In-Situ Chlorine-In-Situ Chlorine-3636

Nicole DixHWR 696T

OutlineOutline

IntroductionIntroduction Production MechanismsProduction Mechanisms Sample Collection MethodsSample Collection Methods Laboratory AnalysisLaboratory Analysis ApplicationsApplications ReferencesReferences

IntroductionIntroduction

Chlorine has three isotopes. Two of which Chlorine has three isotopes. Two of which are stable (chlorine-35 and 37) and the are stable (chlorine-35 and 37) and the third is a cosmogenic isotope (chlorine-third is a cosmogenic isotope (chlorine-36). 36).

However, for this presentation we will However, for this presentation we will only focus on chlorine-36, which is only focus on chlorine-36, which is produced in the solid materials on the produced in the solid materials on the Earth’s surface.Earth’s surface.

http://www.sahra.arizona.edu/programs/isotopes/chlorine.html

Introduction Continued…Introduction Continued…

Because chlorine-36 has a half-life of Because chlorine-36 has a half-life of 310,000 years it is useful in age dating 310,000 years it is useful in age dating ground water and solid materials on the ground water and solid materials on the Earth’s surface.Earth’s surface.

Like all cosmogenic nuclides the production Like all cosmogenic nuclides the production of chlorine-36 depends on the intensity of of chlorine-36 depends on the intensity of incident cosmogenic rays, availability of incident cosmogenic rays, availability of target nuclei in the exposed material, and target nuclei in the exposed material, and the probability with which a nuclear the probability with which a nuclear reaction produces the nuclide of interest.reaction produces the nuclide of interest.

Zreda et al., 2000

Production MechanismsProduction Mechanisms

Chlorine-36 is produced in solid materials Chlorine-36 is produced in solid materials on the Earth’s surface primarily through on the Earth’s surface primarily through cosmic-ray induced reactions with cosmic-ray induced reactions with chlorine-36, potassium-39 and calcium-40.chlorine-36, potassium-39 and calcium-40.

The three mechanisms of formation are: The three mechanisms of formation are: 1) spallation reactions, 2) muon reactions 1) spallation reactions, 2) muon reactions and 3) thermal neutron absorption.and 3) thermal neutron absorption.

Zreda et al., 1991 & http://www.sahra.arizona.edu/programs/isotopes/chlorine.html

Production Mechanisms Production Mechanisms Conti..Conti..

In the top few meters of the Earth’s surface In the top few meters of the Earth’s surface thermal neutron activation of chlorine-35 and thermal neutron activation of chlorine-35 and spallation of potassium-39 and calcium-40 are spallation of potassium-39 and calcium-40 are the dominant means of production for chlorine-the dominant means of production for chlorine-36.36.

Below that depth, slow negative muon capture, Below that depth, slow negative muon capture, by calcium-40, becomes more important than the by calcium-40, becomes more important than the other mechanisms.other mechanisms.

In carbonates chlorine-36 is produced by Ca and In carbonates chlorine-36 is produced by Ca and in silicates it is produced by K, Ca, and Cl.in silicates it is produced by K, Ca, and Cl.

Zreda et al., 1991 & Zreda et al., 2000

Production Mechanisms Production Mechanisms Conti..Conti..

Zreda et al., 2000

Production Mechanisms Production Mechanisms Conti..Conti..

Zreda et al., 1991

Production RatesProduction Rates Case Study (Phillips et al., 1996): Measured the Case Study (Phillips et al., 1996): Measured the

chlorine-36 content in 33 rock samples of well-chlorine-36 content in 33 rock samples of well-constrained exposure histories and ages.constrained exposure histories and ages. Production parameters were made by minimizing the Production parameters were made by minimizing the

squared deviation (squared deviation (22) between the chlorine-36 and ) between the chlorine-36 and independent ages (found through carbon-14, argon, or independent ages (found through carbon-14, argon, or thermoluminescence).thermoluminescence).

Phillips et al., 1996

Production Rates Production Rates Continued…Continued…

Phillips et al., 1996

Production Rates Production Rates Continued…Continued…

The following production rates were found:The following production rates were found: Spallation and muon production from Ca 2940Spallation and muon production from Ca 2940++200 200

atoms atoms 3636Cl (mole Ca)Cl (mole Ca)-1-1 yr yr-1-1.. Spallation from K 6020Spallation from K 6020++400 atoms 400 atoms 3636Cl (mole K)Cl (mole K)-1-1 yr yr.... Neutron production in air 586Neutron production in air 586++40 fast neutrons (g air)40 fast neutrons (g air)-1-1

yryr-1-1.. The new production constants found in Phillips The new production constants found in Phillips

et al., 1996 for the spallation of Ca and thermal neutron et al., 1996 for the spallation of Ca and thermal neutron activation are in agreement with previous constants. activation are in agreement with previous constants. However, the new production constant for K is now However, the new production constant for K is now about 50% larger than previously thought.about 50% larger than previously thought.

