Preliminary SEM and TEM study of pristine samples of Tagish Lake meteorite

Introduction: The Tagish Lake meteorite fell on the frozen lake in northern British Columbia, Canada, on 18 th of January, 2000 [1]. It has been the subject of intense preliminary petrographical, geochemical (both organic and non-organic), lithological and isotopic studies [e.g., 2-4]. However, little is still known about this meteorite. These studies demonstrated that Tagish Lake is an ungrouped Type 2 carbonaceous chondrite, unique among known meteorites, but with affinities to CIs and CMs. Two distinct lithologies were identified by [2], including a carbonate-poor lithology containing abundant phyllosilicates, Fe-Ni sulfides and magnetite with sparse, altered chondrules and CAIs; and a carbo-nate-rich lithology containing abundant Fe-Mg-Ca-Mn carbonates with rare magnetite and poor in CAIs or chondrules. Lithological variations, beyond what has been previously described [2] are evident from casual inspection of the exterior of individual samples of the pristine Tagish Lake meteorites [5]. The origin of these variations are not known and form the basis for the current study. Samples investigated: Two samples representative of the macroscopic lithological variation were selected. Sample 5b is a compact, coherent fragment with relatively abundant chondrules evident in hand sample. Sample 11i is an example of a " dark, dusty " lithology; fragments of it are very friable and tend to shed a residue of very fine black dust. SEM methods and observations: Scanning electron microscopy, including energy-dispersive spectro-metry (EDS) was carried out using the JEOL 6301F Field Emission SEM in the Department of Earth and Atmospheric Sciences at the University of Alberta. In addition to standard observation methods, still-frozen fragments of sample 11i were investigated using the Emitek K1250 cryogenic system, which allows samples to be kept at-40ºC during observation. Sample 5b was examined using the JEOL 6500F Field Emission SEM at the Carnegie Institution of Washington, also with EDS capability. Sample 5b consists of altered chondrules in a matrix of Mg-Fe silicates, some Fe-Ni sulfide and magne-tite grains, the latter present as either individual grains, framboids, or whiskers. The chondrules consist of Fe-Mg olivine and pyroxene, or Mg-rich olivine and ensta-tite. Alteration rinds on chondrules consist of fram-boidal (and occasional whisker) magnetite replacing what may have been interstitial metal, and Mg-Fe (with