Using X-ray Microfocus Spectroscopy to determine
Cometary and Asteroidal Parent-Body Processes
L. J. Hicks1, J. C. Bridges1, T. Noguchi2, M. C. Price3, J. E. Wickham-Eade3,
M. J. Burchell3, J. L. MacArthur1, G M. Hansford1, A. L. Butterworth4
1
Leicester Institute for Space and Earth Observation, Space Research Centre, University of Leicester, UK. (ljh47@le.ac.uk)
2Faculty of Arts and Science, Kyushu University, Japan. 3School of Physical Sciences, University of Kent, UK.
4Space Sciences Laboratory, University of California at Berkeley, USA.
Since the Stardust and Hayabusa missions, we have investigated a selection of these returned cometary and asteroidal samples
using Beamline I-18 at the Diamond Light Source synchrotron in UK, measuring Fe-K X-ray Absorption Spectroscopy (XAS)
and transmission X-ray Diffraction (XRD). Such techniques are essential to determine the origin and evolution of fine grainedplanetary materials, and in particular we have identified evidence for water-rock reaction on the Wild2 parent body.
I-18 X-ray microfocus spectroscopy
Beamline I-18 has an energy range 2.0-20.7 keV and a spot size reduced to ~2.5 μm. X-ray Fluorescence (XRF) can provide
chemical compositions, with XRF maps providing the opportunity to accurately locate features of interest at the micron-scale.
Fe-K XAS measurements typically range 6900-7500 eV at variable energy resolutions 1.0-10 eV, with higher resolution stepsof 0.1 eV focused over the XAS near-edge (XANES) region 7090-7150 eV necessary to accurately determine minor shifts andvariations in the 1s→3d transition pre-edge peak structure and absorption edge. The 1s→3d centroid position is estimated fromthe intensity-weighted average energy position over the pre-edge peaks, and the absorption edge position is the energy at which
intensity is 0.5 of the normalized spectra. Comparisons are made to reference materials, in particular the ferric content of silicatesare estimated by comparing pre-edge centroids with the ferrous-ferric energy shift in standards [1]. Combined with XRF mapping,producing XANES maps can reveal any variations in the ferric content. Additionally, in-situ measurements of transmissionXRD are acquired at 13 keV, with observable 2θ ranging 5.5°-38.4°, corresponding to d-spacings 9-1.5 Å.
Identifying magnetite in Comet Wild 2 samples
Initial investigations of Comet Wild 2 samples returned by Stardust
identified mostly high temperature ferromagnesian silicates [2], but
additional constituents have included CAI’s and chondrule-likefragments, suggesting links between carbonaceous chondrites and theWild 2 material [3].
Hicks et al. (2017) [4] investigated the terminal grains of eight
Stardust aerogel tracks, which include terminal grains identified asnear pure magnetite Fe3O4. Stardust track C2045,4,178,0,0 (#178)
featured a magnetite terminal grain measuring 10 µm in diameter, anda 5 µm magnetite terminal grain in track C2065,4,187,0,0 (#187),
both visibly identified via XAS structure (Figure 1). In particular,absorption edge positions at 7121.5 and 7121.0 eV, and 1s→3d Figure 1. The Fe-K XAS and pre-edge centroid (inset) ofcentroid positions estimated at 7113.1 and 7113.5 eV, closely terminal grains in tracks #178 (Tg1a), #187 (Tg1), and #189
resemble that of a magnetite powder standard with edge and centroid (Tg1), and an analogue CR2 terminal grain (Tg) shot into
positions at 7120.8 eV and 7113.2 eV respectively. A small positive aerogel, compared to powdered magnetite and hematite
shift of up to ~0.5 eV in the Wild 2 grains, compared to the magnetite standard reference material.
standard, could be due to a minor Ni2+ content identified with XRF,
which replaces some Fe2+, resulting in a higher Fe3+/ΣFe content, thus a positive shift in the energy of the Fe-K XAS spectra.
