Non-destructive meteorite classification using Mössbauer spectroscopy

Abundance and speciation of Fe play an important role in meteorite classification. Ordinary chondrites, for example, are grouped based on their Fe content and speciation into H chondrites (H for high total Fe and high metal content), L chondrites (L for low total Fe) and LL chondrites (low total Fe, low metal). In differentiated bodies metallic Fe partitions into the core while oxidized Fe stay in the mantle. Mössbauer spectroscopy is a powerful tool to determine Fe-bearing mineral phases and Fe oxidation states, and to quantify the distribution of Fe between mineral phases and oxidation states. The miniaturized Mössbauer spectrometer MIMOS II is set up in backscattering geometry and thus enables non-destructive measurements of whole rock samples [1]. Creating a database of Mössbauer analyses of meteorites reported in the literature and from our own analyses with MIMOS II, we are investigating whether Mössbauer spectral information allows to distinguish between meteorite groups. It could then be used alongside other non-destructive techniques used for meteorite classification such as magnetic susceptibility measurements [2] and elemental analysis with portable X-Ray Fluorescence (XRF) spectrometers [3].

Mössbauer parameters were assembled from [4-6] and from measurements of meteorites loaned from the collections of National Museums Scotland (NMS). Literature data were obtained by measuring pulverized bulk rock samples in transmission geometry. The NMS meteorites were analysed with a MIMOS II instrument [1] at the Mössbauer Spectroscopy laboratory for Earth and Environment (MoSEE) at the University of Stirling. MIMOS II has a 1.4 cm diameter circular field of view, and we measured interior meteorite surfaces free of fusion crust.

In Fig. 1 we compare ordinary chondrites (OCs) with several achondrite groups where Fe is distributed between Fe(II) in olivine and Fe(0) in the metallic phase (the Fe-Ni alloy kamacite). OC H, L, and LL data are from [4], ureilite data are from [5]. Howardite, eucrite and diogenites (HEDs) have been measured for this work. A set of paired stony meteorites have been investigated with a MIMOS II on board the Mars Exploration Rover Opportunity at Meridiani Planum on Mars [6]. On the basis of their chemical composition they are similar to HEDs but have a higher metal content, which would be consistent with the silicate fraction of mesosiderites [6]. They are labeled as HED/MES in Fig. 1.

Fig. 1

For the plot in Fig. 1 we have made the assumption that any Fe(III) measured stems from terrestrial weathering of most likely the metal phase. We have therefore added that Fe fraction to the kamacite metal fraction. This plot allows for a clear distinction between H and L, and H and LL groups. L and LL groups overlap and cannot be distinguished. However, this complements magnetic susceptibility measurements, which are able to clearly distinguish L and LL groups, but the data for L and H groups overlap [4] A combination of Mössbauer an magnetic susceptibility is therefore able to resolve the groups unambiguously. The two LL chondrites plotting amongst the H chondrites are so severely weathered that olivine was also weathered and contributed Fe to the Fe(III) weathering phases.

Ureilites, a group of primitive achondrites, plot in an area distinct from the H, L and LL OCs. There are two outliers. One near a kamacite + Fe(III) value of ~80% is so severely weathered that all kamacite has been converted to Fe(III), and the Fe(III) fraction likely also contains a significant fraction of Fe from weathered olivine. The other lies near a kamacite + Fe(III) value of ~42% and has an unusually high amount of Fe(II) associated with pyroxene rather than olivine.

The HED/MES meteorites found on Mars plot close to the L and LL OC groups and do not overlap with the HEDs. HEDs generally contain little olivine and metal, if any, and therefore plot close to the origin of the diagram. Excursions to higher kamacite + Fe(III) values larger than 10% are a result of substantial terrestrial weathering. We need to obtain Mössbauer spectra from mesosiderite silicate fractions to establish a grouping with the HED/MES specimens. The HEDs all plotting on the X-axis highlight that different parameters need to be plotted to compare, for example, more evolved stony achondrite groups that do not contain any metallic Fe, and meteorite groups that do not contain olivine.

Mössbauer spectroscopy has not been applied systematically to meteorites so far. As a result, Mössbauer data are not available for all meteorite groups or lack a statistically significant number of analyses of different specimens from each group. This work is therefore ongoing. We could show that certain meteorite groups can be distinguished on the basis of Mössbauer data. Some ambiguities can be resolved by applying other non-destructive techniques, e.g. magnetic susceptibility measurements or XRF. Mössbauer spectroscopy, magnetic susceptibility, and XRF are available in portable form and can be applied in the field or in situ on the surface of asteroids during similar missions, where they would provide an unequivocal link between parent body and the relevant meteorite group. Because these methods are non-destructive, they can also be applied to the small and invaluable sample volumes that will be returned from asteroids in ongoing missions such as the Japanese Hayabusa2 and NASA’s Osiris Rex.

References

[1] Klingelhöfer, G. et al.: Athena MIMOS II Mössbauer spectrometer investigation. Journal of Geophysical Research 108, 8067, doi:10.1029/2003JE002138, 2003.

[2] Rochette, P., et al..: Magnetic classification of stony meteorites: 1. Ordinary chondrites. Meteoritics & Planetary Science, Vol. 38, pp. 251-268, 2003.

[3] Zurfluh, F.J. et al.: Evaluation of the utility of handheld XRF in meteoritics. X-Ray Spectrometry, Vol. 40, pp. 449–463, 2011.

[4] Righter, K. et al.: Non-destructive classification approaches for equilibrated ordinary chondrites, Meteoritics and Planetary Science, Vol. 48(s1), 5232, 2013.

[5] Burns, R.G., and Martinez, S.L.: Mössbauer Spectra of Olivine-rich Achondrites: Evidence for Preterrestrial Redox Reactions. Proceedings of Lunar and Planetary Science, Vol. 21, pp. 331-340.

[6] Schröder, C. et al.: Properties and distribution of paired candidate stony meteorites at Meridiani Planum, Mars. Journal of Geophysical Research 115, E00F09, doi:10.1029/2010JE003616, 2010.