Calibrating Cosmogenic Cl-36 Production Rates in Terrestrial
Rocks for Use in Surface Exposure Age Dating
Maciej Sliwinski
A review of 4 studies
The 4 studies being reviewed:
•
Phillips, F.M., Zreda, M.G., Flinsch, M.R., 1996. A reevaluation of cosmogenic Cl36 production rates in terrestrial rocks. Geophysical Research Letters, Vol. 23, No.
9, Pages 949-952
•
Zreda, M.B., Phillips, F.M., Elmore, D., Kubik, P.W., Sharma, P., and Dorn, R.I.,
1991. Cosmogenic Cl-36 production rates in terrestrial rocks. Earth and Planetary
Science Letters, Vol. 105, pages 94-109
•
Stone, J.O., Allan, G.L., Fifield, L.K., Cresswell, R.G., 1996. Cosmogenic chlorine36 from calcium spallation. Geochimica et Cosmochimica Acta, Vol. 60, No. 4, pp.
679-692
•
Swanson, T.W., Caffee, M.L., 2001. Determination of Cl-36 production rates
derived form the well-dated deglaciation surfaces of Whidbey and Fidalgo islands,
Washington. Quaternary Research 56, 366-382
• Different analytical approaches at different localities were used to
work out Cl-36 production rates, which are discordant.
The basis of cosmogenic isotope exposure age dating
• A geomorphic surface, fixed in
geomagnetic coordinates on the
Earths surface…is bombarded by the
incident cosmic radiation, creating
new (in situ) isotopes, both stable
and radioactive, by an exponentially
attenuated flux in the substrate of the
surface (Cerling and Craig 1990).
http://lyoinfo.in2p3.fr/manoir/lsm_eng.html
The basis of cosmogenic isotope exposure age dating
• If a production rate (atoms per g per yr of exposure)
can be worked out and applied to different settings by
use of scaling factors, then measuring the accumulated
concentrations of cosmogenic nuclides can be used as
a tool to get at rates and dates of geomorphic
processes.
Cosmogenic Isotopes
Isotope Half-life (yrs)
3
He
10
Be
14
C
21
Ne
26
Al
36
Cl
53
Mn
131
Xe
Stable
1.5 x 106
5730
Stable
0.71 x 106
0.30 x 106
3.7 x 106
stable
Principal targets (lithosphere)
O, Si, Al, Mg, etc.
O, Si, Al
C, O
Mg, Na, Si, Al
Si, Al
Cl, K, Ca
Fe
Ba
Table 1: Cosmogenic isotopes for in-situ exposure age studies of terrestrial rocks.
Source: Cerling, T.E., Craig, H. 1994. Geomorphology and in-situ cosmogenic isotopes. Annual Review of Earth and Planet Science 22, 273-317
Applications of cosmogenic nuclide surface
exposure age dating techniques
• Glacial events
• Erosion rates
• Volcanic events, lava
flows
• Alluvial deposits
• Ancient erosion surfaces
• Ice ablation rates
• Meteorite impact
• Ex. Investigating the exposure
frequency of the Antarctic landscape
• Ex. Determining rates and dates of
deglaciation in of West Antarctica
since the LGM
• Ex. Quantifying glacial erosion rates
• Ex. Timing the LGM worldwide
http://depts.washington.edu/cosmolab/ant_web/index.htm
Working out a production rate
• Cosmogenic nuclide production rates are dependent on:
• Latitude
• Altitude
• Erosion rate of surface
• Also need to address and assess the importance of:
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Composition of sample
Sample properties which affect production at depth
Variations in cosmic ray flux
Variations in solar ray flux
Large scale tectonic motions which change sample lat. and alt.
Concentration of radioactive elements in sample (ex. U, Th)
and their contributions to target nuclide production
Variations in the geomagnetic field
Irradiation geometry (how much of the cosmic flux hits the
sample)
Any shielding effects
Also need a independent chronology of surfaces used for
calibration.
