Meteorite Monday: Carbonaceous Chondrites Revisited

A chunk of the Murchison Meteorite. This carbonaceous chondrite is probably one of the most studied rocks in science. (Image from Northern Arizona University)

A little over a year ago I started the Meteorite Monday series with this post about carbonaceous chondrites. Interest in these primitive space rocks exploded with the fireball that produced the Sutter’s Mill meteorite in California. This meteorite is the newest carbonaceous chondrite to be found and it’s generating a lot of excitement. So, with everything I’ve learned (and the realization of how little I actually know) I’ve decided to revisit the topic and expand on some of what I wrote. However, I’m going to handle this post a little differently. Since there’s a lot to be said on these rocks, I’m going to break this up into at least two separate Meteorite Monday posts. I say two because I’ll probably forget something and want to cover it later. If I try to cover everything in one post, things will get messy and I don’t want that.

To start, let’s get a basic understanding of a carbonaceous chondrite. These are what we call a stony meteorite as opposed to an iron or stony-iron meteorite. They are the meteorites we turn to when we want to learn about the conditions of the solar system at its inception. That’s because if we were to strip the sun down of its atmophile elements, such as nitrogen, helium, and hydrogen, then we’d have a chemical abundance that is also found in some of the carbonaceous chondrites. It’s like being able to study a blank canvas before the paint goes on it. In fact, there is one group of meteorites called the Ivuna-type (or CI) that is used as the “blank canvas” or “standard”. When we want to understand the evolution of a meteorite and it’s parent body, we plot it’s element constituents against that of the CI type meteorites (1). This allows us to look at the concentration of elements and get an idea of its thermal history.

A bulk composition graph showing element abundances at certain temperatures. (Image from David Mittlefehldt at the PSRD- University of Hawaii)

This graph is showing us the bulk abundance of three groups of elements at temperatures present in the solar nebula. The lithophile elements are those that go into silicates, or the rocky parts of the meteorite. The siderophile elements are found in iron metals and the chalcophile are found in sulfide minerals (2). On the Y-axis we see the numbers range from .1 to 10 with the 1 line being our blank slate, so to speak. At 1 is the composition of the Ivuna-type meteorite. Generally, any element that falls below that line is considered depleted, and above that line is enriched. Carbonaceous chondrites plot at either 1 or above that line when looking at the abundance of refractory-lithophile elements. These are the elements that condensed first out of the solar nebula at temperatures around 1500-1800 K. These elements then formed minerals such as corundum (Al2O3), melilite, and perovskite.

These minerals are all condensed in one of the hallmark physical features of most carbonaceous chondrites, calcium-aluminum inclusions (CAI’s).

Large prominent CAI’s in the cut face of the Allende meteorite. (Image from NASA) (3)

Another prominent feature in carbonaceous chondrites are chondrules. Carbonaceous chondrites have seen little in the way of thermal metamorphism and this has left the chondrules with distinct boundaries and rims. Thermal alteration degrades and destroys chondrules at progressively higher temperatures.

A thin section of a carbonaceous chondrite. The spherical inclusions are the chondrules. (Image taken by author)

The chondrules formed after the CAI’s in the solar nebula and have a different chemical composition than the CAI’s. Since this is an overview, I don’t want to delve too much into the chemical properties of these two inclusions. Instead I’ll save those for a later post.

Resources:

  1. Weisberg, Michael. McCoy, Timothy. Krot, Alexander. Systematics and Evaluation of Meteorite Classification. 
  2. Mittlefehldt, David. Tagish Lake- A Meteorite from the Far Reaches of the Asteroid Belt. December 12, 2002.
  3. Taylor, Jeffrey G. Solar System Exploration: Origins of the Earth and Moon.
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