Zircon, or zirconium silicate, is a hardy mineral that typically forms in igneous systems like volcanoes. It is hardy because it is not easily broken down by weathering processes but can remain intact for billions of years. In fact, the oldest mineral so far discovered on Earth is a zircon mineral that is 4.4 billion years old. For reference, the Earth is 4.56 billion years old so zircon minerals are capable of being heated and squeezed repeatedly for many years without breaking down. This convenient property of zircon as well as the abundance of radioactive elements incorporated in its structure such as Uranium and Thorium allows researchers to date magmatic systems of all ages. Uranium is an element that decays over time. More specifically, Uranium gives off pieces of its atom, i.e. radioactivity, and transforms into other elements like Thorium or Lead depending on how long the zircon has to sit around. In our case, the part of this decay chain that is most helpful is the transformation from Uranium to Thorium. This process does not take billions of years, and allows us to date relatively young volcanic processes like those that we find at Lassen. In fact, the rocks that we are concerned with range from 67 million years to only a hundred years old.
So what does zircon look like?
The grain in the red circle is a zircon mineral. Zircon can be elongated like this one or stubbier. One of the defining features of zircon is the double pyramidal termination. This means that the ends of the mineral end in a point which, in a 3D view, creates a pyramid. In the picture above, you can get an idea of this shape although the points are rather rounded. Another diagnostic feature is the faceted nature of the crystal faces which means that if you were to rotate the grain you would run into flat surfaces like you see with a prism or rectangle. In the 2D view of this microscope this can be difficult to see but you can see hints of this along the vertical edges of the grain. These edges are slightly thicker than the other sides, implying a faceted shape. Lastly, see the small circles within the crystal? Those are inclusions of various elements within the crystal structure. This particular grain is about 150 by 40 microns- small enough that you need a microscope to see it.
Zircon is not the only thing that we extracted from that pile of rock. There are also glass fragments such as the grain just to the left of the circled zircon.
This next picture has a nice mix of both.
While we would ideally like only zircon in our sample, it is relatively easy to differentiate between zircon and glass. Here, the grains in the yellow circles are zircons and you can see the sharp point of that pyramidal termination better. There are suspect grains in this view but otherwise it is mostly glass which is comparatively shapeless and does not have nice facets. Why do we get all of this glass in our sample? It is possible that the glass had inclusions of denser elements that let it pass through the heavy liquids with the zircon. Expertimental error is another possibility. Whatever the reason, there are plenty of zircons here to analyze.