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10 Be 06NOV2012 Kelly Hughes

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How is it produced? Cosmic Ray Spallation: – 10 Be results from 16 O being bombarded with highly energetic cosmic rays Muon-reduced reactions: – Unstable subatomic (-) charged particle interacting w/ target elements Stopped muons Fast muons

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Spallation-Dominated Fig. 3. Production rate of 10Be in quartz as a function of depth at sea level and high latitude. The total production is a composite of the production by neutron spallation, stopped muons, and fast muons. Ivy-Ochs & Kober, 2008

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Production Rate Ivy-Ochs & Kober, 2008 Fig. 2: Production of rate of 10 Be in quartz as a function of geomagnetic latitude and altitude (based on STONE 2000). The production rates have been normalized to sea level and high latitude. At low latitude, production rates are lower than at high latitude. Production rates increase exponentially with increasing altitude.

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Concentration of 10 Be Where P (0) is the production rate at sampling site (atoms/g/yr) t is the exposure age (yr) λ is the decay constant (yr -1 ) ρ is the density of the rock (g/cm 3 ) ε is the erosion rate (cm/yr) Λ is the attenuation length, length at which the probability has dropped to 1/e that a particle has not been absorbed (g/cm 2 ) C in is the inherited nuclide concentration (atoms/g) Ivy-Ochs & Kober, 2008

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Secular Equilibrium Fig. 5: Increase in concentration of the radionuclides 10 Be, 26 Al, 36 Cl and the stable nuclides 3 He and 21 Ne with time. Secular equilibrium, where production of radionuclides equals radioactive decay, is approached after 3-4 half- lives. The secular equilibrium concentration sets the limit of the maximum exposure age that can be determined with a given radionuclide. Ivy-Ochs & Kober, 2008

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Method of Measurement Accelerated Mass Spectrometry – Pure quartz is obtained by selective chemical dissolution in hot ultrasonic bath and/or on shaker table – Carrier of 9Be (0.5 mg in soln) added to pure quartz mineral separate – Completely dissolved with concentrated HF and HNO3 – Be separated and purified with ion exchange and selective pH precipitations – Ratio of 10 Be/ 9 Be relative to a standard

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Why Date Moraines? Moraines record glacier extent Moraine dates can construct a chronological structure to past glacier fluctuations

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Well-constrained production rate Widely applicable age range (10 1 to 10 6 yrs) Why 10 Be?

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Why target greywacke boulders? Greywacke: poorly sorted sandstone, high quartz content – Quartz is ubiquitous and highly resistant to weathering – Quartz can be cleaned of the meteoric (atmospheric) 10 Be Large boulders are more stable than smaller clasts – Toppling/shifting can result in inaccurate ages

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Improvements to the 10 Be Method? Pinning down an accurate half-life; 1.51 Myr and 1.34 Myr both published Combining nuclides checks for continuous exposure vs. intermittent coverage

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Reference Ivy-Ochs, S. and Kober, F. Surface exposure dating with cosmogenic nuclides. Journal of Quaternary Science. 2008. v. 57. no. 1-2. p. 179-209

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