The Natural History of Meteorites


A member of the Perseid Meteor Shower from National Geographic
Information about the formation of the Solar System comes from two general sources: The purpose of this lecture is to add depth to your knowledge of the last source, meteorites and provide a sense of the kind of Solar System data they supply.

Meteorites in Nature:


The Allende meteorite
Basic terminology


The Bovedy meteorite from Michael Blood Meteorites
Fusion and Ablation: Meteoroids are transformed by their passage through the atmosphere:

Discovery: We obtain access to meteorites in two distinct ways.


Meteorite trajectories from NASA's Cosmos
Origins: We have discussed the origin of many dust grains that are vaporized in the atmosphere as meteors - cometary dust. Most meteorites, however, originate in the asteroid belt. Indeed, in cases where meteorite falls have been tracked by multiple cameras, it has been possible to calculate the meteoroid's orbit, confirming their asteroid belt origins.

General meteorite types:

Note: To assess these percentages, we count only meteorite fall discoveries. This is to counter the collecting bias that favors the finding of iron meteorites because they just look so different. If we were to base percentages on finds, then iron meteorites would make up 40%.

Origins:


Chondrules from Wikipedia
Chondrules: Beads of silicate material containing up to 15% iron that have undergone melting prior to being incorporated in the meteorite. Previously we learned about the condensation of vaporized material in the inner protoplanetary disk - the likely origin of most chondrules. The diversity of chondrule chemistry tells us that condensation proceeded differently in different regions. Interpreting this history based on meteorite chemistry is a primary goal of meteorite studies.

Specimen names: Individual meteorites are named after the localities in which they were discovered. Thus, the Allende meteorite was discovered in Pueblito de Allende, Mexico.

Chondrite types:

Geochemists have developed a complex taxonomy of meteorites. A typical meteorite type designation might look like this:


The Allende meteorite - a CV3 chondrite

CV3

Separating the elements, this tells us that the following:

The Allende meteorite (right) is a type CV3.

Learning all of these types is beyond the scope of this course, but here are some examples:


A Type-C ordinary chondrite from Guy Worthy Washington State University

Sound familiar? These chondrite types largely match up with asteroid types described previously, but not entirely. Type C asteroids resemble carbonaceous chondrites, but not ordinary ones.

Primitive meteorites: By primitive we mean "unaltered since the beginning of the Solar System." The most primitive meteorites are those whose elemental concentrations most closely resemble those of the Sun. These are type CI carbonaceous chondrites. Remember from early in the semester that the relative concentrations of refractory elements in type CI chondrites are the "chondritic concentrations" that are the standard against which we compare other objects using spidergrams.

Meteorites as forensic evidence for the early Solar System

Consider some examples of the application of different methods of geochemistry to meteorites to help us answer questions about the Solar System's history:

The first condensation: Recall that as the protosolar nebula first formed, much (if not all) of its material was vaporized, then recondensed from that vapor. What were the first solid objects to condense?


Chondrules and CAIs from Joe Orman
The Allende meteorite, among others, contains Calcium-aluminum-rich inclusion - CAIs - little giblets of refractory material that are not chondrules. How did they form? There are two hypotheses:
Fortunately, meteorite geochemistry enables us to choose the more reasonable hypothesis: If CAIs formed during the first condensation of the early Solar Nebula, we would expect their ages to be similar and uniform. If they originated in older solar systems, there would be no reason to expect them to have similar ages.

Radiometric dating:

Several radiometric systems can be used on CAIs, including: Both systems indicate that CAIs began forming roughly two million years prior to chondrules (at roughly 4.571 ga). But note: The uniformity of these ages implies that they formed through the same underlying process. The only reasonable hypothesis is that they condensed from the Solar Nebula.

As nice as this sounds, we need confirmation from independent lines of evidence.

Stable isotope analysis:

Recall that a given element may have several stable isotopes (i.e. non-radoactive). Natural processes tend to sort or fractionate such isotopes in distinct ways. Consider the example of oxygen: Oxygen has three natural stable isotopes, 16O, 17O, and 18O. Their concentration ratios show meaningful patterns.

First, a convention: Rather than using absolute concentrations, it is customary to plot stable isotope ratios with respect to a standard. If:

the sample ratio will be expressed as:

δ18O expressed all pretty from Alessondra Springmann

1000*((Rs/Rr) -1)‰ = δ18O

Three things to note:

Consequences of fractionation:

Imagine a group of samples of a substance that formed at different times, under different conditions. We would not expect their isotope ratios (E.G. δ18O or δ17O) to plot in the same region of a graph of these values. If they do plot in the same place, that tells us they formed at the same time, through the same processes (Time 1 - right).

If we now subject these samples to a fractionation process of some sort, they will:

Eventually, they will fall on a fractionation line. The fact that they line up says that had a common origin and have been subject to the same general processes.

So, how do true premolar grains differ from CAIs when we plot their isotope ratios?

Presolar grains: The ratios of 17O/16O and of 18O/16O from unambiguously genuine presolar grains shows substantial variation. Thus, if CAIs are presolar grains, we would expect them to show considerable variation in oxygen isotope ratios.

CAIs: In fact, they don't. Samples of CAIs from the Allende meteorite plot along a clear fractionation line. (For comparison, samples of Earth rocks tend to plot along the terrestrial fractionation line. that is different but represents the same process.) Different meteorite types, likewise, plot along their own sloping lines. The CAIs of the Allende meteorite are similar, plotting in a simple linear manner. Like the radiometric dates, this strongly suggests that they formed similarly and in the same environment from the material of the solar nebula, not previously, in different stellar environments. (Remember, when they formed the protoSun probably resided in an open star cluster near other stars.)


Key concepts and vocabulary:
Additional reading: