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ES2001 > Geochronology > Flashcards

Geochronology Flashcards

1
Q

What are way to determine relative age?

A

Sedimentary packages younging upwards
Fossil assemblages
Cross cutting relationships - intrusions

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2
Q

How do you determine absolute age?

A

Geochronology

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3
Q

What is geochronology?

A

Means of measuring the ages of rocks based on the laws of radioactivity

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4
Q

How is radioactivity used for geochronology?

A

Long lived radioactive isotopes which decay to stable isotopes as a function of time only

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5
Q

What do protons control?

A

Control the number of electrons and shape of electron cloud

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6
Q

What does the electron cloud control?

A

Chemistry of an element

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7
Q

How many elements are there?

A

118 elements (92 naturally occuring)

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8
Q

How many isotopes are there?

A

> 3000 known
254 stable isotopes

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9
Q

What does the chart of the nuclides show?

A

Stable and unstable isotopes plotted on graph of neutron vs proton numbers

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10
Q

What controls nuclear stability?

A

Electrostatic force (like repels like)
Strong nuclear force (when protons overcome their repulsion they stick together)
Quantum mechanical rules (weak force)

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11
Q

What does the ratio of protons and neutrons control?

A

Determines the stability of a nucleus
Neutrons dilute instability

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12
Q

What makes a nucleus unstable?

A

Too many protons - unbalanced p:n, nucleus wants to rip itself apart
Too many neutrons - a free neutron outside the nucleus is unstable, decay into a proton
Too many nucleons - the strong nuclear force can keep the nucleus together

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13
Q

What makes an isotope radioactive?

A

Only specific combinations of neutrons and protons are stable
Isotopes that do not have the right ratio are unstable
Isotope wants to be stable so decays

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14
Q

What are the three types of radioactive decay?

A

Alpha
Beta
Gamma radiation

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15
Q

What is the difference between alpha, beta, gamma radiation?

A

Alpha and beta = reconfiguration of the nucleus
Gamma - reduces the energy of the nucleus

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16
Q

What happens in beta negative decay?

A

Nucleus is too neutron rich
A neutron turns into a proton and electron
No loss of mass
Atomic number + 1

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17
Q

What happens in beta plus decay?

A

Nucleus is too proton rich
Proton turns into a neutron + positron (e+)
Atomic number - 1q

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18
Q

What happens in electron capture?

A

Nucleus absorbs a nearby electron shell
-ve charge balances +ve charge = neutron
Atomic number -1

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19
Q

What is an alpha particle?

A

4 He
2

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20
Q

When does alpha decay occur?

A

In heavy isotopes
Two protons two neutrons are ejected by from nucleus
Mass -4, proton -2

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21
Q

What are the conditions of radioactive decay?

A

Unpredictable and spontaneous

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22
Q

What is the decay equation?

A

P = P0 e -yt
Number of radioactive atoms at time t = Number of radioactive atoms at time t=0 x e to the power of decay constant x time

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23
Q

What is the decay constant?

A

A measurement of the rate of decay of a radioactive isotope
Specific to each radioactive isotope
1/yr

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24
Q

How do you rearrange the decay equation for t?

A

= -ln(P/P0) / y

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25
Q

What is half life?

A

Time it takes for half the number of radioactive isotopes in a sample to decay
T1/2 = ln(2)/ y

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26
Q

What does a larger half life mean?

A

Smaller decay constant
Slower decay

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27
Q

What happens after 8 half life’s?

A

Radioactive isotope is essentially extinct

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28
Q

What is the discovery outcrop, narrower gneiss terrane?

A

Alluvial fan/delta deposits dated at 3.6Ga
Contains 4.4Ga zircons

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29
Q

What are the current views on the hadean era (environment
) based on zircon evidence?

A

Faint young sun counteracted by thicker atmosphere
Solidified mantle insulates surface
Liquid water at surface
Limited bombardment?

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30
Q

How do we date zircons?

A

U-Pb dating

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31
Q

Why are sedimentary rock difficult to date?

