hide
Free keywords:
-
Abstract:
Introduction: Organic Matter (OM) is an important constituent in a variety of extraterrestrial materials such as
carbonaceous chondrites [1] or returned mission samples [2]. Its isotopic composition, morphology, structure, ele-
mental composition, and functional chemistry record a combination of presolar, nebular, parent-body, and terrestrial
processes, leaving a complex fingerprint on the OM. These manifold processes have to be disentangled by either bulk
extraction methods [3] or high-spatial resolution “in-situ” studies [4]. In-situ studies have the advantages that they
also allow study of the petrographic relationship to the surrounding matrix and require only minimal chemical treat-
ment. Moreover, the exclusion or identification of any terrestrial overprint on the makeup of OM is of paramount
importance to understand early Solar System processes. In this study, we investigate OM properties within two recent
observed falls, Tarda and Winchcombe, by a combination of high-spatial resolution analysis techniques, to gain further
insight into OM formation and modification processes.
Samples and Methods: Tarda is an observed fall from Morocco (2020) and has been classified as C2-ungrouped
showing oxygen isotopic and petrographic affinities to the Tagish Lake meteorite [5]. Winchcombe has been classified
as CM2 and is the first recovered observed fall in the UK for 30 years. Both meteorites offer a unique opportunity to
investigate OM within fresh samples that have not been severely overprinted by terrestrial contamination. The C- and
N-isotopic composition of Tarda was studied by NanoSIMS applying standard ion imaging protocols. OM was char-
acterized in thin sections by SEM at NHM London and then FIB-prepared for synchrotron and TEM experiments at
SuperSTEM using a Hitachi Ethos NX5000 FIB-SEM. Scanning Transmission X-Ray Microscopy (STXM) was per-
formed on several lamellae prepared from Winchcombe at the I08 beamline of Diamond Light Source. Further min-
eralogical and low kV STEM-EELS analyses in the vibrational and core loss regimes will be performed with a mon-
ochromated, aberration-corrected Nion UltraSTEM 100MC (60 kV) at SuperSTEM.
Results and Discussion: OM identified in the mildly, intermediately, and highly altered lithologies of the Winch-
combe and Tarda meteorites shows globular, multi-globular, diffuse and vein-like morphologies. Based on SEM ob-
servations, silicates (most likely phyllosilicates) and carbonates are intimately associated with the OM. Nitrogen and
carbon isotopic compositions of OM aggregates and nanoglobules within Tarda show a range of 15N values from
close-to-terrestrial to ~ 600‰ with close-to-terrestrial or slightly heavy 13C values, which is, as expected, more ex-
treme than reported bulk isotopic compositions (15N = 55‰, 13C = 11‰) [6]. Single hotspots can reach >1000‰ in
15N and ~80‰ in 13C. No negative 15N values similar to Maribo OM have been detected [7]. Further isotopic
analyses on Winchcombe OM will be presented at the meeting.
STXM analyses at the CK-edge show that OM in Winchcombe is typical for pristine OM in primitive extraterres-
trial samples, with strong absorption at the aromatic C=C (~285 eV) and the ketone/aldehyde (~286.6 eV) bands. The
COOH carboxyl absorption feature (~ 288.5 eV), however, is diminished or absent in all measured areas. It has been
suggested that the appearance of this bonding feature could be indicative of more advanced OM alteration and the
formation of smaller, more soluble molecules [7,8]. Fine structure at the NK-edge shows two dominant bands at
around 398.8 eV and 399.8 eV, which can be attributed to C-N double (imine) and triple (nitrile) bonding environ-
ments. The functional chemistry of nitrogen in meteoritic OM is very sensitive to aqueous alteration reactions, with
the abundance of oxygenated [9] and hydrogenated, i.e., N-Hx- bonds [10] correlating with advancing alteration. The
presence of the highly reactive double and triple C-N bonding environments is therefore a strong indicator that the
Winchcombe OM is still very pristine.
Acknowledgements: The DFG is acknowledged for funding this project in the course of the SPP 1833. SuperSTEM is the
U.K. National Research Facility for Advanced Electron Microscopy, supported by the Engineering and Physics Science Research
Council (EPSRC). We thank Burkhard Kaulich and Majid Kazemian for help with STXM analyses and acknowledge Diamond
Light Source for time on beamline I08 under proposal #MG30183-1.
References: [1] Pizzarello S. and Shock E. (2017) Origins Life Evolution Biosphere 47:249-260. [2] Yada T. et al. (2022)
Nature Astronomy 6:214-220. [3] Martins Z. et al. (2020) Space Science Reviews 216:54-77. [4] Van Kooten E. M. M. E. et al.
(2018) Geochimica et Cosmochimica Acta 237:79-102. [5] King A. J. et al. (2021) LPSC 52:A1909. [6] Marrocchi Y. et al. (2021)
The Astrophysical Journal Letters 913:L9. [7] Vollmer C. et al. (2020) Scientific Reports 10:20251-20260. [8] Changela H. G. et
al. (2018) Meteoritics & Planetary Science 53(5):1006-1029. [9] Cody G. D. and Alexander C. M. O`D. (2017) LPSC 48:A2747.
[10] Vollmer C. et al. (2020) Meteoritics & Planetary Science 55(6):1293-1319.
