Cosmogenic Nuclides 9/16/10

Lecture outline:1) cosmic ray introduction

2) cosmogenic nuclide formation

3) applications

artist’s rendition of cosmic ray spallation reactionsin atmosphere

Zircon

Cosmic Rays

Victor Hess (1912) discovered cosmic radiation in hot-air balloon

~90% of cosmic rays arenuclei of H (aka ?), 8% are He nuclei (aka ?), rest electrons, or heavier nuclei

Energy

Flu

xspallation:cascade of subatomic particlesassociated 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) thecrust to form cosmogenic radionuclides.

Ex: 14Cformed from 14N

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))• geomagnetic field strength (more particles deflected when field strong)• solar activity (sun’s magnetic field shields E from cosmic flux when active), see below

14N(n,p)14C14N(n,3H)12C14N(n,p α)10Be40Ar(n,p α)26Al40Ar(p,α)36Cl40Ar(p,α)32Si

10Be, 26Al, and 36Cl* Measuring cosmogenic isotopes requires AMS (accelerator mass spectrometry), because they are verylow in abundance compared to their stable counterparts (e.g. 12C is 1012 more abundant than 14C)

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

10Be 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=7.16e5y36Cl 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)

26Al & 36Cl

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=A0e-αt equation, if you know production history

10 100ln( ) ln( ) ( )

dBe Be

S

if t=d/s, can calculate sedimentation rate (s):

Sedimentation Rate

36Cl in Hydrological Applications

Paul et al., 1986

source

destination

In a simple world, 36Clfalls to ground, gets drawninto aquifer, and you can date the water by trackingits decay:

teClCl 03636

But what happensif you have evaporation?or bedrock dissolution?

Solution: measure stablechlorine isotopes; trackimpact of processes usingmass balance

What processes are at work in this system?

What numbers would you need to know tocalculate the age of the Dead Sea?

Other applications of cosmogenic nuclides

10Be in arc magmas was the smoking gun forrecyclying of ocean sediments in subduction zones

control,non-arc

arcsetting

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 36Cl (t < steady state)

Bloody Canyon terminal moraine, CA

Ex: Exposure ages of glacial morraines

Schaefer et al., 2006

Meteorites – measuredecay from “saturation”(clock starts from steady state)

Terrestrial ages of meteorites

Photo of Lewis Cliff, AntarcticaEx: meteorite ALH84001ejected from Mars 13Ma,landed on Earth 13,000ybp;“terrestrial” age dated by 14C