A

Made up of older rocks
Need to date the cement which is very hard
Can date ash beds ie Permian Triassic extinction

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32
Q

What are the quantities of U and Pb in nature?

A

Trace elements
0.01-20ppm in terrestrial rocks

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33
Q

What are the three naturally occurring isotopes of Uranium and what do they decay to?

A

238U to stable 206Pb
235U to stable 207Pb
234U formed by 238U (rare, short lived)

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34
Q

What is the 238U->206Pb decay chain?

A

218U -> 206Pb + 8a + 6B- + Q

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35
Q

What is Q?

A

Energy of decay

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36
Q

What is the 235U->207Pb decay chain?

A

235U->207Pb + 7a + 4B- + Q

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37
Q

How do you calculate the number of original isotopes in a sample?

A

P0 = P + D
Where P=number of isotopes left
Where D=number of daughter isotopes
Or
P0 = P/e^-lamba x t

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38
Q

How do you find the number of daughter isotopes present in a system?

A

D* = P(e^gammap.t - 1)

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39
Q

What makes up the measurement of an isotope?

A

Initial isotope + radiogenic isotope (daughter)
D = D0 + D*
Measured = initial daughter + isotopic decay daughter

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40
Q

Where does initial daughter isotope come from?

A

From they melt, not generated by decay

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41
Q

How do we get around the problem of the initial daughter?

A

Use systems with high P/D ratios (therefore negligible D0)
Employ the isochron method

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42
Q

What are the characteristics of zircons?

A

Orthosilicate
Mostly colourless
Extremely robust - survives chemical weathering and high grade metamorphism
Zr4+ octahedral site and Si4+ tetrahedral sites
In ppl = high relief, clear, pleochroic halos from radioactive damage
In x-polar = 3rd-4th order interference colours

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43
Q

Where are zircons found?

A

In most rock types
Common accessory mineral in felsic igneous rocks
Forms during metamorphism acculumates in sediments

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44
Q

Why is there U in zircons?

A

Uranium ion (U4+) readily substitutes for Zr4+ in mineral structure

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45
Q

Why do zircons not contain Pb?

A

Too large and incorrect charge to fit in mineral structure

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46
Q

What is the benefit of having no Pb in zircons?

A

Simplifies the decay equation )no initial PB

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47
Q

How do you date a granite using zircon U-Pb?

A

Collect granite
Smash the granite up
Find suitable zircon in the rock
Analyse the zircons U and Pb isotopes with a mass spectrometer

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48
Q

What is the benefit in geochronology of uranium having two decay chains?

A

Separate decay chains = independent calculates
Acts as a check of data quality = concordant vs discordant

49
Q

Give examples of a system.

A

Single mineral grain
Single rock
Granite pluton
Metamorphic terrane
Whole planet

50
Q

What is an implicit assumption in the decay equation?

A

Isotopes are only varying through radioactive decay

51
Q

How can we assume a closed system?

A

Choose the right samples - analyse the freshest rocks
Choose minerals and/or isotope systems that are unaffected by weathering (zircon)
Perform statistical checks using the data

52
Q

What is the problem with open systems?

A

Invalidate the geochronological age
Adds/removes daughter/parent isotopes

53
Q

What are possible reasons for zircon discordancy?

A

Result of Pb loss
- heating due to metamorphic event
- fluid interaction with source rock
- radiation due to zircon crystals

54
Q

What is the best case scenario for dating the crystallisation age of granite?

A

Analyse large number of individual zircon grains from the granite
Each grain has concordant ages
Each grain gives the same age

55
Q

What are the 4 assumptions of geochronology?

A

Decay constant is known and remains constant
We have a way to correct for initial daughter
The isotopic system has remained close since t=0 with no addition or loss of either the parent or daughter isotopes
The whole rock/mineral suite is cogenetic and crystallised over a geologically short time and therefore have the same age

56
Q

What does cogenetic mean?

A

Formed by the same event

57
Q

What is Wetherill’s Concordia?

