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Cosmogenic Nuclides 9/16/10

Cosmogenic Nuclides 9/16/10. Lecture outline: cosmic ray introduction cosmogenic nuclide formation 3) applications. Zircon. artist’s rendition of cosmic ray spallation reactions in atmosphere. Cosmic Rays . spallation : cascade of subatomic particles associated with cosmic rays.

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Cosmogenic Nuclides 9/16/10

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  1. Cosmogenic Nuclides 9/16/10 • Lecture outline: • cosmic ray introduction • cosmogenic nuclide formation • 3) applications Zircon artist’s rendition of cosmic ray spallation reactions in atmosphere

  2. Cosmic Rays spallation: cascade of subatomic particles associated with cosmic rays Flux Victor Hess (1912) discovered cosmic radiation in hot-air balloon Energy ~90% of cosmic rays are nuclei of H (aka ?), 8% are He nuclei (aka ?), rest electrons, or heavier nuclei

  3. Muon “shadow” caused by moon, as detected by 700m subterranean Soudan 2 detector, MN. Actual location of moon is marked by crosshairs.

  4. Cosmogenic nuclide formation Cosmic rays interact with atoms in the atmosphere or (more rarely) the crust to form cosmogenic radionuclides. Ex: 14C formed from 14N NOTE: Nuclear bomb testing in the 1950’s created a huge pulse of cosmogenic isotopes - a story for another lecture

  5. Cosmogenic nuclides 14N(n,p)14C 14N(n,3H)12C 14N(n,p α)10Be 40Ar(n,p α)26Al 40Ar(p,α)36Cl 40Ar(p,α)32Si • The rate of production of cosmogenic nuclides depends on: • latitude (charged particles enter E’s atmosphere more readily where field lines are • perpendicular to E’s surface, ie at poles) so production α(cos(θ)) • geomagnetic field strength (more particles deflected when field strong) • solar activity (sun’s magnetic field shields E from cosmic flux when active), see below

  6. 10Be, 26Al, and 36Cl * Measuring cosmogenic isotopes requires AMS (accelerator mass spectrometry), because they are very low in abundance compared to their stable counterparts (e.g. 12C is 1012 more abundant than 14C) 10Be 26Al & 36Cl produced by interaction of cosmic rays with O, N (most abundant atoms in atmosphere), so production rate is fairly large; also generated when spallation products reach crust (O, Mg, Si, Fe) 10Be decays to 10B with t1/2=1.5e6y readily adsorbed onto aerosols in atmosphere, rained out, residence time = 1-2 weeks in atmosphere adsorbed onto clays in ocean; scavenged produced by interaction of cosmic rays with 40Ar; also generated when spallation products reach crust (O, Mg, Si, Fe) 26Al decays to 26Mg with t1/2=7.16e5y 36Cl decays to 36S and 36Ar with t1/2=3.08e5y readily adsorbed onto aerosols in atmosphere, rained out Al relatively immobile (like 10Be, “locked in”) but Cl mobile geochemically… (useful in hydrlogical studies, groundwater ages, etc)

  7. Sedimentation Rate Principle: cosmogenic nuclide production is quasi-constant, so can date sediments, ice cores, etc. using the A=A0e-αt equation, if you know production history if t=d/s, can calculate sedimentation rate (s): But you can get better ages if you combine cosmogenic nuclides for sed rate determination: why?

  8. Paul et al., 1986 36Cl in Hydrological Applications In a simple world, 36Cl falls to ground, gets drawn into aquifer, and you can date the water by tracking its decay: source destination But what happens if you have evaporation? or bedrock dissolution? What processes are at work in this system? What numbers would you need to know to calculate the age of the Dead Sea? Solution: measure stable chlorine isotopes; track impact of processes using mass balance

  9. Other applications of cosmogenic nuclides 10Be in arc magmas was the smoking gun for recyclying of ocean sediments in subduction zones control, non-arc arc setting Tera et al., 1986

  10. Exposure dating Principle: cosmogenic nuclides also created when high-energy particles strike nuclei in rocks (much more rare, but very useful) - track their accumulation (predictable with ‘t’ if you know the rock chemistry, ie quartz,etc) - can also compare the steady in-growth assumption against observed profiles, obtain erosion histories (next lecture)

  11. Ex: Exposure ages of glacial morraines Glacial moraines- measure grow-in of 36Cl (t < steady state) Bloody Canyon terminal moraine, CA

  12. Schaefer et al., 2006

  13. Terrestrial ages of meteorites Meteorites – measure decay from “saturation” (clock starts from steady state) Photo of Lewis Cliff, Antarctica Ex: meteorite ALH84001 ejected from Mars 13Ma, landed on Earth 13,000ybp; “terrestrial” age dated by 14C

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