Submitted by Anonymous on Thu, 12/09/2010 - 06:13
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Glacial History in Antarctica

Greetings, Lesley! What kind of equipment do you use to measure the "age" of the rocks you are studying? If any chemicals are involved, how do you manage to keep them isolated in that fragile environment? Dave Throgmorton

Thu, 12/09/2010 - 16:43

Lesley Urasky

Hi, this is from John Stone, whose group Lesley is working with. There are many steps in the process of determining a cosmic ray exposure age. In the field we select a rock to sample, and collect about 1 kilogram to ship back to the US. If necessary, we may have to remove part of the surface of a large rock using a hammer and chisel. After samples arrive in the lab, we first separate a particular mineral to work on, usually quartz (for Be-10 measurements) or feldspar (for Cl-36). We use dense liquids and a surfactant flotation technique to separate the minerals. The next step is to dissolve the mineral and separate the element of interest, such as beryllium or chlorine. Dissolution requires hydrofluoric acid (HF), the only acid that can attack and break down silicate rock. The subsequent purification involves processes such as ion exchange chromatography, which separates elements by ionic charge and their ability to form complex ions in solution. The end result is a few hundred micrograms of pure beryllium, which may contain less than a part per trillion of Be-10. Now we have to measure the ratio of the rare isotope, Be-10, to the common isotope, Be-9. This is done by a nuclear physics technique, called accelerator mass spectrometry (AMS), in which a beam of beryllium ions is separated into its isotopic constituents using magnetic and electrostatic deflection. By accelerating the ions to very high energies in the accelerator, very sensitive techniques can be used to identify each ion as it enters the detector, and distinguish between those of interest, Be-10, and contaminants such as boron-10. After we have determined the Be-10 concentration in quartz from our sample, we can estimate the sample's exposure age, provided we know the Be-10 production rate. Effectively, the higher the concentration of Be-10, the longer it must have been exposed to cosmic rays. It's actually a bit more complicated, because isotope production rates vary with both altitude and latitude. Fortunately, we have fairly reliable correction factors that we can use to correct for those effects on the rate of Be-10 build-up.