Previous constants were: 4160Previous constants were: 4160++310 atoms 310 atoms 3636Cl yrCl yr-1-1 mol mol-1-1 3939K, 3050K, 3050++210 atoms 210 atoms 3636Cl yrCl yr-1-1 mol mol-1-1 4040Ca, and Ca, and (3.07(3.07++0.24)*100.24)*1055 neutrons (kg of rock) neutrons (kg of rock)-1-1 yr yr-1-1..

Phillips et al., 1996

Production Rates Production Rates Continued...Continued...

However, in Phillips et al., 2001, the value However, in Phillips et al., 2001, the value for the neutron production in air for the neutron production in air increased from 586 to 626 fast neutrons increased from 586 to 626 fast neutrons (g air)(g air)-1-1 yr yr-1 -1 and the value for the and the value for the spallation of Ca decreased by 9% from spallation of Ca decreased by 9% from 73.3 to 66.8 atoms 73.3 to 66.8 atoms 3636Cl (g Ca)Cl (g Ca)-1-1 yr-1, due yr-1, due to the inclusion of production by muon to the inclusion of production by muon absorption of Ca (unlike the 1996 study). absorption of Ca (unlike the 1996 study). The production rate value for the The production rate value for the spallation of K stayed the same.spallation of K stayed the same.

These values seem to be the most These values seem to be the most accurate to date, but will most likely be accurate to date, but will most likely be changed in the future.changed in the future.

Phillips et al., 2001

Production Rates Production Rates Continued....Continued....

Problems in calculating production Problems in calculating production rates include:rates include:

Ø The lack of a perfect calibration site. The The lack of a perfect calibration site. The calibration cannot be more accurate than the calibration cannot be more accurate than the chronology on which it is based. At sites chronology on which it is based. At sites erosion occurs, burial, etc.erosion occurs, burial, etc.

Ø Problems with scaling factors for location Problems with scaling factors for location and geomagnetic strength.and geomagnetic strength.

*If scaling factors were known with an absolute *If scaling factors were known with an absolute certainty or the “perfect” site were to be certainty or the “perfect” site were to be found then production rates would no longer found then production rates would no longer be controversial and ever changing.be controversial and ever changing.

Sample Collection Sample Collection MethodsMethods

First and foremost, determine the rock/mineral First and foremost, determine the rock/mineral type you want to sample, from what surface and type you want to sample, from what surface and how many samples you need to collect.how many samples you need to collect.

Because chlorine-36 is produced from several Because chlorine-36 is produced from several target elements, virtually all rock types are target elements, virtually all rock types are suitable for sampling.suitable for sampling.

The number of samples is related to geological The number of samples is related to geological characteristics of the surface dated, specifically, characteristics of the surface dated, specifically, its history of burial and erosion.its history of burial and erosion.

Zreda et al., 2000

Sample Collection Sample Collection Continued...Continued...

Sampling sites should be assessed due to Sampling sites should be assessed due to their geomorphic stability and geometry.their geomorphic stability and geometry.

Preferably sampling should take place on Preferably sampling should take place on flat, horizontal surfaces that are likely to flat, horizontal surfaces that are likely to have been continuously exposed since the have been continuously exposed since the surfaces formation such as, large tall surfaces formation such as, large tall morainal boulders.morainal boulders.

For chlorine-36 samples should be far For chlorine-36 samples should be far from edges because of a possible leakage from edges because of a possible leakage of thermal neutrons form the sides.of thermal neutrons form the sides.

Zreda et al., 2000

Sample Collection Sample Collection Continued...Continued...

Samples need to be collected from the top Samples need to be collected from the top few centimeters of rock minimizing the few centimeters of rock minimizing the variability of production rates with depth.variability of production rates with depth.

The least weathered surfaces are ideal for The least weathered surfaces are ideal for sampling.sampling.

Once collected samples should be stored Once collected samples should be stored in plastic bags until preparation.in plastic bags until preparation.

Zreda et al., 2000

Laboratory AnalysisLaboratory Analysis

First, samples need to cleared of any First, samples need to cleared of any organic growth.organic growth.

They then need to be ground to a size They then need to be ground to a size fraction smaller than the mean phenocryst fraction smaller than the mean phenocryst size of each rock.size of each rock.

