EOS 335 Flashcards
John Dalton
all matter consists of atoms
1806
William Prout
Law of Constant Proportions
1815
Law of Constant Proportions
atomic weights are integral multiples of the mass of hydrogen
why the Law of Constant Proportions does not always hold
elements with isotopes do not have integer weights
N
number of neutrons
Z
number of protons
atomic number
A
mass number
N + Z
M
atomic mass
I
neutron excess number
N - Z
3H excess = 1
Isotope
same number of protons different number of neutrons
Isobars
same mass number
N + Z
Isotone
same # neutrons, different # protons
amu
atomic mass unit
dalton
defined by 12C = 12 amu
931.5 MeV of energy
types of nucleosynthesis
Big Bang nucleo.
Stellar nucleo.
Explosive nucleo.
Cosmic ray spallation
Big Bang nucleosynthesis
primordial nucleons formed from quark-gluon plasma
first few 100µs
once cooled
length of nucleosynthesis
about 17minutes
after that T and density of universe too low for fusion
Nuclear synthesis of C, O, etc.
in stars (Stellar nucleo.?) by nuclear fusion or nuclear fission up to Fe
formation of elements heavier than Fe
neutron capture (s-, r-processes) fusion of Fe w/ other elements must absorb E rather than release it
simplest atom
proteon
1H
stable
most abundant isotope
hydrogen isotopes
1H - proteon, stable, most common (Z = 1, N = 0)
2H - deuterium, stable (Z=1, N=1)
3H - tritium, unstable, (Z=1, N=2)
atom diameter
about ca. 10^-8 cm (1 Å)
size of nuclei of atom
ca. 10^-12 cm (10^-4 Å)
e-
electron number - # of electrons in atom
neutral atom
Z = e-
Binding energy vs. mass number
binding energy per nucleon (MeV) as a fn of Mass Number, A
Increases straight up, curves to the right = fusion = formation of elements up to (including) Fe
after Fe line is mostly straight across, goes down a bit after A = 110 = Fission
fusion
2+ atomic nuclei form 1+ different atomic nuclei and subatomic particles (neutrons and/or protons)
Difference in mass between products and reactants = release of large amounts of energy
fission
large nuclei breaks apart into two smaller nuclei, releasing a great deal of energy
most tightly bound nucleus
Fe - 8.8 MeV per nucleon binding energy
Isotope notation
^A X
e.g. superscript 13 C = mass number 13, 7 N, 6Z
nuclide
a distinct kind of atom or nucleus characterized by a specific number of protons and neutrons
nuclear isomer
same nuclide, different energy state
types of isotopes
radioactive
stable
radiogenic
radioactive isotopes
spontaneously and predictably change atomic mass
stable isotopes
do not undergo any decay
radiogenic isotopes
may be radioactive or stable
a nuclide that is produced by a process of radioactive decay
Neutron number vs proton number for stable nuclides
follows 1:1 only up to ca. Z = 20-30
N increase more rapidly than Z
need more N for heavier elements to be stable
Nuclear force
holds nucleus together
more powerful than electromagnetic force
only over VERY short distance
radius of nucleon
ca. 10^-13 cm
radius of 4He nucleus
ca. 2*10^-13 cm
radius of 4He atom
ca. 10^-8 cm
Nucleon radius vs force
small radius = repulsion
medium radius = attraction
large radius = 0
Segré chart
protons vs # neutrons
stability in the middle - darker
decreasing stability in both directions out from the dark middle
top right is completely unstable and many undiscovered
Known nuclides
ca. 3000
ca. 275 stable, 270 in nature
ca. 70 unstable (radioactive)
nuclei are stable on what timescale
> 10^15 yrs to
Proton/Neutron energy levels
have energy levels like electrons
Z:N stability depends on energy levels
even numbers most stable
unstable configurations
eventually decay to more stable ones
alpha or beta decay and other processes
magic numbers of protons/neutrons
2, 8, 20, 28, 50, 82, 126
what are magic numbers
magic # of nucleons = higher average binding energy per nucleon
more stable against decay
analogous to filled shells of electrons (e.g. noble gas)
why are there magic numbers
thought that nuclei do not homogenize, stick their component groups like friends in a class
valley of stability
atoms at edge of parabola most unstable (dripline)
centre of parabola stable, atoms w/ highest nuclear binding energy
cross-section across valley of stability
parabola of binding energies
like x-section (isobar) across Segré chart - low stability, high, low
why are atoms on the dripline most unstable
large amounts of energy are released by their decay
β decay
how are isotopes useful in geoscience
relative Pb proportions tell Earths age
U-Th-Pb measurements used to determine age of crystals
how Lb is used to determine age of Earth
relative proportions of Pb isotopes in meteorites used as proxy
age inferred from Earths bulk Pb isotope composition
determining the age of something
paleotemperature from ice cores
tracers of present processes
Age of Earth
ca. 4.56 bya
t = 0 for Earth
when it ‘coalesced’
somewhat arbitrary
using isotopes to tell the age of the ocean
measure 14C ratios
14C comes from atmosphere (fossil fuels)
tells how long since water was at surface
Isotopes as tracers of present processes, migration
you are what you eat
can track butterfly migration from rainwater source
nucleon
proton or neutron in nucleus
atomic mass
number of nucleons
fermi
10 ^ -15 m
strong force
extremely short-range force between nucleons
alpha particle
helium nucleus, commonly emitted in radioactive disintigration
beta particle
electron, emitted in some radioactive disintegrations
gamma ray
a high-energy photon
electromagnetic radiation
extremely harmful to living organisms
geiger counter
device for measuring radioactivity
scintillation counter
device for measuring radioactivity
MRI
magnetic resonance imaging
based on energy levels of H nucleus
N(t)
population at time t
A(t)
activity
number of disintegrations per second
becquerel
one disintegration per second
curie
another unit of activity
number of disintegrations/s/g radium
protons expel each other by
coulombic (electrostatic) force
how do we know nuclear force/strong force has to only act at very short distances
otherwise all matter would collapse into a single nucleus
nuclear force is mediate by
the pion
Pion
type of meson
can exist for a short amount of time
meson
intermediate mass particles which are made up of a quark-antiquark pair
mass decrement of an atom
δm = W - M W = sum of mass of constituent particle (e.g. 6 protons + 6 neutrons + 6 electrons) M = actual mass of atom
binding energy
the mass converted to energy binding the nucleons
measure of nuclear stability
E=δmc^2
magic number features
isotopes and isotopes w/ m.n. are unusually common
m.n. nuclides unusually abundant in nature
heaviest stable nuclides
N=126
Z =83
number of stable nuclei for odd and even Z and N
Z-N # odd-odd 4 odd-even 50 even-odd 55 even-even 165
10^-11 seconds after the Big Bang
universe expanded and cooled enough for quarks and anti-quarks to condense from energy
10^-4 seconds after the Big Bang
cool enough for quarks to associate with each other and form nucleons
10^-2 seconds after the Big Bang
universe cooled to 10^11
neutrinos combined with neutrons to form electrons and protons
s-process
slow neutron capture
neutrons captured slowly (ca. 1000yrs) to produce successively heavier elements, in late generation stars
r-process
rapid neutron capture
tends to form the heavier isotopes
Antoine-Henri Becquerel
discovered radioactivity
placed U salts on photographic plates
produced image by beta particles
Marie and Pierre Curie
discovered polonium and radium by chemical separation from ores
Ernest Rutherford
discovered alpha and beta particles
showed that radioactivity involved transformation of an element in to an entirely different one
J.J. Thomson
discovered the electron
invented the first mass spec. – gave clear evidence of two isotopes of Ne
lowest # element that has natural decay
52
Te
Tellurium
Protons and neutrons are composed of
3 quarks
Proton - 2 up quarks, 1 down quark
Neutron - 1 up quark, 2 down quarks
Stable isotope distribution
relatively mixed in top 5 rows of periodic table
83-118 have no stable configurations (Period 7 and Actinide series)
Proton number vs neutron number, decay
stable region in middle
proton > neutron = beta + decay
proton
Isotope half-life distribution
> 10^15s in the middle
half-life decreases out in each direction from middle
Primary modes of radioactive decay
Alpha decay
Beta decay - positron decay, negatron decay, electron capture
Other forms of decay that we will not worry about
gamma decay
proton decay
cluster decay
Alpha decay is predominantly
in higher atomic number elements
also in Li, Be
Nuclide chart
normal plot of Z vs N
shows radioactive decay processes
nuclide has coordinates Z, N
decay will change coordinates
Alph decay
spontaneous emission of alpha particle from nucleon
occurs for nuclides with atomic number > 58 and 5He, 5Li, 6B
what is happening during alpha decay
a He nucleus is emitted (2protons, 2 neutrons)
