Cosmogenic Nuclides9/16/10 Lecture outline: 1)cosmic ray introduction 2)cosmogenic nuclide formation 3) applications artist’s rendition of cosmic ray spallation reactions in atmosphere Zircon

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

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

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

Cosmogenic nuclides The rate of production of cosmogenic nuclides depends on: 1)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)) 2)geomagnetic field strength (more particles deflected when field strong) 3)solar activity (sun’s magnetic field shields E from cosmic flux when active), see below 14 N(n,p) 14 C 14 N(n, 3 H) 12 C 14 N(n,p α) 10 Be 40 Ar(n,p α) 26 Al 40 Ar(p,α) 36 Cl 40 Ar(p,α) 32 Si

10 Be, 26 Al, and 36 Cl * Measuring cosmogenic isotopes requires AMS (accelerator mass spectrometry), because they are very low in abundance compared to their stable counterparts (e.g. 12 C is more abundant than 14 C) 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) 10 Be decays to 10 B with t 1/2 =1.5e 6 y readily adsorbed onto aerosols in atmosphere, rained out, residence time = 1-2 weeks in atmosphere adsorbed onto clays in ocean; scavenged 10 Be produced by interaction of cosmic rays with 40Ar; also generated when spallation products reach crust (O, Mg, Si, Fe) 26 Al decays to 26 Mg with t 1/2 =7.16e 5 y 36 Cl decays to 36 S and 36 Ar with t 1/2 =3.08e 5 y readily adsorbed onto aerosols in atmosphere, rained out Al relatively immobile (like 10 Be, “locked in”) but Cl mobile geochemically… (useful in hydrlogical studies, groundwater ages, etc) 26 Al & 36 Cl

But you can get better ages if you combine cosmogenic nuclides for sed rate determination: why? Principle: cosmogenic nuclide production is quasi-constant, so can date sediments, ice cores, etc. using the A=A 0 e -αt equation, if you know production history if t=d/s, can calculate sedimentation rate (s): Sedimentation Rate

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

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

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)

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

Schaefer et al., 2006

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