Not so ordinary after all

In a recent post we talked about ordinary chondrites. Why “ordinary”? Well that’s because we have so many of them, nearly 46,000 individuals according to the Meteoritical Bulletin database, or 87% of all officially classified meteorites. No wonder some people think they are a dull. But familiarity can and does breed contempt. They are important samples and have much to tell us about the early evolution of our solar system.

Ordinary chondrites are divided into three distinct groups each designated by a letter: H group (High total iron); L group (Low total iron) and LL group (Low total iron and Low metal). With nearly 20,400 individuals, the H chondrites are the largest of these groups (45% of all ordinary chondrites). So where are all these meteorites coming from? It is often difficult to link asteroids and meteorites, but in the case of the H chondrites arriving on Earth today there is strong evidence that they are ultimately derived from the main-belt asteroid 6 Hebe.

A further interesting aspect of the H chondrite story is their potential link to another meteorite group, the IIE irons. Unlike the majority of irons, which probably come from the cores of melted asteroids, most IIEs contain abundant silicate inclusions. This is an important clue to their origin because silicates and iron have very different densities. Molten silicate and iron should rapidly unmix. The fact that many IIEs contain abundant silicates is one line of evidence that suggests this group are not core-related, but rather formed by some form of impact process. A further interesting feature of IIE silicates is that they have very similar oxygen isotopic compositions to the H chondrites and some even contain chondrules.

Polished slice of the Weekeroo Station IIE iron containing rounded silicate-rich inclusions image: Kathryn McDermott

So are the IIE silicates really melted H chondrite material, and could the IIEs and H chondrites be from the same parent body, and if so could that parent body also be 6 Hebe? Lots of questions and a new oxygen isotope study from the Open University has just been published that looks at all of them in some detail. The full results are now online.

Barred olivine chondrule in the IIE iron Netschaevo (Mg X-ray map). image: Kathryn McDermott

This new study strengthens the case for a genetic link between the H chondrites and the IIE irons. So are both from 6 Hebe? Well, the new study suggests that this is not a forgone conclusion. While 6 Hebe may be supplying the vast majority of H chondrite meteorites arriving on Earth today a recent study by Vernazza et al. (2014) concluded that numerous compositionally similar ordinary chondrite parent bodies were probably formed early in solar system history. There are also some subtle isotopic differences between the H chondrites that we have in our meteorite collections and the silicate fragments found in the IIE irons. These differences can be explained if the parent asteroid to the IIEs was compositionally similar to 6 Hebe, but not identical. Perhaps it formed from similar starting materials and in a similar region of the Solar System.

However, whether they are from 6 Hebe or another H chondrite-like asteroid, there is little doubt that the IIEs must have formed during a major impact event that resulted in catastrophic mixing of silicate and metal. The scenario favoured by McDermott et al. (2015) involves a hit-and-run style collision between a smaller molten iron-rich asteroid and a larger H-like body. The early solar system was a chaotic environment and there is mounting evidence that such catastrophic encounters were relatively commonplace.

If you think this all sounds far-fetched it is worth bearing in mind that our home planet probably formed as the end product of a vast multitude of such collisions and mergers. So you see, in the end, ordinary chondrites and related meteorite groups have a lot to tell us about the processes involved in planet-building. Did someone say they were dull?