Mapping a meteorite as never before


Rebecca Pool

Thursday, September 26, 2019 - 19:30
Image: Carbon channel EDS tilt series, learn more below [Science Advances].
US-based researchers have mapped a meteorite particle to 15 nm spatial resolution, using advanced X-ray and electron microscopies at the Advanced Light Source and National Center for Electron Microscopy.
Taking a multi-modal imaging approach to meteorite analysis, Professor Jianwei (John) Miao, leader of the Coherent Imaging Group, and colleagues combined X-ray ptychography and scanning transmission X-ray spectromicroscopy with 3D energy-dispersive spectroscopy and electron tomography. 
By using textural and quantitative elemental information, the researchers could understand the mineral composition and processes before and after accretion; the coming together of matter as  influenced by gravity.
Miao and colleagues used X-ray ptychography and STXM in 2D to investigate a grain from the Allende meteorite, observed in Mexico on February 8, 1969 as a CV3 carbonaceous chondrite.
They combined the setup with energy dispersive spectroscopy (EDS) and high-angle annular dark field (HAADF) imaging in 3D.
LEFT: Multimodal x-ray and electron nanoscopic spectral imaging scheme. Allende meteorite grains deposited on a TEM grid were transferred between a Titan 60-300 electron microscope and the COSMIC soft x-ray beamline for tomographic, ptychographic, and spectromicroscopic imaging. COSMIC’s TEM-compatible sample holder enabled the same meteorite grain to be imaged using both imaging modalities. RIGHT: HAADF and EDS GENFIRE tomography reconstructions. Representative 14-nm-thick layers in the reconstructed 3D HAADF (A) and EDS (B) volumes of the Allende meteorite grain. The red arrow points to melt pockets, and the green arrow points to shock veins that were embedded. [Science Advances]
Compared to past studies, the researchers significantly enhanced spatial resolution with results revealing many internal textures and channels to suggest shock veins and melt aggregates on the meteorite.
Using the spectroscopic measurements, they classified the major meteoric components as silicates, sulphides and oxides.
Their multidimensional work also provided hints to the origins and transport of the Allende meteorite within the early solar nebula and highlighted the potential to combine X-ray and electron imaging to study diverse heterogenous materials.
New information
Miao and colleagues deposited the Allende meteorite grain on a carbon-coated TEM grid and transported it to the COSMIC beamline for X-ray imaging.
They conducted multimodal electron and X-ray spectral imaging using HAADF and EDS tomography.
When they sliced through the generalized Fourier iterative reconstruction (GENFIRE) HAADF of the grain, they observed a variety of internal morphologies that revealed different phases of assembly.
X-ray ptychography and STXM absorption spectromicroscopy. (A to D) Localization of major elements in the meteorite revealed by dividing pre-edge and on-edge ptychography images at the absorption edges for Al, Fe, Mg, and Ni. The absorption quotient maps, displayed in logarithmic scale, show the presence of Fe in the shock veins of the silicate that is barely observable in EDS images (red arrows). (E and F) Scattering quotient (fq) maps derived from ptychographic Mg pre-edge and Al pre-edge images, respectively. This region of interest is a zoomed-in view from the dashed red rectangle shown in (B). (G and H) Ni-Fe ratio maps from Mg pre-edge and Al pre-edge scattering quotient maps, respectively. These ratio maps are converted using the SQUARREL method, given a fixed amount of sulphur. The colour bar indicates the Ni-Fe ratio and 100% implies a pure nickel sulphide region. (I to L) Absorption spectra generated from STXM energy scans across the four absorption edges. [Credit: Science Advances]
In the larger meteorite matrix, they observed long internal channels ranging from 20 to 50 nm in diameter to suggest shock veins.
Adjacent to the matrix, they observed two high-intensity spherical granules representing melt pockets.
Electron tomography data was collected using a Titan 60-300 equipped with a HAADF detector (Gatan) and four windowless silicon drift EDS detectors (FEI Super-X).
Using EDS tomography, the team determined the elemental composition of the grain and observed the presence of carbon, oxygen, magnesium, aluminium, silicon, sulphur, chromium, iron and nickel.
They superimposed the EDS and HAADF GENFIRE reconstructions to reveal the main mineral domains, including iron magnesium silicate, aluminium-chromium iron oxide and iron-nickel sulphide.
The research team also used differential X-ray absorption contrast to study elemental locations and abundances in more detail to complement the electron microscopy results.
For this, they collected 2D ptychography images of the grain to pinpoint locations of each element with high contrast and spatial resolution.
Upon closer inspection, the team observed regions with higher Fe concentrations in the silicate to coincide with the Al melt pockets and veins.
The research team developed a SQUARREL (scattering quotient analysis to retrieve the ratio of elements) method to retrieve quantitative information on elemental composition of the complex ptychography images.
The scientists obtained two scattering quotient maps for Mg pre-edge and Al pre-edge images as a region of interest.
These maps showed new and different image contrast compared to conventional XAS images or phase contrast images.
The work distinguished different mineral polymorphs as a strong advantage of the XAS technique compared to conventional EDS.
According to the researchers, this combined information will help them to identify possible mineral phases present in the meteoric grain with nanometre spatial resolution.
The researchers also plotted the compositions of major elements in the iron-nickel sulphide, iron-magnesium silicate and aluminium-chromium iron oxide regions of the grain.
Experimental HAADF tilt series. All 69 processed HAADF projections used as input for GENFIRE reconstruction. Each image was masked, normalized to reference projection, background subtracted, and aligned by center-of-mass and common line alignment. Background subtraction was iterated with alignment to minimize common line differences. Horizontal direction is the rotation axis. [Science Advances]
Using analyses from both X-ray and electron microscopy data, they narrowed the identities of the various phase groupings to provide a detailed nanoscopic petrographical picture of the meteorite grain.
They quantified the Ni and Fe composition in the iron-nickel sulphide region using the SQUARREL method to reconfirm the identity of the compound, previously only predicted using EDS.
The HAADF tomography and X-ray ptychography provided high-resolution textural information on the possible processes that affected the Allende parent body during and after accretion.
The researchers explained the possibility of shock melting as the cause for localised shock veins and pockets to generate the highly deformed material.
Experimental carbon channel EDS tilt series. All 20 processed carbon EDS projections used as input for GENFIRE reconstruction. Each image was masked, normalized to reference projection, background subtracted, and aligned by centre-of-mass and common line alignment. Background subtraction was iterated with alignment to minimize common line differences. Horizontal direction is the rotation axis. [Science Advances]
As Miao highlights, the evidence that was gathered using the two imaging techniques - HAADF and EDS with X-ray ptychography and XTSM absorption spectromicroscopy - agreed well with previous 2D studies of Allende, which were comparatively at a coarser resolution.
Research is published in Science Advances.
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