no electrons expelled
change in mass
change in E = heat
change in parent from alpha decay
Z - 2
N - 2
A - 4
daughter product
mass difference between 2Z+2N and 1 alpha particle
equivalent to energy lost in alpha decay:
kinetic energy of alpha particle
kinetic energy of remaining nucleus - conservation of momentum and nucleus recoil)
gamma ray emitted
standard model (element formation)
quark - meson (2quarks), baryon (3q) (both hadrons)
baryon – protons, neutrons
meson – pion
lepton - electron, muon, tau, neutrinos
In nuclide chart what is the direction of change associated with alpha decay
left two, down two
Z - 2, N - 2
why does alpha decay only occur at high atomic numbers
nuclei must have masses above maximum in binding energy curve (56Fe)
proton mass
1.00728 u
neutron mass
1.00866 u
mass of alpha particle
4.00153
(but 2protons + 2neutrons = 4.03188)
mass difference is converted energy
238U alpha decay
238,92U – 234,90Th + 4,2He + Q
A - 4
Z - 2
alpha decay branched reactions
may not go to lowest energy state right away
intermediate levels are unstable
may evolve gamma emission
depends on where the alpha particle is coming from in the nucleus
beta decay
changes charge of nucleus
does not change # of nucleons
daughter product is an isobar
emission of electron or positron
types of beta decay
negatron decay
positron decay
electron capture
beta decay stability valley
stable nuclei exist in energy valley
α-decay moves nucleus down valley axis
β-decay moves nucleus down walls toward valley axis, depends on which side of the valley the parent lies (Z>N on left)
Negatron decay
β- decay
tranform neutron into proton + electron (N – P + e-)
emission of β- from nucleon, antineutrino, energy, γ-ray
results of β- decay
Z + 1
N - 1
A = A
movement on nuclide chart from negatron decay
up 1, left 1
40K negatron decay
40,19K – 40,20Ca + β- + v^ + Q
Z + 1
A = A
Negatron decay daughter
gains one proton, Z + 1
same atomic number as parent, isobaric
Positron decay
β+ decay
transformation of proton into neutron
emission of +charged electron (positron, β+) from nucleon, neutrinos, radiant energy, gamma rays
results of β+ decay
Z - 1
N + 1
A = A
40K β+ decay
40,19K – 40,18Ar + β+ + v + Q
movement on nuclide chart from positron decay
right 1, down 1
Isobaric
Electron capture
capture of extranuclear electron (e.g. K-shell capture)
electron reacts with a proton, forms neutron + neutrino
isobaric decay
results of electron capture
Z - 1
N + 1
A = A
same as β+ decay
how electron capture is fundamentally different than β+ decay
neutralize a charge rather than throwing it out
why is there lots of harmful rays associated with electron capture
excited state = gamma rays
replacement of lost electron = x-rays
125I electron capture
125,53I + e- – 125,52Te + v + Q
Z - 1
A = A
movement on nuclide chart from electron capture
down 1 right 1 Z - 1 A = A same as β+ decay
Adjacent isobars
can not be stable (up/down 1, over 1)
atomic number difference must be > 1
two isobars have
different masses
different binding energies
one isobar is converted in to another by
β decay
stable isobars
must be separate by a radioactive isobar
branching decay
decay of isotope by different methods to 2 or more different daughters
e.g. 40K – 40Ar by β+ decay, or to 40Ca by β- decay
ratio of decay directions is fixed
daughters are diagonal in each direction away from parent on nuclide chart
branched decay equation
40,19K – 40,18Ar (+ β+ + e-) + 40,20Ca (+ β-) + Q
atomic numbers of daughters for the 4 decay processes
alpha = Z - 2
EC = Z - 1
beta + = Z - 1
beta - = Z + 1
U decay series
3 clocks - 238U, 235U, 232Th
U (or Th) is rate determining step
multiple alpha and beta decay steps to reach stability
paths of the diff. clocks do not overlap
238U decays to
206Pb
8 alpha decays (length of chain)
6 beta decays
235U decays to
207Pb
7 alpha decays (chain length)
4 beta decays
232Th decays to
206Pb
6 alpha decays (chain length)
4 beta decays
decay end member features
stable
high enough in atomic number to be able to avoid contamination
semimetals
B, Si, Ge, As, Sb, Te
non-metals
noble gases
halogens
C, N, O, P, S, Se
halogens
group 17
beside noble gases
F, Cl, Br, I, At
noble gases
period 18
He, Ne, Ar, Kr, Xe, Rn
mass spectrometer
separates atoms or molecules according to mass
basic parts: ion source, mass analyzer, detector
application of radioactive isotopes
geochronology tracers medical imaging - trace, treatment energy weapons
radioactive isotopes, tracers
agriculture - plant fertilizers
industry - engine parts
geo-processes and characters
elements that make good radioactive models
not extremely short or long 1/2t
common enough for use
well represented in typical rock groups, abundant
Law of radioactivity, Rutherford and Soddy
rate of decay of radioactive nuclides is proportional to # of that nuclide remaining at any time (t)
lots of parent = faster decay
Basic Decay Equation (exponential decay)
-dN/dt ∝ N_t
λ is proportionality constant so -dN/dt = λN_t
variables in basic decay equation
N = number of nuclides that will decay (parent) -dN/dt = rate of decay λ = decay constant (time^-1) λN = activity (A) or 'rate of decay'
λ
decay constant
probability a given constant will decay at time t
typically independent of T, P
experimentally determined, accepted by consensus, not empirical
integrating basic decay equation
-dN/dt = λN
-dN/N = λdt
-∫dN/N = λ∫dt
-lnN = λt + C
at t_0 N = N_0 – C = -lnN_0
-lnN = λt - lnN_0
N_0
N at t=0
all parent is still present
no decay has taken place yet
Standard/Basic decay equation, working form
N = N_0 e^-λt
half life
time required for half of radio-nuclides to decay t = T_1/2, N = No/2 No/2 = No e^-λT_1/2 1/2 = e^-λT_1/2 -ln(2) = -λT_1/2 T_1/2 = ln2 / λ
rate of decay is proportional to
N (amount of parent nuclides)
larger N = more decay
rate decreases exponentially
half life working equation
T1/2 = ln2/λ
max number of 1/2 lives
5-10
>5 half lives and theres likely not enough parent left for data analysis
low N
lower activity
lower decay
lower quality data - less accuracy
14C T1/2
5730 yrs
exhausted in ca. 70kyr
40K T1/2
1.28Gyr
good for dating 10kyr - 100’s Myr
238U T1/2
4.47Gyr
used to date Myr to Gyr
growth of stable daughter
D* = No - N D* = No - No e^-λt D* = No (1 - e^-λt) D* = how much daughter produced with none present initially
example of when D* is an alright assumption
K-Ar dating, Ar is a gas and therefore escapes before rock solidifies
Growth curve of daughter
number of atoms vs. time, half lives
exponentially increasing
opposite to decay curve, exponentially decreasing
Geochronology equation
D* = N(e^λt - 1) D = Do + N(e^λt - 1)
If there is daughter material to start with that is not accounted for
rock will ‘appear’ older than it is
Graphing D = Do + N (e^λt -1 )
87Sr = 87Sro + 87Rb (e^λt - 1)
y (D) = c (Do) + mx (e^λt - 1)(N)
87Sr is y axis, 87Rb is x, (e^λt - 1) is slope, 87Sro is intercept
If 87Sro = D* = no daughter to begin with, then intercept is at 0
If 87Sro≠D* then there was daughter to start with
How to graphing D = Do + N (e^λt -1 )
take multiple samples in same outcrop
rock heterogeneity will give different values
plot all the values (parent, daughter)
if rocks are same age, should plot along straight line and have same to
In graphing D = Do + N (e^λt -1 ), the slope (e^λt -1 ) is what
an isochron
it means all rocks that plot along that line have the same age
overcoming mass spec measurement constraints
difficult to measure amounts discretely so isotope measurements are generally made as ratios (R)
use stable, nonradiogenic isotope for normalization
87Sr normalization for mass spectrometry
86Sr is the stable isotope = normalizer
R = Ro + R_P/D (e^λt - 1)
87Sr/86Sr = 87Sro/86Sro + 87Rb/86Sr (e^λt - 1)
just divide each term by the normalizer
normalizer must be common and not in the decay system
86Sro should technically = 86Sr (Stable)
R P/D plot
R = Ro + R_P/D (e^λt - 1) y = c + xm y is the 'now' ratio x is the Parent/Daughter (e^λt - 1) is the slope = isochron c is the intercept
Assumptions for R vs. P/D plots
isotopic equilibrium of system at t=0, i.e. homogeneous value of Ro, generally thermal/diffusional constraint (blocking T)
closed system, i.e. no loss or gain of material (parent or daughter) with time
Rb-Sr commonly used to date
Rb-rich minerals: muscovite, biotite, k-feldspar
these minerals do not incorporate much Sr at time of formation
Rb-Sr dating compared with K-Ar
Rb-Sr has a greater blocking T - usually gives somewhat older age thank K-Ar (minerals formed slightly later)
minerals form at cooler T according to Bowens reaction series
Using isochron method for Rb-Sr
plot multiple rock samples 87Sr/86Sr vs 87Rb/86Sr at to
should be a horizontal line
wait… plot values again at t1
wait.. plot again at t2
should plot co-linearly to the same intercept (if same age)
In isochron dating methods why does the isochron become more steep with time?
