Thursday, April 8, 2010

Ghosts under pressure

Callan has a great (and evocatively titled) post about "crystal ghosts" - metamorphic minerals that have been replaced by something else. Callan's examples are, like most of the ones that I've seen, "retrograde metamorphism" - replacement of one mineral as it cools. Textures like these form because we live on Earth's surface, and it's colder here than where metamorphism happens. In fact, we only see metamorphic minerals because it's hard for those retrograde reactions to happen - you've got to add water to get many of the minerals that form at lower temperatures.

But my favorite metamorphic ghost tells a different story:

This is a microscopic image, rather than a field photo - it's of a thin slice of rock with light passing through it. And it contains the same minerals that Callan discussed: kyanite, sillimanite, and andalusite. Three minerals with exactly the same formula, formed under different combinations of temperature and pressure.

Except they're all here in the same rock:

If all these minerals had formed at the same time, we would know the exact temperature and pressure at which they formed. But, as with most places where all three of these minerals are formed together, they probably didn't form at once. In this case, it looks like the kyanite replaced the andalusite. (In another sample from the same outcrop, kyanite fills the entire andalusite-like square.)

And that's really cool, partly because it's a texture that you don't see very often, and partly because it means that this rock got really hot first, and then was buried. (Andalusite forms at high temperature and low pressure; kyanite forms at higher pressure.)

And it fits with the story that my students and I had been working out before we found this rock: that a granitic magma worked its way up a fault zone while the fault was active, first heating the rock and then burying it.

Ghosts can tell fantastic stories, if you listen to them.