The importance of being Ivuna
The Ivuna meteorite has been in the news recently, and is a very, very important sample when it comes to the ancient Solar System, including the Earth, Moon and Mars. But why is it so important, and what does it tell us about how our Solar System formed?
The British Museum of Natural History recently came into possession of a 20 g sample of the Ivuna meteorite. Ivuna is the class-type sample for the incredibly rare subset of CI, or Ivuna-like, carbonaceous chondrites. The CI chondrites are extremely important to the field of geochemistry, cosmochemistry and working out how and why bodies in the solar system, like the Earth, are the way they are.
Chondrites & Carbonaceous Chondrites
Figure 1. A slice of the NWA 2364 CV3 carbonaceous chondrite, with a white CAI visible in the top left. The CAI is ~5 mm across the longest axis. This is one of the CAIs I used for my MSc research. Click to enlarge.
Chondrites are ancient stoney meteorites and direct remnants of the processes that formed the solar system. Broadly ultramafic in composition, they contain mostly iron, magnesium, silicon and oxygen (Scott & Krot, 2004). They were originally named due to the high concentrations of chondrules, mm-sized spherical droplets of silicate glass with minor metal and sulphide contents. There are plenty of chondrules visible in Fig. 1, they’re the red/brown/black circles that make up a large portion of the meteorite slice. The important thing about chondrites is they have not experienced the alteration you’d expect where they part of a planet with sufficient mass to begin differentiating. That is, forming a crust-mantle-core configuration like the terrestrial planets.
The chondrite above is what’s known as a carbonaceous chondrite. The name carbonaceous chondrite is a bit misleading, as this class of meteorite are not universally rich in carbon. The defining property of these carbonaceous chondrites is that they contain concentrations of refractory lithophile elements, such as the Rare Earth Elements (REE), that are equal to or exceed that of the Ivuna-like sub class. That equal to part means that Ivuna is also classified as a carbonaceous chondrite.
Ivuna and Ivuna-Like Chondrites.
Table 1: Data on the types of carbonaceous chondrites. Adapted from Scott & Krot (2004) with additional data from Greenwood et al. (2004). Click to enlarge.
Ivuna was a witnessed “fall” which landed in Tanzania in 1938 as a single 705 g stone. As it was the first meteorite of its type classified, the Ivuna-like group (CI) is named after it.
The Ivuna group is comprised of the Ivuna, Orgueil, Alais, Tonk and Revelstoke meteorites, making up a total of ~3% of discovered carbonaceous chondrites (Table 1). Whether this is indicative of the true proportion of meteorites or not, is up for debate. Ivuna is reasonably fragile, and if a sample makes it through the atmosphere, it faces the prospect of erosion, due to the Earth’s helpful weather systems. The proportions of all samples shown in Table 1, therefore, could be indicative of the proportions of the particular types of meteorites, or simply indicative of the likeliness of each type to survive passage through the atmosphere and/or erosion before being discovered.
Ivuna has very low refractory inclusion (RI) and chondrule concentrations relative to other carbonaceous chondrites. Refractory inclusions include Calcium-Aluminium Inclusions (CAIs) and Ameboid Olivine Aggregates (AOAs).
Why a Sample with Solar Composition is Useful
The sun makes up a total of 99.8 % of the Solar System’s mass. That a lion’s share of the material that’s constituted the original solar system. It’s therefore perfectly reasonable to assume the Sun is representative of the original composition of the gaseous pre-solar nebular the planets, plutoids, moons, meteorites and you and I formed from. And though you can make pretty good measurements of solar composition using spectral analysis, there’s nothing better than having a nice big chunk you can chip a little of into a mass spectrometer. This has the distinct advantage of allowing geochemists to determine the isotopic, in addition to elemental, composition.
Isotopes are important because the cloud the solar system formed from was generated by a supernova or supernovae, generating huge quantities of stable and radioactive isotopes. Many people in the western world will have direct experience with some of these radioactive isotopes – Uranium and Thorium are the elements on which all nuclear power technology is built. So switching on a light in places is almost directly extracting power from a supernova.