increasing across x-axis we have rock A, B, C
rock A has the lowest N (initial amount of parent) so its Activity will be lowest
rock C has highest N which equals larger A
more atoms = more decay
the daughter product from rock C is increasing faster than the daughter product from rock A
blocking T
temperature below which a mineral becomes a closed chemical system for a specific radioactive decay series
what do we get from the isochron dating method
the age of the rock (from the slope)
the initial value, Ro (from the intercept)
what does it mean for rocks to be co-genetic
derived from same parent material
how do we know if rocks are co-genetic
same, single initial isotope ratio, Ro
Assumptions in radiometric dating
N, D have changed only as a result of radioactive decay (closed system)
there was an isotopic equilibrium within the system at the outset (homogenous 87Sr/86Sr)
Parent isotope composition not altered by fractionation at time of formation of rock
decay constant is known accurately
the isochron is not a mixing line
the analytical data are accurate
disintegration rate
A ≡ λ N also activity can be measured with scintillation counter A = Ao e^-λt ln A = ln Ao - λt y = c +mx
Problems with isochrons
two sets of rocks with same age may have different Ro
difficult to fit lines of best fit
metamorphic events after formation of rocks
Meaningful isochrons require
large P/D (e.g. 87Rb/87Sr of 5) large range in P/D in suite or minerals (i.e., Ca + K minerals) closed system homogenous D no fluids and/or metamorphic resetting
dN/dt depends on
number of nuclides available to decay
λ is fixed
-dN/dt = λN_t
why K-Ca-Ar work focus on Ar
40K/40Ar ratio much higher than 40K/40Ca
Easier to measure differences
Why is 40K/40Ca ratio so low (0.00011)
40K least abundant K isotope (0.012%)
40Ca most abundance Ca isotope (96.92%)
Ca is more abundant than K
40K has one of shortest t1/2 of long-lived radio isotopes
∴ ratio is small, signal is hard to detect
Potassium
K, Z = 19
alkali metal, group 1A (w/ Li, Na, Rb, Ce)
1 of 8 most abundant crust elements
key component in rock forming minerals
K isotopes
39K - stable, 93.3%
40K - radiogenic, 0.012%
41K - stable, 6.73%
Argon
Ar, Z = 18
noble gas, group 8 (w/ He, Xe)
mostly gaseous in atmos.
Ar isotopes
40Ar - 99.6% (radiogenic)
38Ar - 0.063%
36Ar - 0.337%
all stable
Ar tracer
36Ar - not formed from decay
40Ar/36Ar = 295.5 - any deviation = radioactivity
K branching decay
40K –> 40Ar + ß+ (positron, e.c.)
40K –> 40Ca + ß- (negatron)
40K –> 40Ar
electron capture + gamma rays (11%, 1.46MeV)
e.c. directly to ground state (0.16%)
positron + 2gamma (0.001%)
total ∆E=1.51MeV
40K –> 40Ca
negatron emission (∆E = 1.32MeV, 88.8%)
K-Ar-Ca λ
λ_T = λec (Ar) + λ_ß (Ca)
fraction of 40K that goes to 40Ca, 40Ar
40Ca = λ_ß / λ_T*40K 40Ar = λ_ec / λ_T*40K
branching ratio
R = λ_ec / λ_ß = 0.0117
K-Ar-Ca equations w/ no initial daughter
40Ar* + 40Ca* = 40K(e^λ_t - 1)
40Ar* = (λ_ec/λ)40K(e^λ_t - 1)
40Ca* = (λ_ß/λ)40K(e^λ_t - 1)
40Ar_0
often assume Ar_initial = 0 b/c gas escapes to atmosphere
K-Ar benefits
Ar is noble gas - escapes, volatile, not bound in lattice
can measure Ar-Ar
Measuring 40K/40Ar
40Ar - melt rock - measure gas composition w/ MS
40K -sample content measured by flame photometry, atomic absorption, ICPMS
key principle behind K-Ar dating
40K/40Ar ratio related to t since rock was cool enough to trap Ar
Rock-forming minerals suitable for dating by K-Ar
feldspars, micas, amphibole (hornblende)
K-Ar assumptions
40K decays independent of P, T
40K/K_T constant in nature
40Ar* produced by in situ 40K decay since crystallization
corrections can be made for nonradiogenic 40Ar
sample in closed system since t_o - i.e. no losses or gains
violation example of 40Ar* produced by in situ 40K decay since crystallization assumption
partial melting
corrections for nonradiogenic 40Ar
36Ar from atmos. not decay - amount diffused in is proportional to 40Ar that diffused in (contamination)
40Ar/36Ar = 295.5
K-Ar additional assumptions
1 no 40Ar* has escaped
2 mineral closed quickly to 40Ar* after formation
3 no 40Ar_initial or 40Ar incorporated later
4 correction for atmos. 40Ar* leak into mineral
5 normal 39K, 40K, 41K abundances, no fractionation during formation
6 λ, λ_ec, λ_ß accurately known
7 [40K], 40Ar* accurately determined
40Ar* escape
remelting solution/precipitation alteration mechanical weathering metamorphism burial P/T
40Ar* is measured how
on mass spec by isotope dilution - enriched with 38Ar so there is enough material to measure
40Ar/38Ar
38Ar/36Ar (to correct for atmos. contamination)
Correction for Ar contamination
40Ar* = 40Ar_t - (295.5)(36Ar)
note that if there is no contamination 40Ar* = 40Ar_t
Ar example, why you need large # samples
Radiogenic 40Ar content vs. distance inward from pillow rim
decrease in 40Ar inward from rim - 40Ar contamination added to pillow rim
40Ar/36Ar vs 40K/36Ar R-P/D plot, y-intercept
y-int = (40Ar/36Ar)o = initial ratio = atmospheric contamination
Ar diffusion
K-Ar thermally reset if T high enough to allow Ar diffusion
Ar diffusion dependent on
diffusion coefficient (D) - material dependent temperature (T) E_a = activation energy Arrhenius equation: D = Do * e^(-E_a / RT)
Ar diffusion consideration
closure T
cooling rate, closure rate
dependence on mineral used for dating
Diffusion of Ar, Temperature
higher T = faster diffusion (good)
in melted rock - 36Ar should escape
in cool rock Ar should stay put
Ar Blocking temperature
T wt which mineral becomes ‘closed’ w.r.t. Ar loss
Problem w/ Ar blocking T
date obtained will be less than true age unless rock cooled very rapidly
blocking T dependent on
diffusion
cooling rate
grain size
grain shape
87Sr normalized by
86Sr
87Sr parent
87Rb
40Ar normalized by
36Ar
radiogenic argon
40Ar
accumulated from decay of 40K
also 39Ar but very short t_1/2
non-radiogenic argon
blank, trapped, cosmogenic, neutron induced Ar
not from decay
Relic Ar
40Ar
remains following partial resetting (partial melting) event
Blank argon
unavoidable surgical Ar introduced into MS
excess Ar
all contamination
trapped Ar
incorporated within mineral
atmospheric Ar w/ or w/o an excess 40Ar component (e.g. H2O)
Excess Ar
released from older K-bearing minerals
typically during heating event - trapped as mineral cools
Atmospheric argon
Ar from EARTHs atmosphere (different extraterrestrially)
40Ar/36Ar = 295.5
Neutron-induced Ar
produced by irradiation of sample in nuclear reactor
mostly synthetic
Inherited argon
radiogenic + non-radiogenic Ar introduced by contamination w/ older material (e.g. inclusions)
