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1
Q

Radioactivity

A

Decay of unstable elements through the emission of nuclear particles and radiant energy

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2
Q

Components of atom

A

Protons (Z), neutrons(N), electrons

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3
Q

Atomic number (Z)

A

Number of protons

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4
Q

Mass number (A)

A

Protons (Z) + Neutrons (N) = (A)

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5
Q

Nuclide

A

A distinct kind of atom with a specific number of neutrons and protons. Can be uncategorized as, isotopes, isotones, and isobars.

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6
Q

Isotope

A

A nuclide with the same atomic number (Z) ( the same number of protons, but a different number of neutrons (N))

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7
Q

Isotone

A

A nuclide with the same number of neutrons (N), but a different number of protons (Z)

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8
Q

Isobar

A

A nuclide with the same mass number (A), but a different number of neutrons (N) and protons (Z)

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9
Q

Atomic weight of protons (amu)

A

1.007276467

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10
Q

Atomic weight of neutrons (amu)

A

1.008664916

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11
Q

Atomic weight of electrons (amu)

A

0.0005485799

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12
Q

Stable nuclides

A

N = Z

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13
Q

Discovery of radioactivity

A

1896 - Antoine Becquerel, Marie and Pierre Currie, Nobel Prize Physics in 1903

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14
Q

Nucleosynthesis

A

Process where the first nuclides form

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15
Q

Oddo - Harkins Rule

A

Isotopic abundance relative to Be, the average galactic occurrence frequency of chemical elements, even atomic numbers are always higher compared to neighboring elements eg. 5B <6C> 7N

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16
Q

Polygenetic Hypothesis

A

Cosmological, stellar, explosive, galactic

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17
Q

Cosmological polygenetic hyp.

A

Hydrogen fusion produces Helium, 0.0001s after the Big Bang nucleons formed 0.01s – 99 999 999 970oC ≈ protons and neutrons.

(n+p=2H) -> PP I

(2He + p = 3He) -> (3He +n = 4He) -> PP II

(3He +4He = 7Be) -> PP III

7Be -> 8Be -> 12C …NO
[Oddo Harkins (galactic abundance relative to Be]

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18
Q

Stellar polygenetic hyp.

A

Fusion in stars, Hertzsprung-Russel diagram: relative luminosity vs surface temps of Stars. Hydrogren burning (H->He). Our sun is class G

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19
Q

Explosive polygenetic hyp.

A

Supernovae

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20
Q

Galactic polygenetic hyp.

A

Li + Be, cosmic ray interaction

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21
Q

Nuclear stability is a balance of what?

A

Strong nuclear force and coulombs repulsion. Strong = long-range, Coulombic = short-range

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22
Q

Nuclear binding energy eq

A

E = mc^2

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23
Q

Mass defect

A

Difference between observed atomic mass and expected atomic mass, ectothermic due to release of energy dM = Me - Mo

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24
Q

What does binding energy per nucleon tell you?

A

A metric for how tightly bound nucleons are within a nucleus (nuclide stability. Peaks at 56Fe, beyond that mass number fusion of heavier nucleons, requires energy input. (fusion <56

