¹Antonio Lanzirotti,^1,2Stephen R. Sutton,¹Matthew Newville,³Adrian Brearley,⁴Oliver Tschauner
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14130]
¹Center for Advanced Radiation Sources, The University of Chicago, Argonne, Illinois, USA
²Department of the Geophysical Sciences, The University of Chicago, Argonne, Illinois, USA
³Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA
⁴Department of Geoscience, University of Nevada Las Vegas, Las Vegas, Nevada, USA
Published by arrangement with John Wiley & Sons

This study describes the application of new synchrotron X-ray fluorescence (XRF) and diffraction (XRD) microtomographies for the 3-D visualization of chemical and mineralogical variations in unsectioned extraterrestrial samples. These improved methods have been applied to three compositionally diverse chondritic meteorite samples that were between 300 and 400 μm in diameter, including samples prepared from fragments of the CR2 chondrite LaPaz Icefield (LAP) 02342, H5 chondrite MacAlpine Hills (MAC) 88203, and the CM2 chondrite Murchison. The synchrotron-based XRF and XRD tomographies used are focused-beam techniques that measure the intensities of fluorescent and diffracted X-rays in a sample simultaneously during irradiation by a high-energy microfocused incident X-ray beam. Measured sinograms of the emitted and diffracted intensities were then tomographically reconstructed to generate 2-D slices of XRF and XRD intensity through the sample, with reconstructed pixel resolution of 1–2 μm, defined by the resolution of the focused incident X-ray beam. For sample LAP 02342, primary mineral phases that were visualized in reconstructed slices using these techniques included isolated grains of α-Fe, orthopyroxene, and olivine. For our sample of MAC 88203, XRF/XRD tomography allowed visualization of forsteritic olivine as a primary mineral phase, a vitrified fusion crust at the sample surface, identification of localized Cr-rich spinels at spatial resolutions of several micrometers, and imaging of a plagioclase-rich glassy matrix. In the sample of Murchison, major identifiable phases include clinoenstatite- and olivine-rich chondrules, variable serpentine matrix minerals and small Cr-rich spinels. Most notable in the tomographic analysis of Murchison is the ability to quantitatively distinguish and visualize the complex mixture of serpentine-group minerals and associated tochilinite–cronstedtite intergrowths. These methods provide new opportunities for spatially resolved characterization of sample texture, mineralogy, crystal structure, and chemical state in unsectioned samples. This provides researchers an ability to characterize such samples internally with minimal disruption of sample micro-structures and chemistry, possibly without the need for sample extraction from some types of sampling and capture media.

¹José C. Aponte,^1,2,3Frédéric Séguin,^1,4Ariel J. Siguelnitzky,¹Jason P. Dworkin,¹Jamie E. Elsila,¹Daniel P. Glavin,^5,6,7Harold C. Connolly Jr,⁵Dante S. Lauretta
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14118]
¹Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
²Department of Physics, The Catholic University of America, Washington, DC, USA
³Center for Research and Exploration in Space Science and Technology, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
⁴Sig Engineering LLC, Laurel, Maryland, USA
⁵Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
⁶Department of Geology, School of Earth and Environment, Rowan University, Glassboro, New Jersey, USA
⁷Department of Earth and Planetary Science, American Museum of Natural History, New York, New York, USA
Published by arrangement with John Wiley & Sons

Volatile organic compounds (VOCs) are carbon-containing chemicals that may evaporate rapidly at room temperature and standard pressure. Such organic compounds can be preserved inside carbonaceous chondrite matrices. However, unlike meteoritic soluble organic matter (SOM) and insoluble organic matter (IOM), VOCs are typically lost (at least in part) during sample processing (meteorite crushing) and exposure to terrestrial atmosphere and/or solvents. Like SOM and IOM, VOCs can provide valuable insights into the chemical inventory of the meteorite parent body and even the presolar cloud from which our solar system formed, as well as the composition and processes that occurred during the early formation of our solar system and the asteroidal stage. Thus, in this work, we designed and built an instrument that allowed us to access the VOCs present in samples of the carbonaceous chondrites Murchison and Sutter’s Mill after mineral disaggregation by means of freeze–thaw cycling. We simultaneously evaluated the abundances and compound-specific 13C-distributions of the volatiles evolving after meteorite powdering at ~20, 60, and 100°C. Carbon monoxide (CO) and methane (CH4) were released from these meteorites as the most abundant VOCs. They were combusted together for analysis and showed positive δ13C values, indicative of their extraterrestrial origins. Carbon dioxide (CO2) was also an abundant VOC in both meteorites, and its isotopic values suggest that it was mainly formed from dissolved carbonates in the samples. We also detected aldehydes, ketones, and aromatic compounds in low amounts. Contrary to Murchison, which mostly yielded VOCs with positive δ13C values, Sutter’s Mill yielded VOCs with negative δ13C values. The less enriched 13C isotope composition of the VOCs detected in Sutter’s Mill suggest that they are either terrestrial contaminants, such as VOCs in compressed gas dusters and common laboratory solvents, or compounds disconnected from interstellar sources and/or formed through parent body processing. Understanding the relative abundances and determining the molecular distributions and isotopic compositions of free meteoritic VOCs are key in assessing their extraterrestrial origins and those of chondritic SOM and IOM. Our newly developed technique will be valuable in the study of the samples brought to the Earth from carbonaceous asteroid Bennu by NASA’s OSIRIS-REx mission.