Argon gains
- Inherited, 40Ar/36Ar greater than 295.5, overestimate of age
- 40Ar doesn’t escape during thermal event - redistributed - disproportionately situated on crystal edges - amongst first to diffuse out during weathering
Argon Losses
- Extended cooling period - enhanced differences in edges
- Differential diffusional loss during reheating
- In 39K(n,p)39Ar, 39Ar lost from crystal rim during recoil following radiation
underestimation of true age
Trouble with K-Ar
chemical differences
measured differently
avoiding K-Ar problems
Use 39Ar as 39K proxy by irradiating sample - turn K into Ar
39K(n,p)39Ar
benefit to using 39Ar as proxy for 39K
measured in same machine at same time
can also measure 36Ar at same time to correct for Ar*
Argon step heating
In situ conversion of 39K - 39Ar
incremental heating of sample over ‘total fusion’
allows liberation of Ar in stages
melt sample from outside-in
benefit of step-wise heating
permits age determination and identification of domains
sample domains
anomalous zones/region
e.g. outside edge of sample may be different than inside
if sample closed throughout history no discrete domains, age same at each increment
step-heating spectrum corrected for
non-radiogenic 40Ar (at every step) using atmospheric ratio from 36Ar measured
result of step heating correction
excess 40Ar not detected
all 40Ar not corrected for is assumed to be from decay
Ar release spectra of samples taken at varying distances from intrusion
intrusion = heating = Ar loss
samples closest to intrusion show lowest age corresponding with Ar loss
If no plateau is reached in spectral image than data not trustworthy
Schematic 40Ar-39Ar age spectra representations
undisturbed - flat line (1 age)
slight disturbance - initial step is lower, increases to plateau
disturbed - no plateau over spectra
reset - plateau in first stages then increase
saddle-shape - presence of excess 40Ar
Ar plots
heating plots/heating release spectra
Isochron plot
inverse isochron plot
Ar inverse isochron plot
36Ar/40Ar vs 39Ar/40Ar
like isochron plot, does not assume non-radiogenic 40Ar/36Ar ratio, can be useful for recognizing excess 40Ar in addition to atmos. Ar
higher precision than isochron plot
more commonly used (than isochron plot)
measured value in stepwise heating
(40Ar/39Ar)m = 40Ar* + 40Ar_c / 36Ar
radiogenic (40Ar*)
contamination (40Ar_c) - atmospheric and Ar entering system since closure
Why inverse isochron has lower error
[39Ar] is lower than [40Ar] – any measurement inaccuracy in 39Ar/40Ar will produce large error
‘pure’ radiogenic component of Ar
where 36Ar/40Ar = 0
36Ar is not formed radiogenically
extrapolating on inverse isochron plot
where 39Ar/40Ar = 0 gives trapped, non-radiogenic component
39Ar ∝ 39K and 40K
corresponds to 40K/40Ar = 0
Data in inverse isochron plot represent
diff T steps from one mineral (if step-heating)
or diff. spots w/i one mineral (if laser-ablation)
inverse isochron plot, measurements w/ high radiogenic component
will plot close to 39Ar/40Ar axis
Measurements of Earths age
Biblical - 6016yrs Lord Kelvin, heat flow, 1862 - 20-400Ma John Joly, Ocean Na, 1899, 80-100Ma Rutherford (radioactive decay), 1913 - >400Ma Hubble, 1929, 2Ga Patterson (Pb isotopes), 1953, 4.55Ga
How John Joly calculated age of Earth
how much weathering must have had to occur for the Na in the ocean
didn’t account for evaporites
µ
238U/204Pb
238U/235U
137.88
κ
232Th/238U
238 decay
238U – 206Pb + 8(4He) + 6ß- + Q
∆E = 47.4 MeV
235 decay
235U –> 207Pb + 7(4He) + 4ß- + Q
∆E = 45.2 MeV
ω
232Th/204Pb
232 decay
232Th –> 208Pb + 6(4He) + 4ß- + Q
∆E = 39.8MeV
Early methods for U, Th decay
chemical method - assume all Pb in mineral/rock is radiogenic (often invalid)
Pb-alpha method - optical spectroscopy to measure Pb, alpha-counting to measure U, Th
U-He method - assumes U, Th minerals retain He released by decay (not always valid)
Current U, Th methods
U, Th-Pb isotopic method or concordia
Common-lead (Pb-Pb) method
Uranium
high incompatible - large ionic radii, large charge
mobile among last species to crystallize out
concentrates in crustal rocks
thorium
high incompatible element - large ionic radii, large charge
relatively immobile
U, Th minerals
Zircon (more U)
Monazite (more Th)
Apatite
zircons
abundant
chemically resistant
incorporate U, Th
Pb
incompatible
immobile except at low pH and high T
Pb minerals
Galena
substitutes for K in feldspar, biotite
sulphides (come out of hydrothermal vents)
U, Th level of incompatibility
U more than Th more than Pb
homogeneous loss
238U/235U remains constant, 238U is lost in the same ratio as 235U
this relationship does not hold for U or Pb gains
U, Th rate determining step
parent is rate determining - by far longest 1/2 life
matching parents with daughters, U, Th, Pb
238U 235U 232Th
206Pb 207Pb 208Pb
even-even odd-odd smallest-largest
Secular equilibium
T_1/2 parent much greater than T_1/2 daughter
e.g. 10-1000X
equilibrium
quantity of radioactive daughter isotope remains constant b/c decay rate = production rate from parent decay
transient equilibrium
T_1/2 parent greater than T_1/2 daughter
e.g. few X’s
main difference btw transient, secular equilibrium
10 half-lives of daughter:
Secular - essentially no parent decay takes place
Transient - significant parent decay takes place
secular equilibrium characteristics
no initial (or fixed initial) radioactive daughter (rd) material
if no initial then no initial rd decay
rd forms at ca. constant rate from parent
no substantial loss of parent over time
decay of rd once material is accumulated
increased rd material = increased activity
activity increases until it reach rate of formation
after ^ amount remains constant over time (equilibrium)
why does increased material = increased decay
A Ξ λN
activity is proportional to amount
if you have more you can lose more, probability of any one particle decaying increases
time to reach specific A (activity)
dependent on λ
after n half lives, activity will be
f - some faction of rate of formation or saturation activity
f = 1 - (1/2) ^n
Secular equilibrium rule of thumb
Secular equilibrium is typically reached by 4-5 half lives of the daughter
real-life example of secular equilibrium
ca. half of U created at creation of universe is left
only Ra that exists today is a result of U decay
Transient equilibrium characteristics
parent will undergo significant decay
A_d will increase and establish eq. w/ parent activity
A_d ≠ A_p; A_d = A_p * T_p / (T_p - T_d)
as T_d approaches T_p, A_d > A_p - transition eq.
