A Brief Review of the Lithology and Petrology of the Tagish Lake Meteorite Type Specimen
The mineralogical and textural characteristics of the type specimen of the Tagish Lake meteorite were described previously [1,2]. These initial descriptions recognized two lithologies, one relatively carbonate-poor, and the other relatively carbonate-rich. In addition numerous lithic fragments within the main lithologies were noted, essentially brecciated fragments within brecciated fragments [2,3]. Most research on the meteorite has generally assumed that only two lithologies were present. Many preliminary mineral and organic analyses done on pristine and non-pristine samples cannot be related to a particular matrix -- neither to the two supposed to be present, nor to any others. Distinction has sometimes been made between chondrule-bearing lithologies or fragments, and non-chondrule-bearing, or so-called dark dusty, lithologies preserved in the pristine specimen suite. To aid in a systematic approach to understanding this unique meteorite, the main petrographic features of the type specimen polished section are reviewed, based on back-scattered electron (BSE) images and electron- microprobe (EM) analyses. What is still needed is a consortium study of the pristine and non-pristine samples, that would define all the lithologies present and their inter-relationships, within the complete specimen suite, as the context and framework for ongoing research.  Brown, P.G. et al. (2000) Science, 290, 320-325.  Herd, R.K. et al. (2001) LPS XXXII, Abstract 1928.  Herd, R.K. and Herd, C.D.K. (2007) LPS XXXVIII, Abstract 2347.
Mineralogical Investigation of an Unidentified Ordinary Chondrite
Classification of meteorites is the fist step to a comprehensive study of extraterrestrial material on Earth and in determining their relationship with the parental bodies. The subject of this study was to classify an unknown meteorite from Northwest Africa from the Royal Ontario Museum collection by using non-destructive techniques such as electron microprobe analysis (EMPA), scanning electron microscopy (SEM) and petrography to identify the mineral assemblages, degree of shock and degree of weathering. Results of this study suggest that the meteorite is an L6 ordinary chondrite with an average olivine composition of Mg1.52Fe0.48SiO4 and shock stage S5 to S6.
The Wood Lake, Ontario H4 ordinary chondrite, a new Canadian meteorite
The 350g Wood Lake meteorite was found by a jogger in July, 2003 as a single fusion-encrusted fragment beside a road in the Muskoka region of Ontario, 20-30 km east of Bracebridge. Mineralogical examination of the Wood Lake meteorite reveals abundant metal and sulphide, numerous sharply defined chondrules with occasional mesostasis chondrule glass, a lack of coarse feldspar but little variation in silicate compositions, indicating Wood Lake to be an H4 ordinary chondrite. Chondrules have barred, radiating, and granular/polycrystalline textures. The bulk of the stone has undergone a moderate level of shock (S3) as indicated in thin section by uneven optical extinction and the presence of planar dislocations in silicate grains, as well as by moderate distortion of silicate crystal structures observed using in situ microXRD. Mineral chemistry for Wood Lake olivine (Fa18.3+/-0.9, n=28), and orthopyroxene (Fs17.5+/-2.6, n=16) agrees well with H chondrite averages for these minerals. The grain density for Wood Lake (3.64+/-0.01 g/cm3) and its magnetic susceptibility (log X = 5.17) are typical of fresh H chondrite falls, consistent with its observed low weathering state (W1). A comparison with known Canadian H chondrites indicates Wood Lake to be a new meteorite, which, given its low weathering grade and high magnetic susceptibility, is likely to have fallen not long before its discovery in 2003.
Olivine Morphology and Trace Element Fractionation in Metal of Main Group Pallasites
Pallasites are stony-iron meteorites consisting largely of olivine macrocrysts in a matrix of iron-nickel alloy in the form of kamacite-taenite intergrowth. Pallasites have been divided into Main Group (PMG), Eagle Station Grouplet (PES) and ungrouped (IrUn) also called pyroxene pallasites. Within PMG, six have anomalous metal contents (PMGam) and five have anomalous olivine compositions (PMGas). The morphologies of olivine macrocrysts in PMG are essentially of two types, angular or rounded. Of 19 normal PMG whose compositions and olivine morphologies are known, 17 have angular olivines. In the remaining two PMG, olivines are rounded as well as those of three PMGam and three PMGas. Experimental studies have demonstrated that rounding of olivines in molten iron-nickel alloy occurs in short times on a geological scale. Metallic cooling rates for PMG have been shown to be rapid at high temperature and slow at low temperature. Detailed analyses have demonstrated that angular olivines are compositionally zoned and therefore not in equilibrium with metal. These conditions imply that molten metal was injected into angular olivines fractured by an impact event. However, the presence of rounded olivine macrocrysts in PMGam and PMGas, as well as in two normal PMG, implies that these pallasites have retained an earlier generation of olivine. A long-standing theory for the origin of pallasites is that they represent the core-mantle boundary of a parent- body, subsequently disrupted by impact and injected by impact-melted metal. A relationship between the trace element fractionation trend in group IIIAB iron meteorites and metal of PMG is evident in that normal members cluster at the end of this fractionation trend. However, the theory does not account for the presence of rounded olivine macrocrysts. Log trace element vs log Au plots of indicate that most PMGam members exhibit the same fractionation trend as that seen for group IIIAB irons, and these correlate with PMGam and PMGas members with rounded olivines. The rounded olivine marcrocrysts may represent pre-impact olivines trapped in fractionating group IIIAB metal. Some scatter in these plots may be attributed to crystallization of melt trapped among olivine macrocrysts. Thus, the core-mantle boundary theory for the original of PMG is consistent with the evidence seen in olivine macrocrysts.
