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The tides of Venus

Shedding new light on an old rock

When thinking about what to kick off this column with, it seemed appropriate to give an insight into a project we’re looking into at the synchrotron.

At coffee a few months ago a fellow researcher Simon, who works on the X-ray fluorescence microscopy beamline, mentioned he was despairing at finding sample to test a new technique to map elements inside a sample on.

Simon’s a brain guy, and mainly works with biological samples which, because they are mainly made of light elements (carbon, oxygen and hydrogen), do not fluoresce all that well under the X-ray beams we use at the synchrotron.

Fluorescence happens when a material absorbs some electromagnetic radiation (such as X-rays from the synchcrotron) and then gives off the excess energy in the form of light. This is easier when a material has more electrons to begin with, for instance is made up of bigger elements than the relatively simple carbon, oxygen and hydrogen.

Hence Simon needed a sample that was made of heavier stuff – silicon, iron and the like – to be able to hone the new technique. If it was interesting, that would be a bonus.

I’ve had in my rock collection (you can’t go through a geology degree without ending up with a bit of a rock collection) a small piece of the Allende meteorite that I picked up from a museum shop in Germany.

After hearing Simon’s plight, I realised this would be a really good sample for him, it has lots of the heavier elements in it, and it is a very interesting piece of rock. I broke off a small chunk (about 3 mm by 3 mm) and handed it over.

A false colour image of fluorescence from a small piece of the Allende meteorite. Red show the presence of calcium, green shows iron and blue is where chromium is. X-ray fluorescence beamline, Australian Synchrotron

Beautiful isn’t it. What the X-ray fluorescence microscopy (XFM) beamline is very very good at is mapping where particular elements are with fantastic detail. In the image I’ve put in here Simon has picked out the positions of three elements: calcium, iron and chromium.

Thousands of tonnes of rock fall from space every year, but why is the Allende meteorite is so special?

It’s often referred to as “the most studied meteorite in history” and came to Earth as a fireball, breaking up on entry over Mexico in 1969. Landing as it did, right in the Apollo era, nearly 3 tonnes of material from the fireball were strewn over a small village, Pueblito de Allende.

As US laboratories were preparing for the first moon rocks to be returned, many were happy to have such an interesting specimen land almost on their doorstep.

It was soon realised that the Allende was the largest carbonaceous chondrite to be found on Earth. This is a particularly rare meteorite type, making up only 4% of the meteorites we have found. Carbonaceous chondrites are especially interesting because they contain some of the oldest and most primitive material in the solar system.

A slice of the Allende meteorite. The lighter grey round patches are known as chondrules and are thought to have formed at the beginning our our solar system. Shiny Things, Flickr

Much of the scientific interest in the Allende meteorite has fallen on the tiny specs of carbon in the form of diamond that has been found within it. These diamonds have a very strange chemical signature, which point to their origins being from outside our own solar system.

With the XFM beamline we unfortunately can’t trace these tiny diamonds. We can, however, accurately map how the heavier elements are spread about this ancient rock.

Hopefully this work, as well as developing a great new technique, could provide clues as to how this amazing rock came together. Who knows what this could tell us about how our solar system came into being?

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