at transition- A_d, A_p change w/ time
once transition equil. attained daughter decays according to T_p
T_p
T_1/2 parent
T_d = T_1/2 daughter
If T_p»_space; T_d
A_d = A_p
secular
Transition equilibrium
typically reached quicker than secular equilibrium
Initial lead values on earth
known from meteorites
206Pb/204Pb = 17.21
207Pb/204Pb = 15.78
208Pb/204Pb = 37.43
238U half life
4.468x10^9 yrs
4,468,000,000 yrs
4.468Ga
235U half life
0.7038x10^9 yrs
703,800,000 yrs
703Ma
232Th half life
14.010x10^9 yrs
14,010,000,000yrs
14Ga
elements in U, Th decay series
3 series
12 elements
43 isotopes
no overlap
changes in 207Pb, 206Pb with time
as rock ages, 207 grows faster
207/206 vs time = increasing hyperbola
235U decays faster
concordant
if ages obtained from independent U, Th Pb chronometers are the same
(usually do not agree- discordant)
Dicordancy often a result of
Pb loss
intermediate daughter loss
caused by radiation damage from alpha decay
common lead method
206-207 method
Pb-Pb method
combine 2 U-Pb geochronometers
lessen effects of open-system
Pb-Pb method characteristics
does no assume Pb_0 is insignificant
often yields older date than individual system - ratio of Pb isotopes not as sensitive to Pb loss
why is Pb-Pb ratio not as sensitive to Pb loss
losses tend to be isotopically homogeneous
238U/235U
137.88
Pb-Pb system
207Pb/206Pb = (e^λ_235t - 1) / 137.88(e^λ_238t - 1)
too complex to solve- L’Hopital
use Wetherill table
what we can see from Pb-Pb equation
only dependent on age
not on P/D b/c that is a fixed ratio
Why Pb-Pb type system is not often used with Th
U, Th different elements - different behaviours, processes
unequal gain/loss
Pb-Pb system value
∆-relationship does not assume negligible/small starting Pb
not affected by recent alteration (Pb-loss or U-loss)
only Pb addition or aging after alteration affects measured age
Why is Pb-Pb method not affected by Pb-loss or U-loss
depends only on ratios not []’s
232Th-238U method
similar equation to Pb-Pb method
different elements, behave differently - unequal loss/gain
works if kappa is constant (not given)
Where does U come from
supernova (collapsing star) - high E - accretion on to planet
238U half life
4.468Ga
Pb-Pb equation
207Pb/206Pb = (e^λ_235t -1)/137.88 (e^λ_238t -1)
Where did Pb come from
R&S processes
decay
super-enriched in crust due to decay
Where is U on Earth
enriched in crust relative to mantle
incompatible - stays in melt longer - doesn’t want to crystallize
Closure temperature
blocking T
pt where system becomes ‘closed’
A system is open if
T is is high enough for atoms to diffuse in/out of crystals
Blocking T depends on
compatibility duration of heating grain size pore fluids mineral chemistry
Example blocking T’s
U-Pb, zircon- >900ºC U-Pb, titanite- >650ºC Sm-Nd, garnet- 600ºC U-Pb, apatite- 500º K-Ar, hornblende- 500º Rb-Sr, feldspar, biotite- 500º
If T_1/2 of daughter is greater than T_1/2 of parent
equilibrium will never be reached
238U/235U
137.88
Homogenous loss
isotopes lost in the same proportions as in the rock before losses, e.g. Pb
If closed system, 207Pb/204Pb vs 206Pb/204Pb
= straight line
207Pb/204Pb vs 206Pb/204Pb shape of growth curve
distance point moves along the line
238U/204Pb (µ)
mineral suitable for U-Th-Pb methods
Zircon
what is needed in a mineral for U-Th-Pb methods
retain radiogenic Pb
be common
why zircon?
retain radiogenic Pb- resistant to mechanical/chemical weathering, metamorphism, remains closed system, robust
be common - found in igneous, metamorphic, sedimentary
Zircons in sedimentary
do not tell age of sedimentary deposit! they are inclusions
Kober-method
single zircon Pb evaporation
hit Zircon w/ high T - vaporize - 2step heating
why 2-step heating is necessary for zircon
rim most likely to lose Pb or have overgrowth (Ga weathering)
core most likely to preserve Pb isotope signature
Zircon formula
ZrSiO4
Zircon mineralogy
very hard
resistant to weathering
permits substitutions of U, Th for Zr
concentrates U/ Th, not Pb - high U/Pb
Concordia diagram
206Pb/238U vs 207Pb/235U
both ratios proportional to t
basically 238U-206Pb* age vs 235U-207Pb* age
Shape of Concordia curve
root curve
initially 207Pb/235U increases more rapidly, as 235U used up trend inverts 206Pb/238U increases more quickly
When to use Concordia
whenever you can- gold standard
rocks/minerals w/ extremely high 238U/204Pb
Why do 207Pb and 206Pb grow at different rates
differences in λ’s
low abundance of 235U
235U decays much faster (shorter T1/2) - 207 produced faster
concordant dates
238U-206Pb age = 235U-207Pb age
On Concordia diagram, the locus of all points yielding concordant dates is called
the concordia curve
ages are concordant
235U half-life
0.704Ga
Why is a Concordia curve not a straight line
207Pb ‘grows’ faster
Any point along Concordia curve
represents equal age
evolution of Concordia curve
doesn’t just grow along the x-axis like if you were drawing a curve
the curve actually moves up the y-axis
x-axis is not age
What if there is complete Pb loss? (Concordia)
then zircon is reset to time of loss
U-Pb dates can not distinguish a ‘reset’ zircon from a crystallized zircon
232Th half-life
14.01Ga
If partial Pb loss in Concordia
data plot on chord that connects true age of zircon w/ age Pb loss occurred - if sufficient data
Concordia ‘chord’
discordia line
Better if Pb loss is continual or episodic
episodic
continual causes greater difficulties, uncertainties
Pb gain, Concordia
less likely
more difficult - not homogenous
cannot be predicted - no specific age relationship
makes dates uncertain
U gain, Concrdia
similar to Pb loss - discordia line (beneath concordia)
Discordia line, intercepts
upper can represent age of formation of rock
lower may represent date of Pb loss if single stage, not continuous
Where would Pb gain plot
above Concordia
Metamict texture
fracturing in crystal
natural radiation damage
amorphous crystal
increases/allows Pb mobility
The location of a point of the Concordia depends on
ONLY age
Concordia =
simultaneous coevolution of 206Pb and 207Pb via 238U and 235U decay (respectively)
Rocks/minerals that do not plot on the Concordia
yield discordant dates
As rocks age (Concordia)
move along concordia (if no Pb, U mobility)
If different Zircon samples from same rock lost differing amounts of Pb during same episode
would all plot along discordia
decay rate is dependent on
# of atoms independent of physical characteristics (T,P)
activity is measured in
atoms / unit time = disintegration rate = total activity
N
number of particles
oldest known rock in Canada
4.28Ga
bedrock, eastern shore of Hudson Bay
Jonathan O’Neil, McGill University
Sm/Nd technique
2nd oldest known rock in Canada
4.03Ga
Gneiss, NWT
Oldest Earth material
Zircon
4.37-4.41Ga
Jack Hills, W. Australia
oldest cratons
ca. 4.3Ga
- materials older than cratons (e.g. zircons)
differentiation of Earth
layering (e.g. mantle, crust, core..)
importance of Earths differentiation
understanding ore deposits
Earth processes that shape and determine compositional nature of planet
metamorphism, erosion
destroy ancient features
systems may retain some information
Chemical differences between elements
valance state, ionic radius, etc.
