundant carbonate followed by dolomite and rare siderite.…”

The Chromium Isotopic Composition of the Ungrouped Carbonaceous Chondrite Tagish Lake

“…In our opinion, this reflects the high proportion of carbonates in Tagish Lake (Zolensky et al 2002) which buffers rapidly the dissolution medium at a close to neutral pH. Since carbonates are depleted in Cr, very little Cr is extracted.…”

“…The procedure followed in each step (proportion of acid mixed, duration of the dissolution procedure, and the temperature at which the dissolution took place) is represented in the 3rd column. (Zolensky et al 2002), and chemical (Pearson et al 2001) characteristics, which place it between the CI1 chondrites and the CM2 chondrites. Moreover, its reflectance spectra seem to indicate that it would come from a D-type asteroid (Gounelle et al 2008;Hiroi et al 2001), which might be related to comets and sample a very pristine reservoir of the solar system (Gounelle 2011).…”

“…Tagish Lake is a unique ungrouped, anomalous C2 carbonaceous chondrite (Brown et al 2000) and consists of two major lithologies, one carbonate-rich and another carbonate-poor that are thought to be genetically related (e.g., Zolensky et al 2002). The Tagish Lake meteorite harbors (1) unusual petrologic and mineralogical characteristics in between those of CI1 and CM2 chondrites such as a fine-grained opaque matrix consisting mainly of phyllosilicates (intergrowth of serpentine and saponite), sulfides, and magnetites and within this matrix, the presence of altered CAIs and chondrules, isolated olivine grains, magnetite, carbonates, and Fe-Ni sulfides (Zolensky et al 2002); (2) mineralogical indications of high degree of aqueous alteration such as the presence of altered CAIs, and of carbonates ranging in composition from pure calcite (CaCO 3 ) to siderite (FeCO 3 ) and magnesite (MgCO 3 ), the last two endmembers not present in any other type of meteorites (Zolensky et al 2002); (3) a bulk refractory lithophile element abundances similar to the one of CM2 chondrites, while bulk moderately volatile and volatile element abundances are between those of CI1 chondrites and CM2 chondrites (Mittlefehldt 2002); (4) an oxygen isotopic composition near those of CI1 chondrites and some metamorphosed carbonaceous chondrites but different from either of these and from the O-isotopic field of the CM2 chondrites (Engrand et al 2001;Leshin et al 2001); and (5) a bulk rock carbon isotopic composition distinct from those of CI1 chondrites and CM2 chondrites, although the carbonates in Tagish Lake have carbon isotopic compositions similar to those of these two meteorite groups (Grady et al 2002).…”