TCN dating summary

Terrestrial Cosmogenic Dating

The information in this summary is referenced to works authored, or
co-authored by S. Aciego, D. DePaolo, B. Kennedy, M. Lamb, K. Simms,
W.Dietrich (Aciego et al. 2007)(1), T. Cerling, R. Poreda, S. Rathburn
(Cerling et al. 1994)(2), C. Hill (2007)(3), and the Encyclopedia of
Quaternary Science (EQS).

Terrestrial cosmogenic nuclide dating (TCN) is a method utilized by
geologists and archaeologists to measure the amount of time a surface or
feature has been exposed. This is useful because information regarding the
formation of surfaces, or exposure of surface materials, can be dated
directly (direct dating) rather than estimated by association to other
events or features (indirect dating). Cosmogenic nuclide dating measures the
amount of nuclides, atoms that exist for a measurable length of time, which
are produced by the nuclear interaction of cosmic rays. Common nuclides used
in TCN dating are helium [He], neon [Ne], beryllium [Be], aluminum [Al],
chlorine [Cl], argon [Ar], and carbon [C] (EQS).

Primary cosmic rays interact with atoms in the upper atmosphere, causing
secondary rays, which cascade towards the earth resulting in further nuclear
interactions in the atmosphere and upper lithosphere. These interactions
fragment nuclei producing transformations that result in new forms. Some of
these new forms are not always initially present in surface target
materials. Measuring this interaction in surface minerals requires knowledge
of the build up and decay rates of nuclides, penetration depths, and
temporal variations (EQS).

Calculating the estimated production rates is based on the known decay rates
and concentrations of nuclides present in independently dated surfaces.
Penetration depths by nucleons and muons (atomic particles created by the
nuclear interactions of primary and secondary cosmic rays) are based upon
the different reactivity rates and kinetic energies of each. These are also
useful for calculating erosional rates. Solar activity, atmospheric
shielding, variations in the geomagnetic field, and the variation in the
amount of primary cosmic rays modulate local production of nuclides. Thus,
the accurate use of TCN requires calibration by correlation with
independently dated materials, previous samples, known local production
rates, or ratios between radiogenic nuclides (inherently present in target
material) and cosmogenic nuclides (EQS).

Materials and methods used for TCN measures include; 3He from olivine,
pyroxene, Fe/Ti-oxide; 21Ne from quartz, olivine, pyroxine, sanidine; 38Ar
from calcium; 10Be from quartz, calcite, sanidine; 26Al from quartz, and;
14C from quartz, olivine (EQS).

Adequate sample selections for TCN dating are exposed materials with a clean
surface containing suitable minerals. Additionally the target should be
unobstructed by other objects (overhangs, foliage, etc.), uncontaminated by
chemical weathering or other contaminative processes, and be in an
established state of rest (non-moving). Collection of a sample should be
from the uppermost area of best exposure in order to achieve the most
accurate exposure date. Collecting of the sample can consist of nothing more
than hammer and chisel and breaking off an amount needed (3).

TCN 3He dating of Box Canyon basalt from the Snake River Plain consisted of
collecting samples of 2-5 grams of basalt. This was crushed and sieved
before the separation of olivine by magnetic separation and handpicking to
achieve several hundred grains of ~500-1000 mg. These are then air abraded,
rinsed with methanol, acetone and ethanol, and crushed again to release
trapped 3He nuclides in the form of gas, then heated to release the
remaining 3He gas. Gas is extracted and measured using a mass spectrometer,
then calculated using formulas for 4He (radiogenic) and 3He (cosmogenic) to
achieve a series of TCN 3He dates ranging from 19,000 to 50,000 ka. (+/-3
ka) for the exposure of the Box Canyon basalts (1). These dates relate to
the exposure of the basalts, which formed roughly ~3 million years ago, to
cosmic rays, meaning the erosion of the canyon is of relatively recent
origin.

Uses of TCN dating include the study of rocks and sediments of glacial
landscapes, volcanic terrains, and alluvial and lacustrine landforms for
exposure geochronology. TCN is also useful in assisting in the evaluation of
landscape evolution by determining erosion rates (EQS). Pleistocene flood
events of the Snake River Plain have been TCN dated through 3He and 21Ne and
have been used in correlating global climate changes as indicated by flood
deposited boulders and scour features (2).

Other local applications of TCN dating could be used in helping to
understand the evolution of terraces along the Western Snake River, for
example how long they took to form, when they became stable, rates of
deposition/deflation. This information would be useful in understanding the
chronology of human behavior in these contexts.


References
(1) Aciego, Sarah M., Donald J. Depaolo, B.M. Kennedy, Michael P. Lamb,
Kenneth W.W. Sims, and William E. Dietrich. 2007. Combining [3He] cosmogenic
dating with U-Th/He eruption ages using olivine in basalt. Earth and
Planetary Science Letters. 254, pp.288-302.

(2) Cerling, Thure E., Robert J. Poreda, and Sara L. Rathburn. 1994.
Cosmogenic 3He and 21Ne age of the Big Lost River flood, Snake River Plain,
Idaho. Geology. 22, pp.227-230.

(3) Hill, Christopher L. 2007. Associate Professor, Department of
Anthropology, Boise State University.Personal communication. September 19,
2007.

(EQS) Encyclopedia of Quaternary Science. 2007. S.A. Elias (ed.), Elsevier,
Amsterdam, Netherlands.
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