Working out a production rate
• There are different production rates worked out by
different research groups for the various cosmogenic
isotopes. There is a general production rate consensus
for some isotopes (ex. Be-10 and Al-26), but not for
others (ex. the production rate of Cl-36 is not agreed
upon. Further calibration studies are needed and are
currently underway).
Working out a production rate
Scaled to sea level and
latitude >60° for crosscomparison.
Swanson and Caffee (2001)
Polygenic origin of Cl-36
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Spallation of K-39 and Ca-40
Thermal neutron activation of Cl-35
Thermal neutron activation of K-39
Muon capture reactions (important with increasing depth)
• U and Th decay produces Cl-36. This must be quantified
and separated from the cosmogenic component.
• Ti and Fe spallation?
Swanson (2001) Cl-36 Production Rate Calibration Location
• “The well-dated retreat history of the
Cordilleran Ice Sheet from the northern
Puget Sound regions of northwestern
Washington (~15,500 cal yr B.P.)
provides an exceptional opportunity to
develop a set of production rates for the
major Cl-36 production pathways.
Because all of the calibration samples
were collected from a closely limited
geographic setting (47-48 deg and near
sea level), the effects of latitude and
altitude variation require little scaling,
thereby eliminating one source of
uncertainty.”
Swanson (2001)
Production rate equation
P  Ca (CCa )  K (CK )  n ( 35 N 35 /  i N i)
Production rates and elemental
concentrations of Ca and K
Thermal neutron capture rate: dependent on
frac. and abundance of neutrons stopped by
Cl-35 and all other absorbing elements
Each component is solved for independently. For ex.: to solve for the production
rate due to Ca, a high Ca/Cl and Ca/K set of samples would be analyzed. Similarly
for solving the other Cl-36-producing components.
Swanson (2001)
Application of the Swanson calibration
• Used to analyze “whole” rocks.
• Need to determine major elemental composition of
rock samples
• Need to determine boron an gadolinium
concentrations
• Need to determine U and Th concentrations
• Need to determine Cl content
• What about Cl-36 production from Fe and Ti?
• Can then apply Swansons production rates to arrive at
the exposure age of a sample.
Swanson (2001)
Production rates
• Spallation of Ca: ~86±5 atoms Cl-36 per (g Ca) per year
• This is 60% greater than the value obtained by Stone et al
(1996) and 20% than that obtained by Phillips et al. (1996).
• Spallation of K: 228±18 atoms Cl-36 per (g K) per year
• This is 35% greater than the value obtained by Phillips et al.
(1996).
Swanson (2001) Production rates
Swanson and Caffee (2001)
Testing the calibrated
production rates
The Swanson (2001)
Cl-36 production rates
used to calculate the
exposure ages of the
sample sets chosen to
test their validity yield
age in close agreement
with independent C-14
ages.
Swanson and Caffee (2001)
Stone et al. (1996)
Production rates
•
This study focused on calibrating the Cl-36
production rate from Ca spallation for use in dating
calcite.
•
Why calcite? Limestone is geologically abundant,
and so cosmogenic Cl-36 measurements in calcite
for exposure dating has widespread applicability.
•
Applicable to the study of karst landform
development.
•
Also applicable to dating basalts too young to be
dated by K-Ar and 40Ar/39Ar techniques (target
minerals are plag. and pyroxenes).
•
A Ca-feldspar from a well-dated basalt lava flow
was used to calibrate a Cl-36 production rate.
http://www.speleogenesis.info/archive/print_save.php?Type=publication&PubID=3287
Stone et al. (1996)
Production rates: whole rock vs. specific mineral analysis
• Stone et al. calibrate the production of Cl-36 from Ca spallation and
muon capture. They choose particular mineral (Ca-fspar) analysis over
whole rock analysis. While whole-rocks dating methods have a wider
applicability, particular mineral methods are simpler, more sensitive and
seem to be a more direct way of arriving at a production rate.