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25
Valley of stability
Nuclear binding energy is the greatest near the bottom = right combination of protons and neutrons to be stable. A cross section of the nuclide chart, there the center are stable nuclides and as you move outwards to transition to unstable
26
Major Elements
Reported as oxides >1.0 wt.% O, Si, Al, Mg, Fe, Ca, Ti, Na, K, P
27
Trace Elements
<0.1 wt.% reported as ppm or ppb
28
Goldschmidt Classification scheme
The affinity of elements to form various types of compounds. Lithophile, Chalcophile, Siderophile, Atmophile
29
Lithophile
bound to oxygen (silicate) (ex. Li, Be, Al, Si)
30
Chalcophile
bound to sulfur (ex. Sn, Zn, Pb, S)
31
Siderophile
bound to iron (ex. Mn, Fe, Co, Ni)
32
Atmophile
gaseous behaviour ~ atmosphere (ex. H, He, C,N)
33
Types of decay modes
alpha decay, beta - decay, beta+ decay, e- capture, spontaneous fission
34
Alpha decay
A heavy nuclide ejects a +He or (a particle) Atomic mass changes ( ex. U238 ->Th234 He4)
35
Beta- decay
a neutron in the nucleus decays to a proton, an electron (b- particle and antineutrino) example (C14 -> N14 + e-) Atomic mass remains the same
36
Beta + decay
a proton in the nucleus decays to a neutron, a positron (b+ particle and neutrino are ejected) atomic mass remains the same (example Na22 -> 22Ne + e+)
37
e- capture
An electron is captured by the nucleus, converting a proton to a neutron and releasing a neutrino (example: K40 -> 40Ar)
38
Spontaneous fission
spontaneous breakdown of large nuclei into two smaller nuclei (U235 -> Cs140,n,200MeV,n,Rb92)
39
The probability that a given atom will decay in some time dt: eq
dN/dt = -lamdaN => decay rate = decay constant x parent atoms
40
Behaviour of parent and daughter through time
Parent (radioactive) atoms decrease exponentially with time. Daughters (radiogenic) increases
41
Parent eq
N = Noe^(-lamda t)
42
Half-life eq
t(1/2) = ln 2 / lamda, (ln2 = 0.69314718056)
43
Daughter eq
D = Do + N((e^lamdat)-1)
44
The CNO cycle
``` 12C + p = 13N +gamma 13N = 13C +e + n 13C + p = 14N + gamma 14N+p 15O + gamma 15O 15N +e+n 15N+p 12C +4He ``` Occurs after He-burning
45
e-process
Si burning (stops at mass 56, 1 day) 28Si + gam = 24Ne+4he 28 +4He 32S +gam 32S +4He = 36Ar+gam
46
s-process
Slow neutron capture in red giants, every 1000 y, makes heavier isotopes: 22Ne +4He = 25Mg +n 141Nd +n 142Nd +gam
47
r-process
rapid neutron capture, explosive nucleosynthesis
48
p-process
Proton capture, lightest isotopes of an element, | less abundant than s and r process
49
Benefits of zircon and chemical structure
ZrSiO4, very resistant to erosion, burial, high-temperature metamorphism, Hardness = 7.5, incorporates large amounts of uranium but no lead during crystallization (Do ~0)
50
Decay eq of 187Re to 187Os
Half-life = 42.3 by 187Os = 187Oso + 187Re(e^-lamdat -1) Isochron eq: (187Os/188Os) = (187Os/188Os)o + (187Re/188Os)(e^-lamdat -1) y=mx+b
51
Decay rate definition
immutable nuclear process
52
U-Pb-Th system
Parents: 235U (0.7G) -> 207, 238U (4.5G) -> 206, 232Th (14G) -> 208 Daughters: 204Pb, 206Pb, 207Pb, 208Pb
53
U-Pb Two decay sequences
Concordia plot Y axis: 206*/238 X axis: 207*/235
54
Secular Equilibrium
Product of the abundance of an isotope and its decay constant are equal among all intermediate daughter products and parent isotope N1Lam1 = N2Lam2 = N3Lam3 (ex. one 206Pb is created for every atom of 238U that decays)
55
When will secular equilibrium be reached in a closed system?
In a time proportional to the longest half-life of the intermediate daughter product. Will remain in SE until one or more isotopes in the chain is fractionated from the others (ex chemical partitioning in mag system or low temp fractionation during chem weathering
56
U-Pb age eq
206: (206/204) = (206/204)o +(238/204)(e^-lamt-1) 207: (207/204) = (207/204)o +(235/204)(e^-lamt-1) 204 is stable and removes systematic uncertainty of calculated moles
57
Pb-Pb age eq
(207/206) = (235/238)([e^-lamt-1]/[e^-lamt-1]) Solar sys formation and meteorite evolution Concordia plot Y axis: 207*/206 X axis: 204*/206
58
What are the three decay systems of U-Pb
238U-206Pb- HL 4.468G 235U-207Pb- HL 0.704G 232Th-208Pb- HL 14.01G
59
238/235 ratio
137.88
60
Concordant
When a mineral crystallizes and begins accumulating Pb* in a closed system 207/235 and 206/238 ratios evolve such that they follow the curve (which is curved because they decay at different rates
61
Discordant
Disturbance or Pb loss resulting the ratios evolving away from the curve. t1 = igneous(crystallization), t2 metaphoric (Pb loss)
62
Concordia plot axis ranges
Y = 0.0 - 0.7 X = 0 - 25 Points along curve = 0, 1000, 1500,2000, 2500, 3000
63
Tera-Wasserburg
238/206 X axis, | 207/206 y axis
64
Define accuracy
measurement of the difference between an experimental result and the truth
65
Define precision
measurement of the reproducibility of an experimental result, without regard to the truth
66
What are the components of Error?
Random and systematic
67
The random component of error
Random fluctuations in the signal you're measuring can be reduced by increasing the number of observations, propagate into a value for it to be accurate
68
Systematic components of error
remains constant or varies predictably no matter how many measurements you make. only propagate into a value when that value is to be compared with other values that are not subject to the same sys errors. Eg decay constant different amongst sys
69
Metamict
radiation damage to zircon, these zones are richest in U and lost Pb = discordant. abrasion and annealing
70
Wetherill Concordia plot
206*/238 vs 207*/235
71
Isotope dilution
process of spiking a sample with a known quantity of one or more tracer isotopes in order to convert ratios measured by mass spec to moles in sample isotope, reference is most abundant isotope
72
Internal random errors
affect only a single analysis, propagate through age eq (1 sigma 1-2%
73
External systematic error
applied to set of ages ~1%
74
Discordance filter cutoff
Tight filter 0-10%: good accuracy, age distribution bias Loose filter 20-30%: less accuracy, less bias
75
Steps to determine ages ID-TIMS vs LC-ICPMS
ID-TIMS: (analyze solution) Dissolve crystal, add spike with non natural isotope, chemically purify, calibrate by comparison with spike (ID), run LA-ICPMS: (analyze solid material in situ), ablate/sputter crystals, measure isotope ratios, calibrate by comparison with sample standard bracketing
76
Comparison of U-Pb techniques
idtims: 1 analysis/hr, 100-300$, 0.1-0.3 % ac, best precision and accuracy SIMS: 4 analyses/hr, 25$, 1-2%ac, best spatial resolution laicpms: 40-100 analyses/ hr, 4-8$, 1-2 % ac, highest efficiency
77
Sample standard bracketing
Each sample needs to be measured more than three times, and the mean value of the measurements is calculated as the Si isotopic composition of the sample to obtain stable and reliable results
78
Compatible material
remains in the solid component of the mantle, ex Peridotite enriched with Ni, Os, Cr
79
Incompatible material
elements that do not fit into the crystal lattice of mantle minerals, ex Rb, K Th, Sr, Zr, are extracted from the mantle into the new basaltic crust, mantle cooling
80
Depleted mantle
Earth's mantle from which basaltic melt has been extracted during melting events ex at mid-ocean ridges, hot spots or island arcs
81
Bulk silicate earth
Mantle =2/3 of Earth's mass, 99.5% silicate earth primitive mantle
82
How to calculate the atomic weight:
(atomic mass x abundance)n1 + (atomic mass x abundance)n2 + ... amu or Da

Isotope Geochemistry (2 decks)

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