Sulfur-rich Carbonaceous Nanoglobules in the Tagish Lake Meteorite

Introduction: Understanding the distribution, chemistry, and structure of C in carbonaceous chondrites (CC) is fundamental to revealing information on early solar chemical evolution. The Tagish Lake CC has generated much interest partly because of its pristine nature and high C content [1,2,3]. Unique to the Tagish Lake CC are hollow carbonaceous nanoglobules [4]. These particles are morphologically similar to materials produced by UV pyrolysis of interstellar ice analogues, and thus these hollow organic globules have been interpreted as extremely primitive organic materials that formed before or during the formation of the solar system [4]. Using electron energy-loss spectroscopy (EELS) we determined the composition of carbonaceous nanoglobules in Tagish Lake. This spectroscopic method together with transmission electron microscopy (TEM) provides unique chemical and crystalchemical information at high spatial resolution, free of spectral interferences from surrounding materials. Materials and Methods: A few crumb-sized pieces of pristine Tagish Lake were kindly donated by Sandra Pizzarello of ASU. The pristine material was from the unaltered interior, free of fusion crust collected 2 to 3 days after the fall. For comparison, we also examined carbonaceous materials from Murray, Mighei, and Murchison CM chondrites. Samples were prepared by crushing a ca. 0.5 mm piece of sample on a clean glass slide and dry spreading the powder onto a Cu TEM grid coated with laceyC. In this way, the samples were kept as pristine as possible. No water or organics were used in the sample preparation. High-resolution transmission electron microscopy (HRTEM) was undertaken with a 002B Topcon operated at 200 keV with a LaB6 filament. EELS spectra were acquired with a Gatan 766 DigiPEELS attached to a Philips 400ST operated at 100 keV with a cold field-emission gun. Spectra were acquired with an energy resolution of ca. 0.8 eV with operating currents of 1 to 2 nA, from regions ca. 10 nm in diameter. Results: Carbonaceous material was recognized by its large C K edge and distinctive spectral shape. We observed three morphologically distinct carbonaceous materials: flakes, rounded particles with a hollow core, and rounded single and agglomerated particles without hollow cores. The carbonaceous nanoglobules are typically <200 nm in diameter (fig. 1a), although larger clusters occur (fig. 1b). Several of the spheres exhibit a central region with lower brightness in the TEM images (fig. 1a), consistent with previous studies and indicative of a hollow core [4]. The spherical hollow nanoglobules are relatively rare compared to the solid ones. A few carbonaceous flakes were found. F r t l s