• Whole rock analysis vs. target mineral:
• Whole rock analysis: the major elemental
composition of samples is determined by X-ray
fluorescence spectrometry
• Target mineral: Rock samples are crushed and specific
minerals are isolated by density separations (ex.
isolating quartz out of granites for 26-Al and 10-Be
analysis or plag. from granites for 36-Cl).
http://www.windows.ucar.edu/tour/link=/earth/geology/min_calcite.html
Stone et al. (1996) Production Rate Calibration Location
• The age of Tabernacle Hill
basalt (in Utah) is closely
bracketed by C-14 dates at
17.3±0.5 cal. ka. It is well
preserved and is the site of
the Stone et al. (1996) Cl-36
production rate calibration.
• Samples were collected
from well preserved
pahoehoe surfaces. Such
surfaces eliminate the
variable of erosion from the
calibration.
Stone et al. (1996) Production Rates
Zdreda et al. 1991 and Phillips et al. 1996
•
Phillips et al. (1996) is a reevaluation of Zdreda et al.
(1991).
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Calibration samples for these studies were collected from
a late Quaternary moraine sequence at Chiatovich Creek
in the eastern White Mountains, from the Tabernacle Hill
basalt flow and from late Pleistocene moraines on Mauna
Kea.
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To test the obtained production rates, samples from
Meteor Crater, Arizona and from moraines in the Sierra
Nevada, California were collected.
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× Unfortunately, the original calibration (Zreda et al.
1991) was based in part on a independent chronology
developed using the falsified varnish radiocarbon dating
“method.” ×
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The study wasn’t a total loss, however, since the varnish
C-14 dates were in close agreement with dates obtained
using the “traditional” C-14 dating method.
•
A reevaluation was done by Phillips et al. 1996.
Mauna Kea moraine photo from: http://satftp.soest.hawaii.edu/space/hawaii/vfts/bigisle/bigisle.ground.photos8.html
Zdreda et al. 1991 and Phillips et al. 1996
• This study also used the “whole-rock” analysis procedure
(similar to that of Swanson and Caffee (2001)). In essence,
Cl-36 production rates were obtained by working with the
multivariable production equation:
P  Ca (CCa )  K (CK )  n ( 35 N 35 /  i N i)
as with a optimization problem. They obtained the following
production rates:
• Spallation of Ca: 72.5±5 atoms Cl-36 per (g Ca) per year
• Spallation of K: 154±10 atoms Cl-36 per (g K) per year
Cl-36 Production Rate Comparison
Scaled to sea level and
latitude >60° for crosscomparison.
“No consistent pattern of variance is seen between each respective research
group’s production rates.” (Swanson 2001).
Swanson and Caffee (2001)
Sources of error in determining a production rate
•
“The lack of consistency between the various production rates reflects the
numerous physical and geological processes affecting the production of Cl-36”
(Swanson and Caffee 2001).
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Analytical error (but this doesn’t account for the large differences).
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Uncertainty in the independent chronology used to determine the age of surfaces
used to calibrate a Cl-36 production rate (ex. C-14 dating uncertainties: reservoir
effects and calibration methods?).
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There are 3 different latitude-altitude scaling systems in use worked out by different
researchers.
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Variability of the Earth’s magnetic field: this could be a additional source of error
for Phillips et al. (1996), who use samples from 19-70° latitude.
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Chemical extraction procedures?
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Whole rock analysis vs. mineral separates? It seems that the whole rock analysis
method and the resulting optimization problem may underestimate the significance
of other production pathways, i.e. Fe and Ti spallation?
Objectives of CRONUS-Earth
The objective of the CRONUS-Earth Project is to
simultaneously address the various uncertainties affecting
the production and accumulation of in-situ cosmogenic
nuclides, with the goal of producing a widely accepted and
internally consistent set of parameters that can be used in
calculating ages and erosion rates. With properly designed
experiments it should be feasible to reduce the overall
uncertainty in results to approximately ±5%, regardless of
location, and to produce consistent results with differing
nuclides.
Visit: http://www.physics.purdue.edu/cronus/