Sawing of Meteorites in the US Antarctic Meteorite Collection
Meteorite samples from the US Antarctic Meteorite Collection have been sawed three different ways over the course of the program since 1977. Samples from the 1976-1977 field season were cut with a circular saw, as can be seen in some of the photos of these 9 meteorites (Fig. 1). All iron meteorites have been sent to the Smithsonian for classification, which involves cutting in order to make an etched surface to examine the Widmanstätten structure in the FeNi metal. The Smithsonian has a large wire saw for enormous meteorites, but the Antarctic irons have not been so large and have been cut using composite blades on their smaller saws (Fig. 2). Finally, the Johnson Space Center has a bandsaw in a nitrogen cabinet and it has been used to split samples collected jointly by the US and Japan (NIPR) during the 1977-1978 and 1978-1979 field seasons, as well as many meteorites from later seasons (Fig. 3). The total number of meteorites cut using the bandsaw at JSC is ~100, including lunar and martian meteorites, HED meteorites, and many chondrites; view a complete list of bandsawed samples. For many of these samples the fines materials generated during cutting has been saved and is available for study as well. Cutting using the JSC bandsaw is done dry so that the meteorites are not contaminated or transformed by use of water or oil. However, there is some potential for contamination as some detailed studies in 2006 showed.
Figure 1: Circular saw marks in the face of ALHA76005, a polymict eucrite.
Figure 2: Two rocks saws at the Smithsonian Institution used to cut iron meteorites for the US Antarctic Meteorite Collection.
Figure 3: Bandsawing of LAP 02200 in 2007 (left) and EET 79001 in 1979 (right), both at JSC.
Contamination from the bandsaw blade
Stepped combustion analyses of bandsaw fines from EET A79001 revealed several different carbon releases (Wright et al., 1993). Although a few of these can be ascribed to indigenous carbon from the meteorite, there were also two distinct releases due to teflon and diamonds. Teflon can be traced back to a possible introduction during handling and processing. And the diamond is undoubtedly from the bandsaw blades which have diamonds embedded onto the front of the steel saw blade. The diamonds are electroplated onto the steel blades, and the electroplating medium can be nickel, copper or other metals; these metals will be discussed below, based on recent measurements on bandsawed samples.
Most Antarctic meteorites cut in the JSC meteorite bandsaw cabinet have dark streaks across the cut surfaces (Fig. 4). The material in these streaks has been unknown until recently. Some scientists have wondered if the dark streaks could contain organic compounds derived either from the rubber wheels at the bottom and top of the bandsaw, or from residue from cleaning the blades. To understand the nature of the dark streaks several Apollo 17 lunar samples were selected for in depth study (Fig. 5). Lunar samples were chosen for this study because the background levels of organics in lunar materials are relatively well understood compared to meteorites (Allton et al., 1999; Clemett et al. 2005; Burlingame et al., 1970; Science Magazine, 30 January, 1970; DesMarais, 1983).
Several splits of aphanitic impact melt breccia 73235 were allocated to Gary Lofgren, and then characterized by Sue Wentworth (HRSEM and EDS) and Simon Clemett (UV laser desorption and raman). SEM images, EDS (Fig. 6) and raman spectra revealed that the black streaks are thin layers of amorphous or glassy material (Fig. 6), containing embedded Ni particles and diamond fragments (Fig. 7). Some images show what appears to be thin glassy or amorphous material, even like wicking in some parts (Fig. 8), indicating that there may have been localized melting. The melting point of 73235 is close to 1250 °C (based on published bulk compositions, and MELTS liquidus calculations), and the nickel particles have the morphology of a solid rather than a liquid. This suggests that the sample may have achieved temperatures between 1250 and 1500 °C locally, along the cut. Images of a diamond embedded blade (Fig. 9), show that the matrix is electroplated Ni along with the large diamonds. Clearly there has been smearing of this blade material, as well as localized heating of the sample, to produce the black streaks. Laser desorption has revealed only a very weak signal for styrene (Fig. 10), a known plasticiser that has been observed in other samples curated at JSC, and most likely coming from the bags or containers.
Figure 4: Photographs of two meteorites cut with a bandsaw – martian orthopyroxenite ALH 84001 (left) and diogenite EETA 79002 (right). Both exhibit the dark streaks from the bandsaw; streaks are present on many meteorites cut with the MPL and lunar bandsaws
Figure 5: Aphanitic impact melt breccia 73235 before and after processing and bandsawing. Many lunar and meteorite bandsawed samples have these dark streaks on sawn faces.
Figure 6a: Low magnification secondary electron and back scattered electron images of the black streak material on 73235.
Figure 6b: Spectra of the unaffected rock (lower left) and of the black streak material (lower right), showing that Ni is a prominent peak in the black streak material.
Figure 7: Secondary electron and back scattered electron images of nickel metal and diamond fragments embedded in the black streak material.
Figure 8: Secondary and back scattered electron images of an ovoid feature in the amorphous black streak material. Note the "wicking" along the edges suggestive of molten material or glass.
Figure 9: SEM image of the front edge of the bandsaw blade showing dark angular diamond fragments, along with gray fine-grained Ni electroplating matrix. The nickel appears to be smeared on the left side of the image and thus could easily be smeared onto a cut surface on a rock.
Figure 10: Mass spectrum obtained using laser desorption mass spectrometry (ultra-L2MS) on the black streak material. Mass 104 styrene is the only significant organic peak, suggesting the black streaks do not harbor any organic compounds.
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- Burlingame, A.L., Calvin, M., Han, J., Henderson, W., Reed, W. and Simoneit, B.R. (1970) Proc. Apollo 11 Lunar Sci. Conf. 1779-1791
- Clemett, S.J., Keller, L.P., and McKay, D.S. (2005) Lunar organic compounds: search and characterization. MAPS 40, #5300
- Des Marais, D.J. (1983) Light element geochemistry and spallogenesis in lunar rocks. Geochim. Cosmochim. Acta
- Wright, I.P., Hartmetz, C.P., and Pillinger, C.T. (1993) An assessment of the nature and origins of the carbon-bearing components in fines collected during the sawing of EET A79001. Jour. Geophys. Res. 98, 3477-3482