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Stony meteorites are constituted mostly of various sorts of silicate, though small quantities of iron and nickel are usually present, plus other substances. □ What do you think are the main constituents of a stony-iron meteorite? A stony-iron meteorite is a mixture of roughly equal amounts of iron-nickel alloy and silicates, with small quantities of other materials (Plate 25(c)). They are thought to come from transition zones in asteroids that had formed a core of iron and a mantle of silicates, and were then disrupted by a collision.

Stones comprise about 95% of all falls (Figure 3.14) and presumably of all meteorites. The two subclasses are chondrites and achondrites. Achondrites are defined on the basis of something that they (and the other two classes) have not got, namely chondrules. Their rather uniform silicate compositions indicate that they are from the mantles of asteroids that have cores of iron and transition zones.

Chondrites

The great majority of stones do have chondrules (Plate 25(d)), and they are accordingly called chondrites. A chondrule is a globule of silicates, up to a few millimetres in diameter. They are thought to have been formed by the flash melting of dusty silicate clumps, raising temperatures to greater than about 1500 K, followed by the rapid cooling of the liquid droplets. Flash melting could have been caused by shock waves spreading from the spiral density waves that were present during the formation of the Solar System (Sections 2.1.2 and 2.2.5), or by electrical discharges in the sheet of dust in the solar nebula. However, some chondrules postdate these possible mechanisms (Section 3.3.3). Therefore, impacts between planetesimals or embryos have been evoked. By contrast the silicates outside the chondrules formed by condensation of nebular gas directly to the solid phase. Chondrules are not found in terrestrial rocks.

Ordinary chondrites (OCs) are the most abundant sort of chondrite (Figure 3.14). In the matrix in which the chondrules are embedded there are more silicates, including fractured chondrules, minerals that form at less than 1000 K, and 5-15% by mass iron-nickel alloy. The alloy further distinguishes the OCs from terrestrial rocks. The carbonaceous chondrites (CCs) are distinguished by a few per cent by mass of carbonaceous materials, and up to about 20% water bound in hydrated minerals. Among the carbonaceous material are many compounds of biological relevance, such as amino acids, which are the building blocks of proteins. There is evidence that a proportion of many of these biomolecules predate the formation of the Solar System. This is from the hydrogen isotope ratio 2H/:H, where :H is the common isotope and 2H, deuterium D, is much rarer. In interstellar molecules this ratio is higher than general Solar System values - the clouds are very cold, which favours incorporation of D into molecules.

The presence of volatile components suggests that the CCs have suffered little heating since they formed. Moreover, they are not fully compacted, indicating that they have never been greatly compressed. These are two of the indicators that CCs have never been in the interiors of bodies more than a hundred kilometres or so across. They are therefore primitive, in that they have been little altered since their formation.

The most primitive of all are the C1 chondrites. The matrix is particularly rich in water and in other volatiles. C1s consist of little else but matrix - they are nearly free of chondrules, so presumably predate chondrule formation. Further evidence that the C1s are primitive bodies comes from the relative abundances of the chemical elements in them. Apart from the depletion of hydrogen, helium, and other elements that would have been concentrated in the gas phase of the nebula, the abundances in the C1s are similar to those in the observable part of the Sun. This indicates that these meteorites are not from differentiated bodies, because on fragmentation this would lead to non-solar ratios in each fragment. A particularly well-preserved primitive meteorite is the Tagish Lake meteorite that was seen to fall in Canada on this frozen lake in January 2000, in pieces totalling 56 000 kg. It is intermediate in type between C1 and another primitive subclass CM. Its orbit shows that it came from the outer asteroid belt.

The other CCs also give close composition matches to the Sun, but not as close as do the C1s. Therefore, in C1s it seems we have the least altered samples of the materials that condensed from the solar nebula when the Solar System was forming.

Because the C1s are available for laboratory study, they have been used to refine the relative abundances of all the elements in the Solar System, except those that are very volatile or reside mainly in very volatile compounds.

As well as volatile compounds, CCs also contain irregular white inclusions, typically 10 mm across, that are rich in non-volatile calcium and aluminium minerals such as corundum (Al2O3) and perovskite (CaTiO3). Unsurprisingly these are called calcium-aluminium inclusions, CAIs, which are thought to have condensed from the solar nebula. They are rare in the OCs. Radiometric dating (Section 3.3.3) shows that the chondrules generally solidified a few million year after the CAIs, so the melting of CAIs might be a further source of chondrules. Some CAIs show evidence of partial melting.

Question 3.11

In what sort of meteorite would you expect the ratio of carbon to iron to be much the same as that of the Sun? Why are the helium to carbon ratios far smaller in such meteorites than in the Sun?

3.3.3 Dating Meteorites

There are various events in the life of a meteorite that can be dated, but we shall concentrate on two important ones: first, the time that has lapsed since a meteorite, or a component within it, last became chemically separated from its environment, almost always by solidification; and second, the time for which a meteorite was exposed to space rather than protected by some overlying material.

Radiometric dating

Radiometric dating is a powerful technique of wide applicability, as you will see in later chapters. We introduce it here in the context of meteorites.

Imagine that, on chemical isolation, a component in a meteorite contains mineral grains that include, for example, the chemical element rubidium. A small proportion of the rubidium atoms will be of the unstable isotope 87Rb that radioactively decays to form the stable strontium isotope 87 Sr:

where e- is the electron emitted by the 87Rb nucleus, thus converting it into a 87Sr nucleus. The number of 87Rb nuclei versus time t decays exponentially as

where the zero subscript denotes t = 0, and r is the lifetime of 87Rb, i.e. the time at which N(87Rb) has fallen to 1/e(= 36.8%) of its value at t = 0. Assume that initially there was no 87Sr in the component, but that it builds up as the 87Rb decays, and that neither of these isotopes escapes from, nor is added to, any of the minerals in the component. The relative quantities of 87Sr and 87Rb in each mineral thus change with time in mirror fashion as in Figure 3.15(a). If, at some time, we measure the ratio N(87Sr)/N(87Rb), then this will tell us how long ago the component became isolated, provided that we know the lifetime of 87Rb. Such lifetimes are known, and are usually expressed as the half-life tj/2 — the time for half the atoms to decay. We have t^2 = 0.693r. For 87Rb,t:/2 = 48 800Ma, with a precision of a few per cent. □ What would be the value of N(87Sr)/N(87Rb) after 48 800 Ma, and after twice this time? After 48 800 Ma, there would be an equal number of the two isotopes, so the ratio would be 1.0. After a further half-life 87Rb will have again halved and N(87Sr) would have increased by half, so the ratio would be 1.5/0.5, i.e. 3. This general method of dating is called radiometric dating. Thus, by measuring an isotope ratio, and knowing the half-life of the unstable isotope, we can calculate the time that has elapsed since the component became isolated.

In practice things are more complicated because strontium is likely to be already present in the component on separation. All four of its stable isotopes, including 87Sr, will be there. This isotope builds up as 87Rb decays, but the amounts of the stable isotopes, including 86Sr,

50 100 150

Time/ 103 Ma

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