Research - Research units A‑Z - SUERC - NERC Facilities - Cosmogenic Isotope Analysis Facility
The cosmogenic nuclide sample preparation laboratory is used for the initial also for cosmogenic isotope analysis and exposure dating on a quasi- commercial or commercial basis. Please contact Tim Barrows for further details and prices.v. Analysis of the long-lived cosmogenic radionuclides 10Be, 26Al and 36Cl used to determine surface exposure ages and denudation rates on timescales of deduced from cosmogenic isotope (10Be and 26Al) surface exposure dating. The other is devoted to mineral dissolution and extraction of the cosmogenic nuclides Al and Be We use these nuclides to measure rates or weathering .
Cosmogenic nuclides such as these are produced by chains of spallation reactions.
Cosmogenic nuclide laboratories
The production rate for a particular nuclide is a function of geomagnetic latitude, the amount of sky that can be seen from the point that is sampled, elevation, sample depth, and density of the material in which the sample is embedded.
Decay rates are given by the decay constants of the nuclides. These equations can be combined to give the total concentration of cosmogenic radionuclides in a sample as a function of age. The two most frequently measured cosmogenic nuclides are beryllium and aluminum These nuclides are particularly useful to geologists because they are produced when cosmic rays strike oxygen and siliconrespectively.
Cosmogenic nuclide dating
The parent isotopes are the most abundant of these elements, and are common in crustal material, whereas the radioactive daughter nuclei are not commonly produced by other processes.
As oxygen is also common in the atmosphere, the contribution to the beryllium concentration from material deposited rather than created in situ must be taken into account. Each of these nuclides is produced at a different rate. Both can be used individually to date how long the material has been exposed at the surface. Because there are two radionuclides decaying, the ratio of concentrations of these two nuclides can be used without any other knowledge to determine an age at which the sample was buried past the production depth typically 2—10 meters.
Because cosmic rays only penetrate the upper few centimetres of a rock, movement of a boulder downslope can result in large errors in the age calculated.
Before sampling a rock, geologists must take detailed and careful measurements of the landsurface, and satisfy themselves that the rock is in a stable position, has not rolled, slipped downslope, been repeatedly buried and exhumed during periglacial rock cycling within the active layer frequently a problem with small bouldersand has not been covered with large amounts of soil, snow or vegetation.
Signs of subglacial transport Scratches striations on a sandstone boulder show that it has undergone subglacial transport and erosion. They want to sample a rock that they are sure has undergone subglacial transport. They will therefore sample boulders that are subrounded, faceted, bear striations, or show other signs of subglacial transport. Accounting for variable production rates Bethan Davies cosmogenic nuclide sampling a sandstone boulder on a moraine.
Ian Hey Cosmogenic nuclide production rates vary according to latitude and elevation. These factors must be measured by the scientist, and are accounted for in the calculation of the exposure age. Topographic shielding, for example by a nearby large mountain, also affects the production rate of cosmogenic nuclides.
This is because the cosmic rays, which bombard Earth at a more or less equal rate from all sectors of the sky, will be reduced if the view of the sky is shielded — for example, by a large mountain that the rays cannot penetrate. Scientists must therefore carefully measure the horizon line all for degrees all around their boulder.
Difficulties in cosmogenic nuclide dating Solifluction lobes on the Ulu Peninsula. Solifluction is common in periglacial environments, and can result in rolling, burial and movement of boulders on slopes. As mentioned above, sampling strategy is the most import factor in generating a reliable cosmogenic nuclide age.
Post-depositional processes, such as rolling, burial, exhumation or cover with vegetation can result in interruption of the accumulation of cosmogenic nuclides and a younger than expected age. Alternatively, if the boulder has not undergone sufficient erosion to remove previously accumulated cosmogenic nuclides, it will have an older than expected age.
This is called inheritance.
- SUERC cosmogenic nuclide analysis price list
- Surface exposure dating
- Cosmogenic nuclide dating
This can be a particular problem in Antarctica, where cold-based ice may repeatedly cover a boulder, preventing the accumulation of cosmogenic nuclides, without eroding or even moving the rock. Rocks can therefore be left in a stable position or moved slightly, without having suffiicient erosion to remove cosmogenic nuclides from a previous exposure.
This can result in a complex exposure history. This is typically characterised by spread of exposure ages across a single landform. Dating just one boulder from a moraine may therefore be an unreliable method to rely on. Scientists may also screen for complex exposure by using two different isotopes, such as aluminium and beryllium 26Al and 10Be. The Production Rate of cosmogenic nuclides varies spatially, but is generally assumed to have remained constant at a particular location.
Published production rates are available for different parts of the Earth. Glacial geologists target elements that only occur in minerals in rocks, such as quartz, through cosmic-ray bombardment, such as aluminium and beryllium 26Al and 10Be. Beryillium is used most widely, as it has the best determined production rate and can be measured at low concentrations. Chlorine 36Cl can also be used to date the exposure age of basalt lavas.
Extraction of quartz Bethan Davies using HF to dissolve rocks for cosmogenic nuclide dating. Note the personal protection equipment! The first stage in the calculation of a cosmogenic nuclide exposure age is to extract the quartz from a rock.
This is quite an involved process and means using some quite dangerous chemicals, such as HF Hydrogen Flouride. HF is an acid with a pH of about 3, but the small molecule is easily absorbed by your skin. Once absorbed, it reacts vigorously with the calcium in your bones, forming Calcium Flouride which may then be deposited in your arteries. All in all, not a substance you want to get on your skin! Scientists must therefore take strong precautions before using this chemical.
The first stage is to crush the rock or rock fragments in a jaw crusher. The crusher must be perfectly clean to avoid contamination. The crushed rock is then sieved to the right size. Magnetic seperation removes particles with lots of iron such as micasleaving you if you sampled granite, for example with a g sample of sand, comprising mostly feldspar and quartz. Preparation for AMS measurement Feldspar is removed by placing the sample in Hexafloursilicic acid or HF on a shaking table for around 2 weeks.
The acids are changed daily. The more durable quartz is left behind. A series of chemical precipitations leaves you with Beryllium Oxide BeOa white powder. It is mixed with Niobium NB and pressed into a copper cathode. Calculation of an exposure age Once the ratio of cosmogenic to naturally occuring isotopes has been calculated, the production rate is used to calculate an exposure age.
This varies with altitude and latitude. Topographic shielding and shielding by snow, vegetation or soil is also taken into account. There are a number of online calculators that can be used to calculate the exposure age.