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.

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).


A visitor from afar

Katie Joy | 21 Jan 2019

I nearly fell of my skidoo with surprise today — well I certainly ducked. On our traverse back from an icefield a visiting skua (a dark grey predatory seabird, similar to a large gull) swooped in. When you don’t expect to see any life out here it makes you pay attention. It flew around us as we came to a stop and then settled in for a sit down on the snow between me and the sledge just hanging out as we paused to enjoy our visitor. I imagine it was as perplexed as we were to see it. We are some 450 km from open ocean, and about 270 km from where we know there is a bird roosting site to the northwest of us. Whatever the skua was doing this far south is a mystery — there’s not a lot of food to be had in these parts… We left it sitting on the snow to continue its journey and drive on home.

Our visiting skua. [Credit: K H Joy]

Work wise — we have had a couple of broken days. The wind has been quite high blowing between 15 and up to 20 knots at times. When it is like this the snow blows across the surface making it hard to spot tiny blueberry sized meteorites or even the not so tiny cantaloupes. It is not that cold (probably –10ºC to –15ºC or so with wind chill) in our big down jackets. We did manage to get out yesterday afternoon and this afternoon after waiting for the snow to stop blowing a bit, and found a meteorite each day. Both times a fragment from what must be bigger stones. The one today looks interesting — it has a very shiny (glass-like) black fusion crust and a pale coloured interior with no chondrules to be seen. At first guess from these characteristics it looks like an achondrite type of meteorite — which is exciting as it implies it might have originated from a large parent body like asteroid Vesta or maybe even the Moon (I am still holding out some hope to find a lunar this season!). I look forward to seeing what the classification is of this one**.

Ice art: one of the four dragons as Julie calls them. Some spectacular sastrugi on our daily commute route. [Credit: K H joy]

We managed to get out to some of the icefields that are furthest from us, and have no reason to return again there this season. If the weather stays the same for the next couple of days as the forecasters say we will finish off some of our local icefields and hopefully get a few meteorites to top up the 33 we have found so far.

A workplace with a view. Driving around in the sun and wind today. [Credit: K H Joy]

PS In a blog post the other day I implied that the rocks at the local nunatak were probably part of the Beacon Group of sedimentary rocks. Having read in more detail about the Shackleton mountains this probably isn’t the case so I take it back. My bad, I got a bit carried away with excitement being surrounded by non-space rocks for a change 🙂 I look forward to chatting with the guys at BAS when I get to Cambridge next to find out what geological unit our nunatak belongs to!

** How do we classify the meteorites we find?

At the moment in the field it is quite hard to judge what types of meteorites we have found, although I am having fun guessing. I have used the AnMetMet device, which Tom Harvey described in a previous blog post, to check out some of the stones we have found. This test is pretty indicative telling us if we have samples that are one of the different ordinary chondrite groups (there are three main types, that we can tell apart due to their iron content) or if they are iron-rich types or stony-irons.

We need to check the samples in more detail when we have them back in the lab to see what specific type of sample we have and see if they have been thermally metamorphosed (i.e., heated to a high temperature and altered) or have been altered by water on their parent asteroid. We do this by making a geological thin section — cutting the sample and polishing it until it is about 30 microns in thickness, about as wide as a human hair. We can look at this thin slice of the meteorite under a microscope and interpret what crystals it has, the form these crystals are in, and what other meteorites it is similar to. You can look at some amazing meteorite thin section on the Virtual Microscope website (along with Apollo Moon rocks and Earth rocks).

We will also do some chemical tests to check certain elements in minerals that help to tell different groups apart, and use the also the different forms of the element oxygen. Oxygen isotopes are a great fingerprint we can use to link different meteorite groups together, and to relate them to planetary-scale chemical processes.

Once all this work has been done (it might take us a few months after the samples reach us to get through all our analyses), we will submit a report to the Meteoritical Society Nomenclature committee reporting our finds and what type of meteorite we have found. We will request an official name for the meteorite, called after the local site where we found them.