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.

Meteorite Monday: Origins of carbonaceous chondrites

Last week I mentioned this paper that was recently published by Linda T. Elkins-Tanton, Benjamin P. Weiss and Maria T. Zuber of MIT. In it, Elkins-Tanton et al., discuss the possibility that carbonaceous chondrites could have originated from differentiated bodies. For those unfamiliar with this term, it basically refers to any asteroid or planetesimal that grew large enough to develop a distinct core/mantle boundary and a cold crust. If you recall, I wrote a few posts about iron meteorites that originated from such bodies.

The researchers based their hypothesis on the “metamorphic, magnetic, and exposure age data” for the Vigarano type chondrites, with most of the data being taken from the most famous of this type, Allende. To support this hypothesis they created a computer model of a differentiated body with a cold crust. They then used equations that would allow them to model thermal behavior on such a body and also modeled in a core dynamo as a way to explain the magnetic orientation of Allende’s mineral grains. The model was constrained to bodies 500 km in radius with a crust “no thicker than 2% of the bodies radius”.

This crust basically sits on top of a magma ocean. They further speculate that the planetesimal accretes material as it collides with other pieces of rock in the solar system. This process allows for the crust to thicken and undergo varying levels of metamorphism based on it’s proximity to the heat from the magma.

What the researchers have suggested is a significant departure from that of the current thinking behind planetary formation. As I had mentioned last week, it is commonly thought that carbonaceous chondrites originated from undifferentiated bodies. This paper is suggesting that some of these chondrites originated from bodies experienced at least partial differentiation. It’s a pretty neat concept that I hope will see a lot of debate within the meteoritics community. I plan on keeping my eyes open for any responses or research that builds off this work.

I have only one critique for the paper. It’s pedantic really, but I think it makes all the difference in clarifying the purpose of the research. The title of the paper “Chondrites as samples of differentiated planetismals” makes one believe that the research will apply to more than just two or three classes of meteorite. Throughout most of the paper, the researchers rely primarily on Allende, which is part of one class of carbonaceous chondrite. There’s mention of Orgueil, but beyond that one doesn’t see much in regards to Ivuna or any other species of carbonaceous chondrite.

What I would like to see is how this model applies to the other chondrites. Can it be adapted in such a way as to explain the origins of other carbonaceous chondrites? If not, should there be a new classification for those which come from partially differentiated bodies? Those are a couple of questions I’d like to see answered in any follow up papers.

Elkins-Tanton L.T. et al., Chondrites as samples of differentiated planetesimals, Earth Planet Sci. Let. (2011), doi: 10.1016/j.epsl.2011.03.010

Meteorite Monday: Or not…

I’ll admit, I kinda let the ball drop on this week’s Meteorite Monday. Normally, I try to get this post done a few days ahead of time, but this weekend has been filled with work, chemistry, petrology readings and completing scholarship essays so I can get some research money.

However, I do have a neat post planned that talks about a recent paper that was published by some researchers at MIT. In the paper, “Chondrites as samples of differentiated planetesimals”, Elkins-Tanton et al., postulate that carbonaceous chondrites could have originated from asteroids that experienced differentiation. This challenges the current paradigm that such chondrites actually come from bodies that didn’t grow large enough to develop a core and mantle. Some have speculated that if true, this could change the way we view the formation of the planets. It’s a good read and if you have time, I’d recommend giving it a look over.

The Allende Meteorite- One of the meteorites used in the Elkins-Tanton et al., study.

Meteorite Monday: Carbonaceous Chondrites

This post marks the first of, what I’m hoping to be many, a weekly series about various meteorites. I love space rocks as much as I love their terrestrial cousins because they recount the history of our solar system. In the simplest of terms meteorites are basically chunks of natural space debris (i.e., not from man made objects) that survived their descent through the earths atmosphere and the resulting collision. These objects come primarily from asteroids such as Vesta 4, but a few have been found of lunar and martian origin.

Allende, Mexico Carbonaceous Chondrite

Allende, Mexico Carbonaceous Chondrite

A carbonaceous chondrite is basically a big ball of space mud  that can contain up to 20% water (I’m not remembering where I read that, so that number may change when I find the source). They are fairly soft and don’t survive their sojourn on the earth to well. These chondrites also represent the most primitive matter drifting through the solar system and have undergone the least amount of  chemical and physical change when compared to ordinary chondrites. It’s estimated that they represent roughly 5% of the meteorites that are observed and collected upon entry to the atmosphere (1).

The Orgueil Meteorite

The Orgueil Meteorite from the Hoover study

As of last month a carbonaceous chondrite found itself in the center of a controversy when a NASA astrobiologist, Richard Hoover, declared that he found fossilized bacteria in a specimen discovered in the late 1800’s. I won’t go into his claims here because this isn’t  a biology blog and I don’t have the expertise to handle such a topic. If you’re really interested you can click here and read the abstract. Also a quick google search will bring up all the arguments for and against Hoover’s claims.

  1. Bischoff, A.; Geiger, T. (1995). “Meteorites for the Sahara: Find locations, shock classification, degree of weathering and pairing”. Meteoritics 30 (1): 113–122.