Transcript lecture8_cosmo1

Cosmogenic Nuclides
9/16/10
Lecture outline:
1) cosmic ray introduction
Zircon
2)
cosmogenic nuclide formation
3)
applications
artist’s rendition of cosmic ray spallation reactions
in atmosphere
Cosmic Rays
Flux
spallation:
cascade of
subatomic particles
associated with
cosmic rays
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
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: 14C
formed
from 14N
NOTE: Nuclear bomb testing in the 1950’s created a huge pulse of cosmogenic isotopes
- a story for another lecture
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:
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
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
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
26Al
& 36Cl
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 t =3.08e5y
1/2
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
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)
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):
ln( 10 Be)  ln( 10 Be0 ) 
d
( )
S
But you can get better ages if you combine cosmogenic nuclides for sed rate determination:
why?
36Cl
in Hydrological Applications
In a simple world, 36Cl
falls to ground, gets drawn
into aquifer, and you can source
date the water by tracking
its decay:
Cl  36Cl0 e  t
36
destination
But what happens
if you have evaporation?
or bedrock dissolution?
Solution: measure stable
chlorine isotopes; track
impact of processes using
mass balance
Paul et al., 1986
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
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
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)
Ex: Exposure ages of glacial morraines
Glacial morainesmeasure grow-in of 36Cl (t < steady state)
Bloody Canyon terminal moraine, CA
Schaefer et al., 2006
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