Classifying meteorites using Scanning Electron Microscopy

How textural and geochemical analyses can give us a glimpse into our Solar System
Classifying meteorites using Scanning Electron Microscopy

Introduction

Meteorites are solid objects that have travelled through space before surviving atmospheric entry and falling to another planetary surface. There are more than 40 different sub-groups of meteorites (Evans, 2016), and more than 65,000 meteorites in global collections. Before a meteorite can be officially recognised, they need to be classified into one of these groups and submitted to the Meteoritical Society. Determining the classification of a meteorite gives us an idea of the processes that formed them and where they may have come from in our solar system. Classification of meteorites uses their textural and compositional characteristics, and this is where scanning electron microscopy (SEM) as well as analytical techniques such as energy dispersive spectroscopy (EDS) and/or wavelength dispersive spectroscopy (WDS) are useful.
<p>Evans 2016 - meteorite classification</p>

Methods


Planetary scientists at PEMC lead the research group UK Space Rocks, and are actively classifying meteorites in the lab. We classify meteorites by standard scanning electron microscopy techniques, preferentially using the JEOL 7001 FE-SEM equipped with a X-Max 50 mm2 EDS detector and Oxford Instruments AZtec software. Textural analyses of each sample are conducted using backscattered electron (BSE) imaging. Standardised quantification of elements, mineral characterisation and X-ray element mapping of each meteorite are conducted using EDS for major and minor elements, or WDS for low concentrations and/or trace elements.
<p>NWA6141</p>

(left) BSE image of NWA 6414, (right) EDS image of NWA 6414 where red = magnesium, green = calcium and blue = iron

NWA 6414

Our in-house facilities were used to collect backscattered electron (BSE) imagery and chemical analysis. BSE images can highlight the distribution of elements across a sample based on their atomic number, which is very useful when determining where different mineral phases are across a sample. These images also help us distinguish between different textures.
BSE imagery of NWA 6414 showed that the sample was brecciated and composed of multiple different types of rock of different clasts. Combined X-ray element maps and normalised compositional data collected by EDS indicated mineral phases such as pyroxene and plagioclase. These analyses also provided a further distinction between clasts and matrix material across the meteorite.
NWA 6414 was officially classified as a polymict eucrite in late 2021. The type specimen is held at the Museum National d’Histoire Naturelle in Paris. 

Winchcombe Meteorite 

Element maps of Winchcombe were produced and combined into a single image, making it possible to identify the minerals based on their chemistry including olivine, pyroxene, tochilinite and phyllosilicates, plus additional minerals not originally seen. Textural variations visible through combined element maps and BSE imagery ensured the identification of two different rock types within our sample. When combining the results with those from the Natural History Museum and other universities across the UK, it became clear that the meteorite is highly brecciated and consists of multiple lithologies, not just the two observed. Winchcombe has been officially classified as a CM2 Carbonaceous Chondrite and is held at the NHM in London. 
<p>Winchombe</p>

(left) Image of the Winchcombe Meteorite (Trustees of the Natural History Museum, 2021), (right) EDS image of our Winchcombe sample, where red = magnesium, green = iron and dark blue = calcium

<p>meteorite</p>

Mineral map of NWA 15189 where pink = Low-Ca pyroxene, orange = High-Ca pyroxene, dark green = Plagioclase, green = weathering and teal = silica

NWA 15189

Most recently, we have classified NWA 15189 - a polymict eucrite. Similarly to Winchcombe, element maps were combined to create a mineral map for this meteorite, before in-detail chemical analysis was carried out for different mineral compositions. The large area map of NWA 15189 displayed a lack in chondrules and brecciated nature indicating it was an 'achondrite' with a range of lithologies. Chemical compositions of each of the mineral phases (predominantly pyoxene and plagioclase) suggested that this sample was from the HED family.
NWA 15189 was officially classified as a polymict eucrite in 2022. The type specimen is held at the University of Plymouth, UK. 

Conclusion

Meteorites need to be classified chemically and texturally before they can be officially named and recognised. SEM, and particularly EDS, are powerful methods in classifying meteorites, allowing for detailed textural and geochemical analysis of samples that enable us to work out which classification a meteorite belongs to. NWA 6414 was officially classified as a polymict eucrite and the Winchcombe meteorite was officially classified as a CM2 carbonaceous chondrite. These efforts are just two examples of how SEM-EDS techniques at Plymouth Electron Microscopy Centre have been used to analyse and classify meteorites.