lead to differentiation
Uranium chemical associations
lithophile actinide series incompatible concentrated in lithosphere can substitute for other lithophilic elements
Lead Goldschmidt classification
chalcophile
chalcophile
metals and heavier nonmetals that have low affinity for O and prefer to bond with S as highly insoluble sulfides
Rubidium chemical associations
alkali metal (Group 1A)
associates w/ Li, Na, K, Ce
substitutes for K
more incompatible than Sr
Strontium chemical associations
Alkali earth (Group IIA)
associates w/ Be, Mg, Ca, Ba, Ra
Substitutes for Ca (feldspars)
more compatible
Rb/Sr fractionation
by igneous processes
Rb stays in melt longer - enriches in lithosphere relative to Sr
U/Pb, Rb/Sr differences valuable
can explain some of Earth’s fractionation processes
Value in explaining Earth’s fractionation processes, Rb/Sr
differentiation/fractionation of alkali metals from alkali earths
Value in explaining Earth’s fractionation processes, U/Pb
fact./partitioning of compatible vs incompatible elements
Value in explaining Earth’s fractionation processes, Sm/Nd
describe events that lead to fractionation of REE
Granites
lithospheric, surface rocks essential constituent of continental crust mosaic of diff. ages accumulate, brake up, drift 5Ma - 4Ma
5Ma granites
Andes, Alps, Himalayas
Strontium granite ratios
87Sr/86Sr ranges 0.705 - 0.850
not homogenous
older rocks have higher ratio (decay)
Rubidium granite ratios
87Rb/87Sr range: 0.5 - 3, average 1
due to incompatibility of Rb
Basalts
essential constituent of oceanic crust
very young - oldest ca. 200Ma, average 80Ma
from high T mantle-melting
Basalt Sr ratios
87Sr/86Sr: 0.7020 - 0.7070
younger rocks
Basalt Rubidium ratios
87Rb/87Sr: 0.001
due to incompatibility of Rb
Oceanic Basalts
MORB
OIB
MORB
Mid-ocean-ridge basalts
upper mantle
arises from mid-ocean ridges - spreads - subjected - mantle
OIB
Ocean-island basalts
lower mantle
arises from subaerial volcanism - form island chains, archipelagos
upper mantle
depleted mantle material
lower mantle
primary, ‘bulk earth’ material
MORB isotope ratios
87Sr/86Sr - narrow range, ca. 0.7025
87Rb/87Sr - ca. 0.001
OIB isotope ratios
87Sr/86Sr - narrow range, ca. 0.7035 (higher than MORB)
Continental crust Sr ratio
87Sr/86Sr - 0.705 - 0.850 (granites)
Sm decay
147Sm decays to 143Nd alpha decay (A - 4) T_1/2 = 10^6 years - very long lived
Sm, Nd characteristics
7 naturally occurring isotopes only 147Sm impacts 143Nd 143Nd is stable both are intermediate REE similar chemical properties Nd enriched in lithosphere relative to Sm
Sm, Nd similar chemical properties
results of identical outer electron shell configuration similar but importantly different ionic radii Sm smaller (1.04Å vs. 1.08Å)
Sm/Nd ratios
chondritic - 0.32
present day - 0.1967
Nd enriched in lithosphere
more than Sm, opposite of Rb/Sr
during fractionation of magma, Sm/Nd
decreases
fractionation pathway of igneous rocks
ultramafic, deep mantle, high Nd - mafic - intermediate - felsic, continental, crustal, lithospheric, low ratio, low Sm high T (first to crystallize) - - - low T (last to crystallize)
Early stage Sm/Nd crystallization
mafic igneous
high Sm:Nd -close to the 0.32 chondrite end-member
calc-alkaline - basalt, andesite, dacite, rhyolite
crustal rock Sm/Nd
lower than mantle rocks
REE plot
chondrite normalized abundance for REEs
MORB ca. straight line around 10
upper continental crust > than MORB (ca.100) for first 3 REEs - decrease - below MORB and straightens out
-the decrease points are the radiogenic elements, unique (Nd, Sm)
Sm decay equation
143Nd = 143Nd_o + 147Sm(e^λt - 1)
Why does Sm/Nd decrease in fractionation
Sm stays in mantle, more compatible
Sm/Nd normalized by
non-radiogenic 144Nd
Sm-Nd theory
Earth isotopically homogenous at outset - initial 143Nd/144Nd ca. to that in meteorites - deviations btw measured and expected evolution through time
CHUR
Chondritic Uniform Reservoir
chondritic (stony) meteorites (especially carbonaceous)
thought to represent earliest material formed in solar system before planets
CHUR used as
approximation of what Earths accretionary composition was 4.6Ga
Present CHUR 143Nd/144Nd
0.512638
= homogenous bulk earth
Present CHUR 147Sm/144Nd
0.1967
All meteorites have
same age
same 143Nd/144Nd = 0.512638
not same 87Sr/86Sr
refractory elements
behave coherently, consistently
Sm, Nd, Sr
volatile elements
low boiling points that are associated with a planet’s or moon’s crust and/or atmosphere
Rb
Using CHUR to determine Earth’s differentiation
when lines diverge from CHUR line
Partial melt - below CHUR - SM depletion
residual solids above CHUR line - Nd, SM enriched
143Nd/144Nd above CHUR line
depleted mantle (enriched, Sm more compatible then Nd)
143Nd/144Nd below CHUR line
continental crust
ε_ND
= [ (143Nd/144Nd)sample - (143Nd/144Nd)chur) / (143Nd/144Nd) ] x10,000
differences in 143Nd/144Nd are small
εND for CHUR
= 0
εND range
-20 to +14
(+) εND
high Sm/Nd
high 143Nd/144Nd
enriched relative to meteorite/bulk earth
Depaolo and Wasserburg
devised εND scale, 1976
easier to express and show differences
graph easier to interpret
Combining Nd, Sr isotopes
negatively correlated
rocks w/ large Sm/Nd variation = mafic, ultramafic, smallest variation = felsic rock
Rb/Sr = opposite
Nd, Sr incompatibilities
Rb more incomp. than Sr
Nd more incomp. than Sm
εND vs 87Sr/86Sr
Bulk earth plots along 0 εND
MORB, OIB above and to left of bulk earth (+)
CC below and right of b.e. (-)
Elemental differentiation, crust
melt
Sm/Nd less than 1, depleted in Sm, below b.e. line
Rb/Sr greater than 1, enriched in Rb, above b.e. line
( - ) εND
low Sm/Nd
low 143Nd/144Nd
depleted relative to meteorite/bulk earth
BABI
basaltic chondrite best initial
standard for ‘primitive’ mantle
Why doesn’t OIB match bulk Earth?
appears to have undergone mixing (subduction)
also not one specific value
Elemental differentiation, mantle
solid residue
Sm/Nd greater than 1, enriched in Sm
Rb/Sr less than 1, depleted in Rb
Sialic
relatively light rock
rich in silica, alumina
typical of outer layers of earth
Mechanisms for creating continental crust
- Accretion of oceanic crust
- Underplating of magmas
- Continental volcanics
- Subduction
accretion of oceanic crust
not common, most subducted
add depleted material - melt - granite
Underplating of magmas
depleted - remelting - granite
Continental volcanics
flood basalts
300Ma later still large flood basalt provinces
old provinces may be partitioned by dykes - metamorphosed, melted
subduction
accretion of arcs
most likely
now principal mechanism
NA age provinces from
reworking (metamorphism, melting) of crust
addition of new crust
Determining age provinces, use
Rb-Sr
new crust should have lower 87Sr/86Sr, younger
NA Archaen craton
stable over geologic time scale
deep rooted ca. 70km
rifted would release massive CO2
age provinces in NA
Archaen block in the middle, 2350-2700Ma (Hudsons bay) - less age as you move out in either direction
youngest on W coast (less than 440Ma)
evolution of continental crust with time
consensus - crust grown at steady rate through geological time - gradual growth
growth w/ accretion
2 component mixing, proportion present
f_a = A/ (A+B)
f_b = B/(A+B)
f_a + f_b = 1
2 component mixing, end members
if the endmembers are known, then f_a can be calculated for any mixture
Two component mixing hyperbola
Ratio-Element Plot 87Sr/86Sr vs. Sr, ppm two end members component A, component B Mixture 'M' somewhere in the middle with 2 pts can find the 3rd
two component mixing hyperbola, Sr_A / Sr_B less than 1
decreasing hyperbola
two component mixing hyperbola, Sr_A / Sr_B > 1
increasing hyperbola
two component mixing hyperbola, Sr_A / Sr_B = 1
straight line
ONLY if end members are equal
linearize it (2 component mixing)
take reciprocal of hyperbola to make it linear
87Sr/86Sr vs 1/Sr ppm^-1
binary mixing
2 components, 2 isotope ratios
two elements - Sr, Nd
end members - crust, mantle
basalt-granite binary mixing may be from
igneous rocks formed basalt magma assimilated granitic rocks or magma generated by melting of a mixture of granitic and basaltic source rocks
HIMU
high mantle uranium
high µ
high 206Pb
µ = 238U/204Pb
mantle array
graphical plot of 144Nd:143Nd against 87Sr:86Sr for igneous rocks. Rocks which have been derived from the mantle tend to plot on a straight line; those that show evidence of crustal contamination tend to fall off the line
mantle array represents
mixing
and evolution?