Rock typeRock type CharacteristicCharacteristicss

SizeSize

Granites etc.Granites etc. Coarse-Coarse-crystallinecrystalline

0.5-1.0 mm0.5-1.0 mm

Basalts etc.Basalts etc. Fine-crystallineFine-crystalline 0.25-0.5 0.25-0.5 mmmm

CarbonatesCarbonates 0.25-1.0 0.25-1.0 mmmm

Laboratory Analysis Laboratory Analysis Conti…Conti…

The samples are then leached for 24 The samples are then leached for 24 hours.hours.

-Silicates are leached in 5% nitric.-Silicates are leached in 5% nitric.

-Carbonates are leached in deionized -Carbonates are leached in deionized water.water.

The leaching is done to remove any The leaching is done to remove any chlorine resulting from handling in the chlorine resulting from handling in the field or secondary carbonates, in the case field or secondary carbonates, in the case of silicates, from the microscopic pore or of silicates, from the microscopic pore or grain boundaries in the rock sample.grain boundaries in the rock sample.

Laboratory Analysis Laboratory Analysis Conti…Conti…

After leaching, the samples are then After leaching, the samples are then dissolved in airtight capsules or “bombs”.dissolved in airtight capsules or “bombs”.

Silicate samples are dissolved using Silicate samples are dissolved using hydrofluoric acid and are incased in the hydrofluoric acid and are incased in the “bomb” for 6 hours, at a temperature of “bomb” for 6 hours, at a temperature of 130 degrees Celsius.130 degrees Celsius.

The carbonate samples are dissolved The carbonate samples are dissolved using concentrated nitric acid and are using concentrated nitric acid and are incase in the “bomb” for 3 hours, at room incase in the “bomb” for 3 hours, at room temperature.temperature.

Laboratory Analysis Laboratory Analysis Conti…Conti…

Once the samples have been digested, Once the samples have been digested, AgNOAgNO33 is added to the solution to is added to the solution to precipitate out AgCl.precipitate out AgCl.

This solution sits overnight.This solution sits overnight. Next, the liquid is removed and NHNext, the liquid is removed and NH44OH is OH is

added to dissolve the solid.added to dissolve the solid. BaNOBaNO33 is added to the solution to remove is added to the solution to remove

any sulfur present.any sulfur present. The solution sits overnight.The solution sits overnight.

Laboratory Analysis Laboratory Analysis Conti…Conti…

The next day HNOThe next day HNO33 is added until a white precipitate is added until a white precipitate forms and again AgNOforms and again AgNO3 3 is added to precipitate AgCl is added to precipitate AgCl out of solution.out of solution.

This solution stands overnight.This solution stands overnight. The above steps, using NHThe above steps, using NH44OH, BaNOOH, BaNO33, and HNO, and HNO33, are , are

repeated two more times.repeated two more times. Finally, the nearly sulfur-free AgCl is rinsed in Finally, the nearly sulfur-free AgCl is rinsed in

deionized water five times, to eliminate any unwanted deionized water five times, to eliminate any unwanted chemicals, and dried in an oven at 60 degrees chemicals, and dried in an oven at 60 degrees Celsius.Celsius.

The resulting sample is then weighed and sent to The resulting sample is then weighed and sent to Purdue University. The amount of chlorine-36 in Purdue University. The amount of chlorine-36 in each sample will be measured using Accelerator each sample will be measured using Accelerator Mass Spectrometry (AMS).Mass Spectrometry (AMS).

ApplicationsApplications

Chlorine-36 can be used to date impact Chlorine-36 can be used to date impact craters, paleoseismic events, glacial craters, paleoseismic events, glacial moraines, young paleoshorelines and moraines, young paleoshorelines and young volcanic events.young volcanic events.

Impact Craters Impact Craters are formed instantly and are formed instantly and deep enough to ensure that there was no deep enough to ensure that there was no prior cosmogenic nuclide build up at the prior cosmogenic nuclide build up at the base of the crater. Therefore, this allows base of the crater. Therefore, this allows the date of the impact to be determined the date of the impact to be determined using chlorine-36.using chlorine-36.

Applications Continued…Applications Continued…

Paleoseismic EventsPaleoseismic Events can be dated by using can be dated by using samples from the face of a fault scarp. samples from the face of a fault scarp. During an earthquake previously buried rock During an earthquake previously buried rock is brought up to the surface exposing it to is brought up to the surface exposing it to cosmic rays. While dating a fault it is cosmic rays. While dating a fault it is important to recognize the fact that faults can important to recognize the fact that faults can be active more than once and to collect be active more than once and to collect samples accordingly.samples accordingly.

The bedrock that the fault scarp is composed The bedrock that the fault scarp is composed of is ideal for dating using chlorine-36 due to of is ideal for dating using chlorine-36 due to it resistance to weathering.it resistance to weathering.

