One of the more heavily debated topics within the field of meteoritics is the origin of chondrules. These are the small, spherical silicate inclusions from where we derive the technical name for the most common type of meteorites, the chondrites. With few exceptions, chondrules are found in all chondrite groups in varying quantities. Sometimes we’ll see chondrites that are nearly 70% chondrules and in other cases, we’ll see chondrites, such the Ivuna, that contain no chondrules. In the simplest of terms, chondrules are composed of olivine and/or pyroxene, occasionally glass and a smattering of feldspar. In not so simple terms, chondrules are a hot mess of textures and compositions- messy enough that I’m not going to cover it in this post, but I did delve into it a bit in this older Meteorite Monday post about our enigmatic friends. Continue reading
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.
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).
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.
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.
- Weisberg, Michael. McCoy, Timothy. Krot, Alexander. Systematics and Evaluation of Meteorite Classification.
- Mittlefehldt, David. Tagish Lake- A Meteorite from the Far Reaches of the Asteroid Belt. December 12, 2002.
- Taylor, Jeffrey G. Solar System Exploration: Origins of the Earth and Moon.
One of the great things about taking my meteorites course this term is that I’m learning a lot about the different aspects of meteorite studies and how it all ties into what we know about the origin of the solar system. It’s also giving me the opportunity to revisit previous Meteorite Monday posts and flesh out some concepts that were, at that time, beyond my comprehension.
One of the more well-studied aspects of meteorite research is the chondrule. These are one of the most telling aspects of a meteorite. In the simplest of terms, they are silicate spheres that are usually less than 1 mm in diameter and are found embedded in most ordinary chondrites.
For the purpose of this post I’m not going to get into chondrule formation as it’s a fairly involving topic and is deserving of more time than what I can give it. The important thing to know is that they’re considered some of the oldest material in the solar system. Isotopic dating tells us that they formed within the first 2 to 3 million years of the birth of the solar system. They were little molten droplets that were formed in the solar nebula and were heated to about 1900 C and then experienced rapid cooling. This high heat and quick cooling determined their texture and over all chemical components. They’re generally composed of either olivine or pyroxene and sometimes both minerals occur in the same chondrule.
These minerals can display some really beautiful textures. This is a picture of a chondrule with a barred olivine texture. The olivine crystals are the elongated structures and they’re set in a glassy matrix or body. I highly recommend clicking through the link as NAU has some spectacular pictures of other chondrule types and how they form.
This next image is of a chondrule displaying a porphyritic texture. This is a way of describing a course grained chondrule where the olivine crystals are enclosed in pyroxene crystals.
And this is one of my favorite chondrule images. These are two chondrules that collided while still molten in the solar nebula and fused together in a quick cooling process.
As I said earlier, there is a lot more behind chondrules and their formation that I can’t get into yet. At some point in the near future I plan on putting together a post that deals with what chondrules tell us about the early years of our solar system.
One of my first tasks at the Cascadia Meteorite Laboratory is to photograph thin sections (PTS) for the samples that haven’t had any work done on them yet. We have over 600 meteorites in the collection, a large number of which haven’t had PTS work done. I’ve photographed around one hundred of them so far and there’s plenty more work to be done. Doing this has been a great way to learn how to classify meteorites. Just like with any terrestrial rock, meteorite classification starts with nothing more than a hand sample, a thin section and a petrographic microscope.
The first thing you look for are the chondrules. Are they super crisp and well defined? Then you possibly have a type three ordinary chondrite. The presence of glass is also a good sign of a type three. If those chondrules are set in a rather dark matrix, chances are you have a carbonaceous chondrite. Type three’s and carbonaceous chondrites have seen the least amount of metamorphism (or thermal alteration).
Once you get into the type four, five and six you begin to see an increase in metamorphism. Namely, your chondrules become less apparent as they basically get destroyed by the thermal process. At the type six level, the chondrites display relict chondrules. These are faint outlines of where a chondrule used to be- a ghost of its former self.
Beyond the type six, there’s the type seven. I alluded to this in last week’s post, but didn’t really expand on it. The meteoritics community has been debating this one since at least the 1970’s with the publication of Pyroxenes in the Shaw L-7 Chondrite. I’m not going to delve too much into the type seven classification here. It’s a tricky one with the definition changing from researcher to researcher. As such, I’m going to address that in its own separate post.