A

On a plot of 206Pb/238U against 207Pb/235U, the curve of all points yielding concordant dates

58
Q

What are points that do not plot on the Concordia called?

A

Discordant

59
Q

How do we find the ages of discordant zircons?

A

Draw line of best fit and extend it to where it intercepts the Concordia at both sides
Upper intersect = age of zircon formation
Lower intersect = age of Pb loss event

60
Q

How do you identify what metamorphic event a zircon is dating?

A

Use mineral association and zircon chemistry

61
Q

What is the cause of a younger rim in zircons?

A

Result of new melt or metamorphism

62
Q

Why do you have to be careful when dissolving whole zircons?

A

Different zones will have different ages and full dissolving will result in a mean value of age

63
Q

What is orogenesis?

A

Process of mountain building

64
Q

What causes orogenesis?

A

Thickening of crust associated with the melting of crustal rocks that are buried and heated up past their solidus

65
Q

What do melts in orogenesis form?

A

Broad band of granite plutons

66
Q

What are the concentrations of Rb and Sr in nature?

A

Rb 1-150ppm in igneous rocks
Sr 1-450ppm in igneous rocks

67
Q

What are the properties of rubidium?

A

Alkali metal
Rb1+ oxidation state, substitutes for K in minerals

68
Q

What are the properties of Strontium?

A

Alkali earth metal
Sr2+ oxidation state
Substitutes for Ca in minerals

69
Q

What is the formula for olivine?

A

(Mg/Fe)SiO4

70
Q

What allows other minerals to substitute into a mineral structure?

A

Ionic radius and charge similar to the major element they are replacing

71
Q

When does Rb/Sr increase?

A

Increases over the course of fractional crystallisation

72
Q

What are the naturally occurring isotopes of rubidium?

A

85Rb
87Rb - radioactive, long lived

73
Q

What is the decay equation for 87Rb?

A

87Rb = 87Sr + B- + Q

74
Q

What makes a decay equation more accurate?

A

Measuring the ratio of isotopes (by dividing all values by stable isotope of the daughter element with no radiogenic input)

75
Q

What is the benefit of measuring the isotope ratio in Rb/Sr?

A

More accurate, removes the elemental variation

76
Q

What is the normalised decay equation?

A

D/D’ = (D/D’)0 + P/D’(e^lamda.t - 1)

D’ = stable isotope

77
Q

What are the quantities of Rb and Sr when a melt forms?

A

Contains 87Rb and 87Sr inherited from its source, the 87Sr that did not form by decay of 87Rb

78
Q

How do you overcome the issue of the measure of 87Sr/86Sr being a combination of both initial and radiogenic Sr?

A

Use systems with high P/D ratios (negligible D0)
Employ the isochron method

79
Q

When do you use the isochron method?

A

When you use the whole rock and mineral samples when D0=/0

80
Q

How do you solve the decay equation with two unknowns?

A

Solve simultaneously by using a suite of samples from a system with the same age and initial daughter ratio

81
Q

What are the four assumptions of geochronology for isochronology?

A

Decay constant is known and has remained constant
The whole rock or mineral suite had the same initial daughter at t=0
The isotopic system has remained closed since t=0 with no addition or loss of either the parent or daughter isotopes
The whole rock or mineral suite is cogenetic and crystallised over a geologically short time so have the same (approx) age

82
Q

What is the normalised decay equation in terms of straight line equation?

A

D/D’ = (D/D’)0 + P/D’ (e^lamba.t -1)
y = c + x m

83
Q

Using the straight line equation how do you find the initial daughter isotope D0?

A

Find c, derived from the graph line

84
Q

How do you date a granite pluton using Rb-Sr?

A
  1. Sample a range of granites from the pluton
  2. Powder each rock and dissolve them
  3. Analyse the 87Sr/86Sr and 87Rb/86Sr ratios of each rock
  4. Plot ratios on a graph of 87Sr/86Sr (y axis) against 87Rb/86Sr (x axis)
85
Q

What are the requirements for successful isochron dating?