In-situ transmission XRD analyses also identified magnetite with 2θ peaks at d-spacings 1.48 Å, 1.61 Å, 1.71 Å, 2.10 Å, 2.42 Å,
2.53 Å, and 2.96 Å for the cometary #178 (Tg1a and Tg2) and #187 (Tg1), closely matching the 2θ peaks a magnetite referencematerial. However, with unit cell dimensions ranging 8.355-8.370 Å for the cometary grains being slightly lower than that of apure magnetite at 8.387 Å, this shift also suggests a Ni-bearing magnetite. Raman analyses performed at University of Kent also
confirmed a Ni-bearing magnetite. Material in other Stardust tracks, identified via XAS analyses on I-18 at Diamond, include
olivine (#177, #178, #187), pyroxene (#189, #190), Fe,Ni-metal (#170, #176), and Fe,Ni-sulfide (#187, #188) [4].

Magnetite grains are prominent in carbonaceous chondrites, if they have been oxidised and aqueously altered [5], thus thepresence of magnetite in the Wild 2 mineral assemblage suggests further affinities to carbonaceous chondrites, probably resulting
from hydrothermal alteration of the co-existing FeNi and ferromagnesian silicates in the cometary parent-body. Exploring thishypothesis further, powdered NWA 10256 CR2 chondrite material was shot into aerogel at 6.1 kms -1, using a light gas gun [6],simulating the conditions of the Stardust collection. Fe-K XAS identified CR2 magnetite terminal grains (Figure 1) establishing
the likelihood of preserving magnetite during capture in silica aerogel.
Itokawa ferromagnesian silicates and LL5 chondrite
Initial analyses of the Itokawa asteroid particles returned by
Hayabusa had already found mineralogical, petrological, and oxygenisotopic affinities to LL5-6 chondrites [7,8,9]. We used Fe-K XAS toinvestigate the relative abundance of Fe3+ and Fe2+ ions in theferromagnesian silicates of the Itokawa grains, comparing them to theTuxtuac LL5 chondrite meteorite.
Noguchi et al. (2014) [10] investigated four samples from the first
Hayabusa touch-down location: RB-QD04-0008; RB-QD04-0011;
RB-QD04-0015; and RBQD04-0024. These samples, each embedded
in epoxy resin, featured olivine with trace amounts of opaque Figure 2. The Fe-K XAS for olivine in three Itokawa particles,minerals (0011 and 0015), olivine and high-Ca pyroxene (0008), and the Tuxtuac LL5 chondrite, and a terrestrial olivine (Sanlow-Ca pyroxene with plagioclase and small (<2 μm) opaque Carlos).
minerals (0024).
Fe-K XAS for the olivines (Figure 2) estimated absorption edges and 1s→3d centroids at 7119.5-7119.8 eV and 7112.5-7112.6
eV respectively, indistinguishable from those estimated for the Tuxtuac olivine and in terrestrial olivine. Edges and centroids forthe low-Ca Itokawa pyroxene (7119.7 eV and 7112.6 eV), and the high-Ca Itokawa pyroxene (7119.4 eV and 7112.6 eV), arealso indistinguishable from Tuxtuac and from Fe2+ in terrestrial augite. EXAFS analyses of the high-Ca Itokawa pyroxene
suggested a disordered structure which may be partial equilibration or shock. A Ni-bearing phase associated with that high-Ca
Itokawa pyroxene (0008) was also measured for Ni-K XAS, finding similarities to taenite in the LL5 Tuxtuac.
The Fe-K XAS analyses suggested a negligible abundance of Fe3+ ions in Itokawa and the LL5 ferromagnesian silicates,consistent with the initial analyses [7,8,9]. However, the EPMA analyses of the four Itokawa grains, that followed the Fe-K
XAS analysis, suggested a wide range of thermal metamorphism corresponding to petrologic chondrite types LL4 to LL6.
Future XAS investigations of Planetary Sample Returns
As we continue to investigate planetary materials further, and as missions such as Hayabusa 2 and OSIRIS-REx return samples
to Earth, more advanced techniques will be essential to analyse micron-sized material, furthering our understanding of planetaryformation. Offering similar techniques to I-18, but with higher spatial resolution down to ~50 nm, the new I-14 hard X-ray
nanoprobe beamline at Diamond offers nanoscale microscopy with an energy range 5-23 keV, capable of XAS, XANES mapping,and XRD. Additionally, the I-08 Scanning X-ray Microscope (SXM) beamline performs XRF and XANES, but in the soft Xray energy range 0.25-4.4 keV, including Si-K XANES, with a spatial resolution down to ~200 nm.
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