mantle array, primitive mantle
origin
ε_UR(Sr) = 0
ε_CHUR(Nd) = 0
mantle array quadrants
I - enriched in Rb, Sm
II - depleted in Rb, enriched in Sm
III - depleted in Rb, Sm
IV - enriched in Rb, depleted in Sm
mantle array, typical depleted mantle
quadrant II
depleted in Rb, enriched in Sm
residual solids
mantle array, sedimentary rocks
quadrant IV
Rb enriched, Sm depleted
much larger range c.w. mantle rocks
very old
Mantle array mixing line
DM, EMII end members
DM
depleted mantle
EM
enriched mantle material
crust, sedimentary rocks end member
contaminates mantle through subduction
FOZO
focus zone
emanating point
EM and HIMU from
subjected ocean crust and continental crust (OC, CC)
Sr isotope signature, Fraser river
runs through terrains of different ages - tributaries carry unique isotope signatures from source land - water signature changes along way
old continental signature ‘diluted’ as younger material is mixed in
global runoff average
0.7119 87Sr/86Sr
10 major rivers mixed together
lower end member
younger/OC and higher [Sr]
Ganges, Brahmaputra
himalayas
high [Sr]
high 87Sr/86Sr
very rapid erosion rates
Ganges 87Sr/86Sr
0.7257
Brahmaputra 87Sr/86Sr
0.7210
Himalayan weathering
rapid uplift = rapid erosion
87Sr rich from terrigeneous sediments, orogenic granites
downstream carbonate signal dominated by high Sr signal
increasing oceans isotopic ratio (slowly)
Himalayan drainage basins
large marine limestone, evaporites
low 87Sr/86Sr (0.706 - 0.709)
Hydrothermal 87Sr/86Sr isotope signatures
seawater component - 0.70916
basaltic component - 0.7025
effects of orogeny on ocean isotopic signal
convergence - orogeny - relief - erosion of old crust - increased ratio
Era when continents moved to current positions
Cenozoic
changes in altitude =
changes in weathering/erosion
Ocean 87Sr/86Sr, Cenozoic
Increasing over most of duration
through the Phanerozoic, Sr isotopes highest during
- times of higher tectonism
- greater uplift
- greater weathering input
Carbonate Sr sources
rivers - large variation
hydrothermal inputs - constant
seafloor spreading - geologically slow
other factors that correspond with carbonate Sr changes
climate changes ocean oxidation (kind of)
cosmic ray
primarily high E H, He nuclei
H = proton
He = alpha particle
incoming cosmic ray particles
89% protons
10% helium
1% electrons (ß-)
collision of particles w/ molecules
nuclear spallation
nuclear spallation
formation of rare isotopes
radioactive or stable
measuring cosmogenic isotopes
AMS
very low in abundance
AMS
accelerator mass spectrometer
cosmic rays interact with atoms
in atmosphere in crust (rarely)
spallation creates
cosmogenic radionuclides
spallation reaction
14N - bombarded by radiation - neutron captured - proton expelled - 14C, Z=6, N = 8
1,0n + 14,7N – 14,6C + 1,1H
14,7N(n,p)14,6C
After formation of cosmogenic radionuclide
eventually reverts back to original state
14C (6p, 8n) - expel ß- particle - 14N (7p, 7n)
Hess
Victor Hess, 1912, balloon flight
[ ] of cosmic rays increases w/ altitude
measured during solar eclipse
why Hess measured cosmic rays during eclipse
nobody agreed with him
solar radiation blocked out
any radiation measured would be cosmic radiation
Cosmic rays at Earth
concentrated at poles
ca. 4X greater near poles
electromagnetic field (dynamo effect)
Dynamo effect
mechanism by which a celestial body or star generates a magnetic field
rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time scales
Isotopes formed by action of cosmic rays on air
3H, 10Be, 14C, 26Al, 32Si, 36Cl, 39Ar, 53Mn, 59Ni, 129I
cosmogenic nuclides governed by
basic decay equation
N = N_o *e^-λt
typical N_o (cosmogenic nuclides)
0
short life
little-no background
Approaches for using cosmogenic nuclides
Radiometric Dating
Exposure Age
Radiometric dating of cosmogenic nuclides
incorporation - isolation -decay
Exposure Age of cosmogenic nuclides
direct irradiation of Si, O especially quartz (10^6 yrs for saturation)
3H useful for
tracing water on ca. 100-yr timescales
short-term water movements
Unique about oxygen, nitrogen
dipoles
absorb majority of cosmogenic energy
most abundant atoms in atmosphere
10Be
cosmogenically produced nuclide
readily absorbed in aerosols - rained out
remains in atmos. 1-2 weeks
adsorbed onto ocean clays
10Be formation
cosmic ray + O/N (atmosphere)
spallation of O, Mg, Si, Fe (crust)
production rate of 10Be
10^-2 - 10^3 atoms/cm^2/sec
0.01 - 0.001
10Be half life
T_1/2 = 1.5x10^6 y
formation of 26Al, 36Cl
intx cosmic ray + 40Ar (99.6%)
spallation products that reach crust (O, Mg, Si, Fe)
26Al decay
26Al - 26Mg, T_1/2 = 7.16 x105 yrs
36Cl decay
36Cl – 36S 00 36Ar; T1/2 = 3.08 x10^5
26Al, 36Cl properties
readily absorbed into aerosols - rained out
Al immobile (like Be)
Cl geochemically mobile
useful in hydrologic studies, groundwater aging
Be evidence
sediments contribute to composition of arc magma
10Be unique because
atmosphere- latitudinally heterogenous due to differences in cosmic ray abundance
oceans- more uniform due to short mixing time
mixing time of oceans
ca. 800 yrs
ocean residence time of Be
ca. 4000 yrs
Be mantle uses
short decay time - shouldn’t exist in mantle - if does, recently subjected
presence of 10Be is a source indicator
10Be in Arc Lava
OIBs = high 10Be MORB = low 10Be
why OIBs have high 10Be
mantle contamination from lithosphere subduction
10Be sedimentation rate
cosmogenic nuclide production assumed constant
using production history can date sediments, ice cores, etc.
how to date using 10Be, constant sedimentation rate
10Be = 10Be_o * e^-λt ln(10Be) = -d/a (λ)*ln(phi/a) d = depth a = constant sed. rate phi = production rate
10Be dating with non-constant sediment rate
10Be = [ phi(t)/a(t) ] * e^ -λt
or e^ -λ(d/a)
combining 10Be
26Al/10Be = (26Al/10Be)_o e^(λ_b - λ_a)t
why coming 16Al, 10Be
improve age dating
date quartz w/ different decay constants (?)
sedimentary quartz
exposed at surface - develop measurable quantities Al, Be - buried - isolated from cosmic-ray flux - nuclides decay at different rates - ratio reflects burial duration
Be-Al also used to date
manganese nodules
radiogenic carbon
14C
one of most commonly known, used cosmogenic dating systems
why 14C is common known/used
high production rate
rapid decay rate (T_1/2 = 5730yrs)
key constituent of organic matter, non-organic compounds
14C dating method
detection, counting of ß rays
ACTIVITY
14C foration
N–>P reaction with 14,7N
or 13,6C(d,p)14,6C - 13C collision w/ deuterium, less common
14C decay
14,6C – 14,7N + ß- + v + Q
Q = 0.156MeV
14C method
A = A_o e^-λt t(BP) = 1/λ ln(A_o/A) t(BP) = -T_1/2 * log_2(A_o/A)
why is it difficult to measure cosmogenic ‘background’
atomic weapons testing - thermal nuclear weapons
BP then, = 1950
can tell pre-post bomb
challenges with 14C
variations in local/secular atmospheric production/contents
Suess Effect
Bomb carbon
Isotope Fractionation
local variations in 14C
production dependent on neutron flux
increases w/ altitude to max 12-15,000 m all
ca. 4X greater at polar regions
changing sun activity
changing intensity of Earths magnetic field
Suess Effect
14C A in 1900s 2% lower than 1900s due to ‘dead’ CO2 from fossil fuel combustion
`Bomb carbon
nuclear bomb additions to atmos. incorporated into other pools, ages need correction
Isotope fractionation
mass differences btw 14C, 12C ca. 16.7%
14C enriched/depleted in certain reservoirs
shared/transported 14C
system will initially share concentration
- dating an organisms C - you are what you eat
- ocean obtaining atmospheres signature
14C, volcanics
eruptions eject large amount of carbonate into air
increased 12C, 13C
varies exchange ratio
magnitude of cosmic radiation depends on
lang altitude
E’s magnetic field strength at given t/place
de Vries effect
sun activity + magnetic field = ca. 2% or more change in 14C activity
14C ‘spikes’ = sun spots
14C dating of water masses
led to thermohaline circulation theory
Depth vs. 10Be
increasing [ ] moving up in agreement with isotopic enrichment of ocean = deposits of old material (mts)
most valuable tracers/technique for water studies
Tritium
what is tritium
3,1H
3H characteristics
useful for freshwater, oceans
T1/2 = 12.43 yrs
very low abundance (3x10^16% of H isotopes)
radioactive, stable H isotopes
radioactive = Tritium, 3 H stable = Protium 1H, Deuterium 2H
formation of 3H
Spallation cosmogenic n,p reaction w/ 14,7N requires fast neutron >4MeV 14,7N + 1,0n --> 3,1H + 12,6C 14,7N(n,p)3,1H
Tritium decay
3,1T – 3,2H + e- + bar + Q
Q = 0.0186 MeV (low E)
Tritium decay energy
low E beta radiation cannot penetrate human skin, only dangerous if inhaled or ingested
cosmogenic production of T
0.5±0.3 atoms 3H /cm^2/sec
natural amount of T
- 65kg in atmosphere
ca. 4kg total
T.U.
