Monday, September 17, 2012

Slice It!

by Stacy Carolin
Here we go! DAY 1! (well, actually this was done a few months ago in Atlanta in prep for this Caltech trip, but let's use our imaginations.... :)

Today we are going to remove from our candidate stalagmites the small rock samples to be analyzed. Remember, we need one from the bottom of the stalagmite, which will give us the oldest date (aka when it started growing), and one from the top, which will give us the youngest date (aka when it stopped growing).

The best way to keep age ERRORS in our final calculations AS SMALL AS POSSIBLE is to (1) not let any of the stalagmite's dirt or mud contaminate our clean pure-rock samples, and (2) to only extract our sample from a SINGLE stalagmite growth layer (so we're not averaging many different ages together). In order to do this, we need to cut open the stalagmite to be able to see the many layers and access the cleanest area of growth: it's center.

I am using a big tile saw with a thin diamond-edge blade for slicing (oh boy!) First, I slice any stalagmites that are too long into smaller sections. Next, I slice each section down the center (longways) to see it's interior, and then finally, I slice the back half of the stalagmite off in order to turn the end product into a "slab." Thin section slabs are very helpful because when you hold them against a backlight the stalagmite's layers, and especially any dirt squished in between, become extremely visible.

Ok, now we have something very beautiful to work with! We are going to need about 250 mg of rock for a sample, which if drilled into powder is about 1/4th a teaspoon in size. In order to drill precisely out of only one of the stalagmite's top layers (and one of the bottom layers), I'm going to attach the stalagmite slab to a micromill that reads out the x-y-z placement of the drill bit. I can then first map out exactly where I'm going to drill into and write down all these coordinates, THEN go back and drill into every point that I picked out. And as I've done this several times before, I know approximately how many points to drill AND how deep to drill in order to have about 250 mg of powder when I'm finished. Just a side note, the drill bit I use is 1.6 mm in diameter, so the layer I'm drilling out, if done perfectly, will be 1.6 mm thick (from top to bottom- remember the stalagmite slab is laying on its side on the drill stage). Stalagmite growth rates vary over time, but just for example, if it grew very slowly at 300 years/mm, that layer that I just removed covers 480 years, which we should keep in mind.

For this exciting (and hopeful!) venture we're going to test 4 different stalagmites, so I repeat this 8 times (top and bottom for each) to get 8 total samples (which took about 8 x 2 hours each = 16 hours, sigh! welcome to science! ;).

Remember, our goal is to determine the AGE of each of these now 250 mg powder samples, and to do that we need to somehow determine the URANIUM and THORIUM atomic ratios that, right now, are invisibly hidden in each. It's time to switch from geology to chemistry... on to Caltech! Yes!