Applications Continued…Applications Continued… Glaciers uplift bedrock ask they move down a Glaciers uplift bedrock ask they move down a

valley. The material is then later deposited, valley. The material is then later deposited, some of which is deposited as some of which is deposited as glacial morainesglacial moraines..

-The assumption in all of this is that the -The assumption in all of this is that the glacial erosion extends deep enough to glacial erosion extends deep enough to

bring bring material to the surface that was material to the surface that was previously previously shielded from comic rays.shielded from comic rays.

Once deposited the material becomes bombarded Once deposited the material becomes bombarded by cosmic rays and chlorine-36 begins to by cosmic rays and chlorine-36 begins to accumulate.accumulate.

It is possible to correlate the deposition of It is possible to correlate the deposition of moraines located in different areas during the moraines located in different areas during the same time interval.same time interval.

Zreda et al., 2000

Applications Continued…Applications Continued… Two types of materials from Two types of materials from ancient lake ancient lake

shorelinesshorelines are suitable for surface exposure dating: are suitable for surface exposure dating: (1) clasts transported by streams and redeposited at (1) clasts transported by streams and redeposited at the shore; and (2) tufa deposits precipitated directly the shore; and (2) tufa deposits precipitated directly from lake water.from lake water.

A problem with this experiment is the fact that some A problem with this experiment is the fact that some fluvial deposits may have already accumulated fluvial deposits may have already accumulated chlorine-36 and this must be assessed before sample chlorine-36 and this must be assessed before sample analysis.analysis.

The tufa, at the time of deposition, will have the same The tufa, at the time of deposition, will have the same concentration of chlorine-36 as the lake water. This concentration of chlorine-36 as the lake water. This amount can be subtracted out when calculating the amount can be subtracted out when calculating the shoreline age.shoreline age.

Zreda et al., 2000

Applications Continued…Applications Continued…

Lava flows are ideal for surface exposure Lava flows are ideal for surface exposure dating for three reasons: (1) they dating for three reasons: (1) they originate deep within the Earth, (2) they originate deep within the Earth, (2) they form almost instantaneously, and (3) they form almost instantaneously, and (3) they have surface structures that help assess have surface structures that help assess their geomorphic stability.their geomorphic stability.

Youngest lava flows= most recent Youngest lava flows= most recent volcanic activityvolcanic activity

Mt. Erciyes, TurkeyMt. Erciyes, Turkey

Zreda et al., 2000

ReferencesReferencesPhillips, F., Zreda, M., Montgomery, F.R., 1996, A Reevaluation of Cosmogenic Phillips, F., Zreda, M., Montgomery, F.R., 1996, A Reevaluation of Cosmogenic Chlorine-36 Production Rates in Terrestrial Rocks, Chlorine-36 Production Rates in Terrestrial Rocks, Geophysical Research Letters Geophysical Research Letters 23 23 No. 9No. 9, , 949-952949-952..

Phillips, F.M., Stone, W.D., Fabryka-Martin, J.T., 2001, An Improved Approach to Phillips, F.M., Stone, W.D., Fabryka-Martin, J.T., 2001, An Improved Approach to Calculating Low-Energy Cosmic-Ray Neutron Fluxes Near the Fluxes Near the Calculating Low-Energy Cosmic-Ray Neutron Fluxes Near the Fluxes Near the Land/Atmosphere Interface, Land/Atmosphere Interface, Chemical GeologyChemical Geology 175, 689-701. 175, 689-701.

Zreda, M.G., Phillips, F.M., Elmore, D., Kubik, Sharma, P., and Dorn, R.I., 1991, Zreda, M.G., Phillips, F.M., Elmore, D., Kubik, Sharma, P., and Dorn, R.I., 1991, Cosmogenic Chlorine-36 Production Rates in Terrestrial Rocks, Cosmogenic Chlorine-36 Production Rates in Terrestrial Rocks, Earth and Planetary Earth and Planetary Science Letters Science Letters 105, 94-109.105, 94-109.

Zreda, M.G., and Phillips, F.M., 2000, Cosmogenic Nuclide Buildup in Surficial Zreda, M.G., and Phillips, F.M., 2000, Cosmogenic Nuclide Buildup in Surficial Materials, Materials, inin J.S. Noller, J.M. Sowers and W.R. Lettis, eds., J.S. Noller, J.M. Sowers and W.R. Lettis, eds., Quaternary Geochronology: Quaternary Geochronology: Methods and Applications, Methods and Applications, AGU Reference Shelf 4, American Geophysical Union, 61-76.AGU Reference Shelf 4, American Geophysical Union, 61-76.