Steps for extracting 10Be from a
granite sample

1. Collection of sample

2. Separation and dissolution of quartz

3. Separation of impurities

4. Beryllium Sample

5. Target preparation for AMS measurement

 

 

From a rock sample(left), crushed sample rock (second from left), dissolved extracted quartz in a beaker(second from right) to a very, very small amount of Beryllium (Right Bottom)

Cosmic rays impact the surface of the earth with sufficient energy to split atoms. Some of the newly created atoms are only made in this way – these are cosmogenic nuclides. Their concentrations in surface rocks can therefore be used as a clock to measure how much time that parcel of rock has been near the surface. Each of the elements in quartz which consists of SiO₂, can be transmuted to a cosmogenic nuclide, in particular Silicon (Si) ->Aluminum (26Al) and Oxygen (O)-> Beryllium (10Be). These two nuclides 26Al and 10Be decay radioactively, and hence are called cosmogenic radionuclides (or CRNs for short). The rate of production of these nuclides is very very low, one to a few tens of atoms per year per gram of quartz. The measurement therefore requires fancy machines, in particular an accelerator mass spectrometer (AMS). The one we use is in the PRIME lab at Purdue.

10Be is extracted from a granite sample through the following steps:

1. Collection of sample – We chisel thin (max. 1 inch) flakes of quartz-rich granite from a bedrock surface. The photo shows sampling of glacial polish.
2. Separation and dissolution of quartz – We crush the rock into very small grains and separate the quartz from all other minerals. Afterwards the extracted quartz is transferred into acid-resistant beakers, where we dissolve the quartz in hydrofluoric acid (see photo).
3. Separation of impurities - Impurities in the quartz such as potassium or iron are separated from the beryllium using cation/anion exchange chromatography.
4. Beryllium Sample - The orange-colored solid in the crucible is the Beryllium that is left from our original sample after the entire chemical processing - not very much.
5. Target preparation for AMS measurement - The Beryllium sample is placed into the little hole of a target; tthe concentration of Beryllium atoms in the sample target will be measured in an AMS machine at the Purdue University PRIME lab.

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There are two types of applications for this work:
1. The timing of a glacier's history can be reconstructed by dating the age of a non-eroding rock surface (e.g., glacial polish, moraine boulders).

2. Erosion rates of a rock surface can be measured at a single point (see photo below). An average erosion rate for a specific part of a landscape (e.g. river drainage basin) can be established when we measure a sediment sample from a location where the rivers exits the mountain range.

 

written by Miriam Dühnforth

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Tor on the north edge of South Boulder Creek Valley, looking south toward Mt Evans on the skyline. 10Be concentrations from these bedrock knobs reveal lowering rates of 5-10 meters per million years.