Magnesium, for example, has 3 stable isotopes: 24Mg, 25Mg and 26Mg. The latter of those isotopes, in addition to being a table isotope produced in supernovae events, also happens to be the daughter product, or the left over material, following the radioactive decay of the short-lived isotope 27Aluminium. 27Al has a half-life of ~700 KYr and when it decays, it produces heat, and lots of it. Knowing how much heat the pre-solar nebula cloud was producing, in addition to the heat-input from the young T-Tauri Sun, is important in understanding how refractory inclusions like CAIs, chondrites, planetesimals and eventually planets, formed. So Having a hand sample of the material that started it all, lacking the more volatile elements such as Hydrogen and Helium, of course, let’s us build a model for where and when things formed in the Solar System.
Figure 2: Generalised Rare Earth Element proportions in refractory inclusions, normalised to the bulk concentrations in Ivuna. Adapted from Taylor (2001). Click to enlarge.
Fig. 2 is a good example of the CI bulk composition being used as a standard. It shows generalised plots one should expect from the Rare Earth Elements (REE) in refractory inclusions, such as the CAI in Fig. 1.
Why Ivuna is so Important
Ivuna is important because it is a sample of the bulk Solar System. Unlike CV (Vigarano-like), CK (Karoonda-like) or CM (Murchison-like) chondrites, which are all enriched in refractory elements, Ivuna is pristine insofar as bulk concentrations go. Containing a bulk composition in close proximity to that of the Sun lets you measure just how enriched those aforementioned meteorites are, in direct comparison to the solar system.
CV3 chondrites, for example, contain a large number of refractory inclusions (Table 1), named so because of their normal enrichment in refractory elements. Those being elements which very high melting and boiling points. That means the highest condensing temperatures too – making them the first elements to condense out of a hot nebulous cloud. Due to the presence of those inclusions, CV3 chondrites are enriched, in bulk, in the refractory elements relative to Ivuna, and therefore the solar system. Knowing this lets you make the assessment that CV3s, or at least the refractory inclusions contained within, formed in conditions that were much hotter than those in which CIs formed. And since this is the early, hot solar system we’re talking about, that means they formed earlier (this is also confirmed by the elevated level of 26Mg, also in relation to those in Ivuna, and good old-fashioned absolute Pb-Pb dating).
The comparisons don’t stop with meteorites, though. CI concentrations are the values many geochemical systems are normalised to (that’s when you divide the value you have in your sample, by the value measured in the standard), allowing for a comparison with the solar system. Any deviation away from the bulk solar system composition denotes a chemical process has taken place, and can tell a story of heat, pressure or aqueous alteration.
Using the most modern equipment, the new samples available from the Natural History Museum purchase will better refine the standard measurement values we have. This will, in turn, allow for higher precision geochemical comparisons and potentially allow us to refine or rethink our models as to how the solar system formed and why the planets, like Earth, got to be the way they are.
References
Greenwood, R., Franchi, I. A., Kearsley, A. T., & Alard, O. (2004). The relationship between CK and CV chondrites: A single parent body source? Lunar and Planetary Science, XXXV.
Scott, E. R. D., & Krot, A. N. (2004). Chondrites and their Components. In A. Davis (Ed.) Treatise on Geochemistry, vol. 1: Meteorites, Comets and Planets, chap. 1.07, (pp. 143–200). Elsevier Ltd.
Taylor, S. R. (2001). Solar System Evolution A New Perspective. Cambridge University Press, 2 ed.
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June 26th, 2008 at 9:15 pm
[...] goodSchist: Hopefully this isn’t too [...]
June 27th, 2008 at 5:56 pm
Great post … this subject is out of my typical realm of understanding … I think I know a bit more about this stuff now.
January 16th, 2009 at 10:13 pm
hello is good photo of chondrite
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http://meteoritesbraik.blog4ever.com
March 6th, 2009 at 9:54 pm
[...] accept that life formed on Ceres, Mars, Venus or any other body from the same raw ingredients (see The Importance of Being Ivuna )the Earth was receiving and was THEN transported to Earth, why not cut out the middle-planet and [...]