Meteorites, individual

The application of the laser probe to meteorite chronology is illustrated by a study of Ca-Al-rich inclusions from the Allende meteorite [7]. This study was able to show that the K in the inclusions studied mainly concentrated in veins and rims with very little, if any, K in the major minerals. The limit obtained is something of the order of 10 ppm. On the other hand, the major minerals do contain appreciable 40Ar. Individual chondrules and the matrix were also studied in the Allende meteorite from places adjacent to the Ca-Al-rich inclusions. For these samples the ages varied from 3.3 to 4.4 G.y. There appears to be evidence that the Allende meteorite has been subjected to numerous metamorphic events, presumably of a collisional origin. 

Nicolussi GK, Pellin MJ, Lewis RS, Davis AM, Amari S, Clayton RN (1998a) Molybdenum isotopic composition of individual presolar silicon carbide grains from the Murchison meteorite. Geochim Cosmochim Acta 62 1093-1104... 

Lee T, Papanastassiou DA, Wasserburg GJ (1977) Mg and Ca isotopic study of individual microscopic crystals from the Allende meteorite by the direct loading technique. Geochim Cosmochim Acta 41 1473-1485 Lee T, Russell WA, Wasserburg GJ (1979) Calcium isotopic anomalies and the lack of aluminum-26 in an unusual Allende inclusion. Appl J Lett 228(L93-L98) 661-662 Marshall BD, DePaolo DJ (1982) Precise age determinations and petrogenetic studies using the K-Ca method. Geochim Cosmochim Acta 46 2537-2545... 

In addition to oxygen isotopes, the volatile elements H, C, N, and S also show extremely large variations in isotope composition in meteorites. In recent years, most investigations have concentrated on the analyses of individual components with more and more sophisticated analytical techniques. 

EmUiani C (1966) Paleotemperature analysis of Caribbean core P6304-8 and P6304-9 and a generalized temperature curve for the past 425000 years. J Geol 74 109-126 Emiich K, Ehhalt DH, Vogel JC (1970) Carbon isotope fractionation during the precipitation of calcium carbonate. Earth Planet Sci Lett 8 363-371 Engel MH, Macko SA, SUfer JA (1990) Carbon isotope composition of individual amino acids in the Murchison meteorite. Nature 348 47-49... 

Watson LL, Hutcheon ID, Epstein S, Stolper EM (1994) Water on Mars clues from deu-terium/hydrogen and water contents of hydrous phases in SNC meteorites. Science 265 86-90 Weber IN, Raup DM (1966a) Eractionation of the stable isotopes of carbon and oxygen in marine calcareous organisms-the Echinoidea. 1. Variation of and content within individuals. Geochim Cosmochim Acta 30 681-703... 

Nicolussi, G. K., Pellin, M. J., Lewis, R. S. et al. (1998) Molybdenum isotopic compositions of individual presolar silicon carbide grains from the Murchison meteorite. Geochimica et Cosmochimica Acta, 62, 1093-1104. 

Nichols, R. H., Hohenberg, . M., Kehm, K., Kim, Y. and Marti, K. (1994) I-Xe studies of the Acapulco meteorite Absolute ages of individual phosphate grains and the Bjurbole standard. Geochimica et Cosmochimica Acta, 58, 2523-2561. 

Electron microprobes can be used in spot mode to measure the chemical compositions of individual minerals. Mineral grains with diameters down to a few microns are routinely measured. The chemical composition of the sample is determined by comparing the measured X-ray intensities with those from standards of known composition. Sample counts must be corrected for matrix effects (absorption and fluorescence). The spatial resolution of the electron microprobe is governed by the interaction volume between the electron beam and the sample (Fig. A.l). An electron probe can also be operated in scanning mode to make X-ray maps of a sample. You will often see false-color images of a sample where three elements are plotted in different colors. Such maps allow rapid identification of specific minerals. EMP analysis has become the standard tool for characterizing the minerals in meteorites and lunar samples. 

The cross section obtained for single fullerenes and buckyonions reproduce the behaviour of the interstellar medium UV extinction curve. A power-law size distribution n(R) R m with in = 3.5 1.0 for these molecules can explain the position and widths observed for the 2,175 A bump and, partly, the rise in the extinction curve at higher energies. We infer ISM densities of 0.2 and 0.1 ppm for small fullerenes and buckyonions (very similar to the densities measured in meteorites). If as expected the cosmic carbon abundance is close to the solar atmosphere value, individual fullerenes may lock up 20-25% of the total carbon in the diffuse interstellar space. 

Cathodoluminescence observations by themselves reveal details of texture among minerals and suggest chemical or structural variation within individual grains. Seldom can the CL observations be interpreted without additional information such as chemical or structural analysis. With this in mind, data obtained from the electron microprobe (EMP), ion microprobe (IMP), scanning electron microscope (SEM), or by thermoluminescence studies complement CL observations and many of the CL examples for meteorites can be best interpreted or related to mineralogy through these data. 

To obtain a quantitative measure of the CL intensity of individual CL emissions, an optical multichannel analyzer was coupled to the optical system of an electron microprobe allowing simultaneous collection of CL spectra and minor element data from a single point (Steele, I.M. Meteoritics. submitted). For CL spectra obtained with a 15 kV focused beam, enstatite from both enstatite chondrites and achondrites showed three distinct peaks (Fig.l) centered at about 742, 664, and 483nm. To allow assignment of these peaks, spectra from synthetic Mn and Cr doped enstatite are shown in Fig. 2 and the emissions from these two samples closely match the two red peaks of meteoritic enstatite neither synthetic sample shows a blue peak. The peak positions of Cr and Mn are not constant for different meteoritic enstatites and are not the same as for the Cr and Mn doped standards. The variation is about 20nm... 

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