Calibrating Cosmogenic Cl-36 Production Rates in Terrestrial Rocks for Use in Surface Exposure Age Dating

Maciej Sliwinski

A review of 4 studies

• Phillips, F.M., Zreda, M.G., Flinsch, M.R., 1996. A reevaluation of cosmogenic Cl-36 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 chlorine-36 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 4 studies being reviewed:

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

• 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.

The basis of cosmogenic isotope exposure age dating

Cosmogenic Isotopes

Isotope Half-life (yrs) Principal targets (lithosphere) 3He Stable O, Si, Al, Mg, etc. 10Be 1.5 x 106 O, Si, Al 14C 5730 C, O

21Ne Stable Mg, Na, Si, Al 26Al 0.71 x 106 Si, Al 36Cl 0.30 x 106 Cl, K, Ca

53Mn 3.7 x 106 Fe 131Xe stable 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:• 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.

• 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

Working out a production rate

Swanson and Caffee (2001)

Scaled to sea level and latitude >60° for cross-comparison.

Polygenic origin of Cl-36

• 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.”

)/()()( 3535 iinKKCaCa NNCCP

Thermal neutron capture rate: dependent on frac. and abundance of neutrons stopped by Cl-35 and all other absorbing elements

Production rates and elemental concentrations of Ca and K

Swanson (2001) Production rate equation

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.

• 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 (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

• 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).

Stone et al. (1996) Production rates: whole rock vs. specific mineral analysis

http://www.windows.ucar.edu/tour/link=/earth/geology/min_calcite.html

• 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.

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).

• 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.

• To test the obtained production rates, samples from Meteor Crater, Arizona and from moraines in the Sierra Nevada, California were collected.

• × Unfortunately, the original calibration (Zreda et al. 1991) was based in part on a independent chronology developed using the falsified varnish radiocarbon dating “method.” ×

• 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:

)/()()( 3535 iinKKCaCa NNCCP

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

Swanson and Caffee (2001)

“No consistent pattern of variance is seen between each respective research group’s production rates.” (Swanson 2001).

Scaled to sea level and latitude >60° for cross-comparison.

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).

• Analytical error (but this doesn’t account for the large differences).

• 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?).

• There are 3 different latitude-altitude scaling systems in use worked out by different researchers.

• 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.

• Chemical extraction procedures?

• 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/