New group paper: Old formation ages of igneous clasts on the L chondrite parent body reflect an early generation of planetesimals or chondrule formation

Barwell

A piece of the Barwell meteorite showing one of the “pebbles”. (Image: Natural History Museum)

The Barwell meteorite is the largest British meteorite. It was seen to fall over Leicestershire on Christmas eve (24th December) 1965, and a total mass of 44 kg has been recovered.

The interesting thing about Barwell is that it contains large “pebbles” or inclusions that are thought to have originated from different asteroids than the Barwell meteorite itself. One particular “pebble” attracted a lot of interest in the 1980s. Its mineralogy and texture suggested that it was an igneous rock (i.e. one that had previously melted), but its oxygen composition was within the range of a group of ordinary chondrite meteorites (which have not melted) – but not the same ordinary chondrite group as the host meteorite. This pebble and other similar inclusions in Barwell are very old, they recorded ages within about 1 million years of the beginning of our Solar System. The old ages suggest that the material remained unaltered for about 4450 million years. But different materials record different ages, suggesting they experienced different histories before being incorporated into the asteroid that ultimately resulted in the Barwell meteorite.

In this study we have examined Iodine-Xenon (I-Xe) ages of further inclusions and samples of the host material from Barwell, to try to determine what range of materials record these ancient ages, and understand what processes in the early Solar System they record. The oldest age we determined was as old as the Solar System itself, and was associated with the formation of a fragment of a chondrule. Some of the clasts analysed record very early ages, within about 2 million years of the start of the Solar System, which are interpreted as dating the formation of those clasts. The ages of other clasts have been reset by processing on the parent body.

The oldest ages record the formation of the samples as they solidified from silicate melts at the very beginning of our Solar System. This may represent rapid cooling of material from an early asteroid that was partially melted, or a distinct process that formed chondrules. Either way, they show that rapid cooling of silicate melts was taking place alongside the very earliest processes in our Solar System.

And that may help us to address another problem. The main source of heat in the early Solar System was radioactive decay of a from of aluminium, 26Al. But there’s controversy as to whether 26Al was uniformly distributed in the early Solar System or not – and that has implications as to when chondrules and asteroids formed. Our data support the suggestion that chondrule ages determined using the Aluminium-Magnesium (Al-Mg) chronometer, indicate that either 26Al was not uniformly distributed in the early Solar System, or the chronometer was reset after chondrule formation.

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The full paper citation is: Sarah A. Crowther, Michal J. Filtness, Rhian H. Jones and Jamie D. Gilmour (2018) Old formation ages of igneous clasts on the L chondrite parent body reflect an early generation of planetesimals or chondrule formation. Earth and Planetary Science Letters, 481, 372-386. doi: 10.1016/j.epsl.2017.10.047

This paper is available open access from the journal website and The University of Manchester library.

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The Natural History Museum have produced a nice video about The Mystery of the Barwell Meteorite. There’s also a piece about The Day a Meteorite Landed on Barwell by the National Space Centre. And the BBC posted an article in 2015 to mark the 50th Anniversary of the fall.

And for further information about the Barwell meteorite also see:

Jobbins, E.A., Dimes, F.G., Binns, R.A., Hey, M.H., Reed, S.J.B., 1966. The Barwell meteorite. Minerallogical Magazine, 35.

Hutchison, R., Williams, C.T., Din, V.K., Clayton, R.N., Kirschbaum, C., Paul, R.L., Lipschutz, M.E., 1988. A planetary, H-group pebble in the Barwell, L6, unshocked chondritic meteorite. Earth and Planetary Science Letters, 90, 105–118.

 

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About Sarah Crowther

I'm a Post Doc in the Isotope Geochemistry and Cosmochemistry group. I study xenon isotope ratios using the RELAX mass spectrometer, to try to learn more about the origins and evolution of our solar system. I look at a wide range of samples from solar wind returned by NASA's Genesis mission to zircons (some of the oldest known terrestrial rocks), from meteorites to presolar grains.
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