UK Antarctic Meteorite Classification – the process

The first of the UK Antarctic meteorites have been classified! Nine rocks retrieved from the Hutchison (HUT) and Outer Recovery (OUT) icefields have been approved and published by the Meteoritical Bulletin. For details of these, please see this table and follow the links.

How were they classified?
The rocks collected in Antarctica were shipped frozen to the UK and kept in temperature monitored freezers. Each rock was thawed in an exisccator chamber to minimise any chemical reaction with residual ice; any ice present should sublimate straight to gas from the solid phase. After thawing, the rocks were all individually weighed and photographed. Measurements were taken of the rock’s magnetic susceptibility and electrical conductivity (see this previous blog post by Tom Harvey, as these properties can give a provisional indication of their possible classification.

Some of the meteorites were chosen to undergo computerised tomography (CT) scanning and photogrammetry. CT-scanning gives a 3D image of the interior of the rock. This provided compositional information and helped us to choose where to attempt to break or cut the rock. Photogrammetry allows a full 3D surface rendering of the meteorite to be made. For more details of this work, please see Tom’s LPSC conference presentation.

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Tom Harvey in the curation lab imaging one of the meteorite samples. Photo: KJoy

Where there were no natural crumbs or fragments of material for further analysis, each rock was split with rock splitters, keeping them inside their sterile bag so that the only contact was with the stainless steel blade. A few meteorites and some of the fragments from the rock splitters were cut with a low speed saw to ensure the material does not get hot enough to cause any alteration.

Fragments were weighed, photographed and then made into epoxy blocks. These have to be polished to a very flat surface, in order to be able to examine the chemical composition using a scanning electron microscope (imaging system) and electron microprobe (used to determine mineral chemistry).

The scanning electron microprobe provides greyscale images (see left below), where the brightness corresponds to the differences in atomic number (number of protons) of the different elements. For example, iron, with an atomic number of 26, will be much brighter compared to sodium, with an atomic number of 11. It can also be used to quantify the difference in this brightness to get an idea of the relative abundance of each element and likely mineral. We use the scanning electron microscope first to get an image and element maps of the whole section (see right image below).

Following this, around 30-40 spots (points) are selected for measurement with the electron microprobe as this allows for more precise and accurate quantification of the mineral composition. In the element map above, the silicate minerals shown in green are usually olivine, and the minerals in light blue are usually pyroxene. The iron content of olivine and pyroxene varies and this is used to distinguish ordinary chondrites between the “H”, “L”, and “LL” classifications (check out where these meteorites sit within the family tree here).  “H” chondrites have the highest iron metal content, but lower iron oxide in pyroxene and olivine. “L” chondrites have lower iron and less metal but higher iron oxide in pyroxene and olivine, whereas “LL” have the lowest total iron but the highest iron oxide in pyroxene and olivine.

Graph showing the iron oxide abundance in pyroxene (Fs) against the iron oxide abundance in olivine (Fa). Yellow crosses show the data collected for five UK Antarctic meteorites, superimposed on a graph of values recommended as distinguishing the H, L and LL ordinary chondrite groups for the Nomenclature committee (Grossman & Rubin, 2006).

Meteorites classified!

We have news…. its a big day for the project as we have our first batch of meteorites that have been formally classified by the Meteoritical Society Nomenclature committee and have now been published in the Meteoritical Bulletin Database!

A list of the newly classified meteorites can be found here https://ukantarcticmeteorites.wordpress.com/meteorite-discoveries/ , and you can click on each meteorite name to learn more and see some pictures of the samples. So far all of those classified from our 1st field season come from parent bodies in the asteroid belt: all are undifferentiated ordinary chondrite types. This means that they come from some from asteroids that represent some of the earliest Solar System building block rocky material that never got big enough to completely melt (hence they are undifferentiated), but they are all are from quite a common type of meteorite group (the ordinary chondrites). So far all the types classified are stony and not metal types – meaning that they are dominantly made up of silicate minerals rather than metal. In case you get lost with all the terms used – an overview of the different types of meteorites can be found at https://ukantarcticmeteorites.wordpress.com/meteorite-classification/

The first classified batch include nine meteorites: two were recovered from from the Hutchison Icefield area (these ones are called HUT), and seven from the Outer Recovery icefield area (these ones are called OUT). The number after the acronym name specifies the particular sample type. If you want to read more about the names of icefields we visited you can read here https://ukantarcticmeteorites.wordpress.com/2021/04/15/new-meteorites-new-names/

This has been the cumulation of a lot of hard work from the field search teams from the 1st 2018-2019 field season (Julie Baum and Katie Joy) and logistics support personnel, the BAS cargo transfer team, the local meteorite lab and classification team – lead by Jane MacArthur with help from Thomas Harvey, Rhian Jones and Katie Joy. A huge thanks to the local analytical lab leads who have kept the instruments we use to image the samples and determine their chemistry (Lewis Hughes and Jon Fellowes), those who support the labs we use to prepare our samples (John Cowpe and Lydia Fawcett) and appreciate key advice from Andrew Smedley, Romain Tartese and Geoff Evatt. Thanks also to all the external help we have had from the Natural History Museum staff in helping these efforts, and to the Meteoritical Society Nomenclature team and Meteorite Bulletin teams for reviewing, approving and sharing the samples’ new names.

You can also read a blog from Jane on the lab curation approach where she will talk about how we go from picking up a sample in the field to working out what type of meteorite it represents.

We also have more samples under review by the nomenclature team so will announce what else we have found in the near future… stay tuned for more meteorites to come…

Photo of the largest sample found in season 1 – now called OUT 18021 (or still affectionately referred to as the melon on account of its large size). the scale cube is 1 cm in size. Image: Lost Meteorites of Antarctica / The University of Manchester