With a total mass of ~650 grams, the meteorites Graves Nunataks (GRA) 06128 and 06129 are two large paired stones found by the US ANSMET team during the 2006 season in the Graves Nunataks area of the Transantarctic Mountains. The GRA 06128/9 meteorites belong to a type of meteorites that we classify as ‘achondrite’, meaning that their formation involved melting processes on their parent asteroids at high temperatures, similarly to lavas on Earth for example. As soon as they were brought back to the lab for examination, researchers realised that these stones were unique, having very Na-rich feldspar and Fe-rich olivine and pyroxene, which is unusual compared to the other achondrite meteorites we have in our collections. Another intriguing feature of these stones was their ubiquitous sulphurous smell. Studying the stones in details, James Day and colleagues notably showed that the GRA 06128/9 meteorites contain much more silicon than other achondrite meteorites, which typically have basaltic chemical compositions not too dissimilar to lavas found in Hawaii for example. Graves Nunataks 06128/9 meteorites are also were very old – they formed around 4565 million years ago, within 2-3 million years of the formation of our Solar System itself. Overall, discovery of the GRA 06128/9 meteorites showed us that crusts formed on large asteroids very rapidly after the start of the Solar System are probably much more diverse in composition than we thought before.
We have been studying the GRA 06128/9 meteorites for a few years now in the group in Manchester. The GRA 06128/9 meteorites formed very early on, but the make-up of the noble gases it contains also indicates that these meteorites recorded numerous post-formation events for several 100’s million years, likely related to collision with impacting objects. In a recently published study aimed at understanding better the origin and processing of volatile elements such as hydrogen and light halogens during the early Solar System history, we have investigated the abundance and isotopic composition of H, F and Cl in Ca- and P-rich phosphate minerals (merrillite and apatite – see image below) in GRA 06128 using secondary ion mass spectrometry. The data we obtained suggest that the water trapped inside the parent asteroid of the GRA 06128/9 meteorites had a similar origin than the water trapped inside other partially melted asteroids and planetary bodies such as Mars and the Moon. The chemical and isotopic compositions of phosphates also indicate that the GRA 06128/9 meteorites interacted with fluids about 100 million years after they formed, interaction that might have been triggered by the collision of an ice-rich impactor with the GRA 06128/9 parent asteroid.
Full paper citation: R. Tartèse, M. Anand and I.A. Franchi (2019) H and Cl isotope characteristics of indigenous and late hydrothermal fluids on the differentiated asteroidal parent body of Grave Nunataks 06128. Geochimica et Cosmochimica Acta, doi: 10.1016/j.gca.2019.01.024.