— Tom Harvey | 07 Dec 2018
One of the important parts of collecting meteorites is detailed record collection which allows for efficient curation and identification when the samples are returned from Antarctica. In addition to where a sample was found, we need to know what it is (and be sure that it is a meteorite after all!). As mentioned in a previous post, in addition to her familiarity with collecting meteorites in the field, one of the ways that Katie might be supplementing her preliminary in-field examination of the samples is by using a CEREGE-built portable combined magnetic susceptibility – conductivity field probe. Magnetic susceptibility is a measurement of the degree to which a material will be magnetised (induced magnetisation) when a given magnetic field is applied. Conductivity is an electrical property which allows a material to conduct electricity.
Following an initial blank air test, the sample is placed close to the probe, reacting with the weak magnetic field generated by the probe. The extent of the reaction is correlated to magnetic susceptibility, and also affected by electrical conductivity. Both properties are then quantified using correction based on sample volume (Gattacceca et al., 2004).
The predominant component of meteorites which can be magnetised is the meteoritic iron (i.e., iron as Fe rather than oxidised phase like FeO or Fe2O3). The meteoritic native iron content of meteorites varies quite reliably with the classification of its type (stony, stony-iron, iron), which depends on its composition, thus linking to its original process of formation. Meteoritic iron is also electrically conductive, whilst terrestrial rocks usually are not. As such, the AnMetMet is able to tell the difference between metal-bearing samples and iron oxide (e.g. Magnetite) bearing samples, a separation that cannot be achieved using a magnet which is sensitive to magnetic susceptibility alone.
Because of this, performing a magnetic susceptibility-conductivity test gives a good estimate of its classification – and can be used to confirm that the rock is indeed a meteorite rather than a terrestrial magnetic rock or mineral. A sample must be above a certain volume for accurate readings, but if they are relatively homogeneous, even large samples (larger than the probe) can be measured.
The magnetic field generated by the probe is weak, similar in intensity to the natural Earth field, so that the natural remanent magnetisation (NRM) of the sample is not disturbed. This is crucial because the NRM of meteorites is of great scientific value as it bears the record of ancient magnetic fields that may have been present in the early solar system, generated within the solar nebula or by dynamo phenomena within planetesimals. For that reason, the use of magnets to test the meteoritic nature of a rock should be avoided at all costs because it both inefficient (non quantitative) and destructive for the magnetic memory of the rock.
To ensure that we understand the numerical output of the probe in the field, we are testing it on some test meteorites we already have here in Manchester (these are the same samples we have been testing our metal detector panels on). Amongst the test subjects are pieces from NWA-869 – an ordinary L chondrite, Gao Guenie – an ordinary H chondrite, Campo del Cielo – an IAB iron meteorite and a few fragments from the ordinary LL chondrite Chelyabinsk fall (which some might remember from the dash-cam videos in 2013).
The aim is to piece together a reliable framework of readings so that we can be sure of how potential finds fit into the meteorite classification scheme. In addition to this, we will be testing the probe at low temperatures to see whether the readings will vary from room-temperature measurements when they are faced with the significant chilliness of Antarctic weather.
If all goes according to plan, we’ll have a useful database to complement Katie’s in-field measurements, which will be a good aid for future sample curation planning and analysis. Big thanks to Jérôme Gattacceca for making sure this explanation is accurate!