tritium units
notation for reporting [T]
1 T.U. =
1 atom 3H / 10^18 atoms H
= 7.1 dissintegrations 3H / min L of water
TU in surface water
10 TU (10^-15)
T synthetic production
- nuclear reactors using neutrons
- particle beam accelerators
why synthesize T
very low natural abundance - impractical
weapons use
T synthesis
- Neutron activation of Lithium-6
2. Neutrons react with 3He in particle beam accelerator
T synthesis, 1. 6Li
N smashes Li in 2
exothermic - does not require high E neutrons
results in T2 gas (like H2)
N - strike Li/Al target - reacts w/ 6Li - produce T
6,3Li + n –> 4,2He + 3,1T
T synthesis, 2. particle beam
N bonks a proton off and replaces it
N react w/ 3He in particle beam accelerator - produce T, H
cascading system - feeds itself - runaway rxn’s
3,2H + n – 3,1T + 1,1H
CANDU
Canadian Deuterium Uranium reactor
T production in heavy water
D captures a neutron, makes deutero-tritiated water
CANDU reactions
double substitution
D2O + n — TDO
singly substituted
DHO + n – THO
how CANDU is special
system bathed in heavy water - buffer = lower production of waste = safer
In the even of a system failure - D2O floods chamber quenching nuclear reaction
US T production
225kg produced 1955-1988 1996 had decayed to 75kg 2003 production resumed 2011 nominal production, maintain equilib. 2016 max production, recover reserve
Tritium practical uses
military applications
flare light source, emergency lights, exit signs, luminous watch/clock dials
fuel for nuclear ‘fusion’ (experimental)
two most common earth science applications of T
dating of relatively recent, short-lived elements
tracing, tracking relatively recent hydrologic/water based processes and events
natural T cycle
3H formed in lower stratosphere - remains 1-10yrs - enters troposphere - oxidizes to form HTO - rains out in 5-20days
change of natural T abundance
nuclear testing
[T] before, after nuclear testing
pre-1953 = less than 25T.U. (typically 5) 1964 = more than 2200T.U. (typically 1000)
post-nuclear testing T geoscience
eliminated use of natural T
pulse-chase type experiment
PTBT
Partial Test Ban Treaty - 1964
US, USSR agreement to stop aboveground testing
then France started, then PRC
HTO ‘Pulse-chase’ experiment uses
reconstruction of T delivery history by identification/measurement of bomb peak
penetration rates of HTO (diffusion, advection, piston velocity)
problems with HTO pulse-chase
T decay means signal decreases rapidly w/ t
natural dispersion of H2O makes difficult to ID peak w/ time and distance from source
solution to HTO problems
use 3He
3He solution
measure simultaneously w/ T
ID T peak as the sum of 3H + 3He
calculate age from 3H/3He ratio
3He
tritiogenic helium
also low abundance, 1.4x10^-4 % of He
produced mostly by T decay
calculating 3H
3H = 3He_o * 3H(e^-λt - 1) 3He/4He = (3He/4He)_o + 3H/4He(e^-λt - 1)
T tracer, ocean water masses
old H2O = low T
young H2O = high Tritium
how ocean circulation was discovered (and nobody believed them!)
can be measured as loss of parent (T) or gains of daughter (3He)
why might 3He be advantageous as the measured species over 3H
not taken up by biological organisms - doesn’t react
how can you measure water recharge rate
age structure
incursions
World Energy use by source
oil 37% coal 25% gas 23% nuclear 6% biomass 4% hydro 3% solar 0.5% wind 0.3% geothermal 0.2%
biggest potential issues with nuclear power
what to do with waste
potential weapon danger
change in energy sources
relatively static over the last 30 years
majority of nuclear reactors
E NA
France
SW Asia
Canada nuclear power
19 reactors, Ontario
16% of Electricity
only half of power generated is used in Canada
used to be world leader - 22% of world output
overtaken by Kazakhstan, 2009
Inequality of nuclear power
non-renewable
U not available in every country
would result in same political battles as fossil fuels
Uranium production in Canada
production from worlds largest McArthur River mine, N SK
expected to increase from 2013, new mine
mostly in WCSB
WCSB
western canadian sedimentary basin
concentrated U deposits
why is U concentrated in WCSB
U deposited in lithos. - rain - leeching - U oxide is soluble (unique) - transports w/ water - concentrates - reduced - drop out
Nuclear fueld cycle
mining and milling -- U concentrate convert concentrate into UO2 or UF6 enrichment fuel fabrication electricity generation optional chemical reprocessing disposal - recovered or permanently stored
mining and milling of uranium
produce concentrated uranium = yellowcake
why does U have to be concentrated in to yellowcake
mined U not directly useable for power
UO2
uranium dioxide
used in heavy water reactors
UF6
uranium hexafluoride
light water reactors
Nuclear fuel cycle, enrichment
increases proportion of 235U
why increase proportion of 235U
rare, fissile
fuel fabrication of U
manufactured in to fuel pellets
typical U pellet
ca. 7g
E = 3.5 oil barrels, 17,000 ft^3 natural gas, 1,780 lbs coal
Uranium fuel manufacturing
last stage before use in reactor compress UO2 powder into cylinder bake at 1700ºC - hard ceramic pellet stack pellets in to thin tubes - fuel rods group into bundle - fuel assembly
typical pressurized water reactor
193 fuel assemblies
51,000 fuel rods
18 fuel pellets
ca 5yrs
fissionable
material can undergo nuclear fission
typically ß decay
mostly actinides
fissile
able to sustain a chain reaction w/ low E neutrons
fissile isotopes
235U
233U
239Pu
241Pu
Fissionable chain reaction
only sustainable w/ fast neutron
unless fissile
Fissile rule
Heavy isotopes
- Z between 90 - 100 (actinides)
- 2Z - N = 43 +/- 2
fertile
not fissile but easily upgraded to fissile
fertile U
238
fissionable process
unstable mix of Z/N
slow, low E N hits - low E
fast, high energy N, (cosmic ray, spallation) - massive destruction - perpetuate reaction
Oklo, Gabon
U deposit behaved as natural nuclear fission reactor ca. 1.8Ga
few 100,000 yrs
100kW power output average
Canadas nuclear power program 2016
5 plants, 3 provinces, 19 active power reactors, mostly ontario, all CANDU design, ca. 16.5% of E
Bruce Nuclear Generating Station, Ont
Pickering NGS, Ont
Darlington NGS, Ont
Gentilly-2 Nuclear Facility, Quebec (shut down)
Point Lepreau GS, NB
Nuclear waste
biggest problem is intermediates - nonactinide radionuclides
short decay times, T_1/2 =50 yrs
= high E = gamma radiation
interim nuclear waste storage
removed radioactively, thermally hot for several yrs
rods in barrels in H2O to cool
the moved to longer term storage
Canada interim storage
on land - dry storage
long term disposal plans
drop from ship into sediment (would leech out of container) vitrify store in Yucca mt in an open system reprocess breeder reactors
Th power
3-4X more abundance than U (in crust)
not fissile but fertile
232Th - 233U is more efficient than 238U - 239Pu
does not require isotopic separation
minimal radioactive waste
anti-theft
abundant in ocean - available to more nations