MA13A-05 [Moved to MA12A]
Mineralogy and Petrology of the Tagish Lake Meteorite: New Lithologies
The Tagish Lake meteorite is an ungrouped type 2 carbonaceous chondrite. Two distinct lithologies were identified in the study by Zolensky et al. (2002), including a carbonate-poor lithology containing abundant phyllosilicates, Fe-Ni sulfides and magnetite with sparse, altered chondrules and CAIs; and a carbonate-rich lithology containing abundant Fe-Mg-Ca-Mn carbonates with rare magnetite and lacking in CAIs or chondrules. Lithological variations beyond these two lithologies are evident from casual inspection of the pristine samples of the meteorite (Herd and Herd 2007, LPSC). The origin of these variations is not known and forms the basis for the current study of the mineralogy and petrology of this unique meteorite. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been carried out on disaggregated fragments of two distinct lithologies; current work includes the study of polished mounts in which texture is preserved. SEM analysis was carried out at the Carnegie Institution of Washington and the University of Alberta. TEM analysis was carried out on microtomed samples at the Naval Research Laboratory with a 200 keV JEOL 2200FS TEM equipped with an EDS spectrometer and scanning-TEM (STEM) based bright- and high-angle annular-dark-field detectors. Two samples representative of the macroscopic lithological variation were selected. Sample 5b is a compact, coherent fragment with abundant chondrules. Sample 11i is an example of a dark, dusty lithology; fragments of this lithology are very friable and tend to shed a residue of very fine black dust. Our SEM observations show that sample 5b consists of altered chondrules in a matrix of Mg-Fe silicates, Fe-Ni sulfide and magnetite grains, the latter present as individual grains, framboids, or whiskers. The chondrules consist of Fe-Mg olivine and pyroxene, or Mg-rich olivine and enstatite. Sample 11i consists almost entirely of very fine-grained material (average grain size less than 5 microns), containing distinct Fe-Ni sulfides, individual magnetite grains, in some cases framboidal, and a compositionally diverse group of Fe-Mg silicates (presumably phyllosilicates). The largest distinct grain found thus far is a refractory forsterite grain, 80 microns in longest dimension. In addition, we have found elongate, football-shaped, Ca-rich (carbonate?) grains of unknown origin. TEM bright-field imaging of sample 11i reveals three types of silicate morphologies that we call ropy, rough and smooth based on their appearance. Their composition, as determined by EDS analysis, shows them to be consistent with Fe-Mg-rich phyllosilicates, although some are apparently amorphous. The difference in texture is likely the result of response of the material to microtome slicing due to differences in hardness. In contrast, sample 5b behaved differently during microtomy, as evidenced by the overall texture, which indicates that the lithology consists of minerals with higher hardness. High-resolution imaging shows that the sheet silicates have a wispy texture and are poorly ordered. Based on our preliminary SEM and TEM observations we conclude that the chondrule-bearing lithology represented by sample 5b is similar to the carbonate-poor lithology of Zolensky et al. (2002), whereas the dark, dusty lithology (sample 11i) has not been previously described. This lithology is notable for its lack of chondrules, fine-grained nature, and complement of amorphous material.
Lead Isotope Investigation of the Tagish Lake Carbonaceous Chondrite
Chondritic meteorites (chondrites) are the most ancient rocks formed in our Solar system providing unique opportunities to constrain physical and chemical processes that were active both in the accretionary disk (Solar nebula) of our early sun, and on the parent bodies of the chondrites themselves. In particular, intense focus has been devoted to the Tagish Lake (CI UNGR) chondrite since its fall and recovery [1,2]. This interest in Tagish is due to the (A) similarity and distinctiveness of Tagish mineralogy to both CI and CM chondrites including at least two lithologies: a dominant carbonate-poor lithology and a less-abundant carbonate-rich lithology [3,4]; (B) unique interstellar and organic features detected in the meteorite [1-5], and (C) correlation of Tagish with D-type outer Solar system asteroids, which have never been previously sampled . Here we present results from a high-precision Pb-isotope study of four different samples of the Tagish Lake chondrite by isotope-dilution thermal ionization mass spectrometry (ID-TIMS). The four Tagish samples (obtained from the University of Calgary c/o Dr. A. Hildebrand) span the carbonate-rich and poor lithologies and are: MM47/66, MG-62, MM-87 and HG-11(1). Results will be discussed in the context of the 'primordial' lead present in the early Solar system. Preliminary results of analyzes on the first whole-rock sample (HG-11) yield slightly radiogenic lead that is consistent with an initial lead isotopic composition similar to that in Canyon Diablo troilite . SEM-BSE and electron microprobe results from thin-microtomed sections of Tagish chondrules obtained from these samples will also be presented. Preliminary SEM-BSE on the chondrules suggest that significant aqueous alteration on the parent body may have disturbed their lead isotope systematics. Therefore Tagish chondrules may themselves be unsuitable for 207Pb/206Pb dating.  Brown et al., (2000) Science 290, 320-325.  Hildebrand et al., (2006) Met.Plan.Sci. 41, 407-431.  Zolensky et al., (2002) Met.Plan.Sci. 37, 737-761.  Simon & Grossman (2003) Met.Plan.Sci. 38, 813-825.  Nakamura-Messenger et al., (2006) Science 314, 1439-1442.  Hiroi et al., (2001) Science 293, 2234-2236.  Tatsumoto et al. (1973) Science 180, 1279-1283.