A

Decay constant or T1/2 needs to be appropriate for expected age
Sufficient variation in P/D ratio, preferably some phases having P/D~0
The elements must be present in sufficient abundance to allow precise and accurate isotope measurements

86
Q

What are you dating with Rb/Sr in different types of rocks?

A

Igneous = time of crystallisation
Sedimentary = time of deposition vs age of sediment provenance
Metamorphic = time of crystallisation vs time of recrystallisation

87
Q

What does it mean if the plots on an isochron aren’t in a straight line?

A

The systems hasn’t remained closed?

88
Q

What does a closed system depend on?

A

The retentivity of the minerals for the isotopes in question
Physical and chemical properties of the isotopes
Cooling rate
Subsequent metamorphic and altercation events

89
Q

What is closure temperature?

A

The temperature below which the isotopes of interest become fixed in specific minerals in a rock

90
Q

What is the closure temperature of the Rb/Sr system?

A

300-500 degrees C
Comparable to medium to high grade metamorphic conditions

91
Q

What is affected during a metamorphic event in a Rb/Sr?

A

Each rock Sr isotope composition os homogenised
The pluton is not homogenised

92
Q

Using Rb/Sr system on a isochron, how do you tell the age of crystallisation of a rock?

A

Based on the whole rock sample

93
Q

Based on an isochron diagram, how do you identify the age of metamorphism?

A

1 whole rock and all its grains

94
Q

What are the three types of short lived radionuclides?

A

those generated constantly by decay of more long-lived systems
extinct radionuclides
cosmogenic radionuclides

95
Q

How do extinct radionuclides form?

A

formed via primordial processes - stellar and supernova nucleosynthesis
short half lives compared to the age of our solar system so are now ‘extinct’

96
Q

How are cosmogenic radionuclides formed?

A

formed by interaction of high energy cosmic rays with particles in our atmosphere
cosmic rays collide with N2 and O2 in the upper atmosphere causing spallation reactions which generate protons, neutrons and muons
these particles cause additional nuclear reactions generating new stable and unstable isotopes

97
Q

What is the energy of cosmic rays and solar winds?

A

cosmic rays - primarily H and He atoms. several GeV to 10^9 GeV
solar wind - lower energy particles, but don’t have to travel as far. ~0.5keV/amu - generates aurorae

98
Q

what are the isotopes of Carbon?

A

12C
13C
Both Stable
14C unstable

99
Q

what is the formation of radiocarbon?

A

cosmic rays bombards atmospheric nucleus producing a neutron which is absorbed bt atmospheric 14N which ejects a proton and this becomes 14C
14C decays back to 14N with a half life of 5730 years

100
Q

what is the radiocarbon cycle?

A

14C is constantly being formed in the atmosphere
14C is oxidised to form CO2 which is taken up by organic matter
14C travels through the food chain
life stops taking in new 14C when it dies

101
Q

what is the decay of radiocarbon?

A

life stops taking up 14C when it dies
the 14C contained in that organism will decay over time as a function of its decay constant

102
Q

to what extent is 14C measurable?

A

in a sample ~ 8 times the half life = 45,000yrs
with accelerator mass spectrometer ~60,000 - 75,000 yrs

103
Q

what is the value we use for carbon 14 halflife?

A

libby = 5568yrs

104
Q

what is the decay constant used for 14C?

A

Libbys constant = 1.244x10^-4

105
Q

How are radiocarbon ages reported?

A

In years before 1950

106
Q

What are the three assumptions of calculating radiocarbon ages?

A

carbon being dated was in equilibrium with atmosphere when alive and has remained a closed system since
troposphere is well mixed in terms of 14C
the activity of 14C in the atmosphere has remained constant over the last ~70,000 years and is defined as 13.56 dpm/g of C

107
Q

What is the activity of 14C?

A

13.56 dpm/g of C
disintegrations per min

108
Q

What is the reason of using the Libby values instead of true values?

A

so data is comparable

109
Q

What causes variation in atmospheric 14C?

A

natural and anthropogenic forcing

110
Q

What are the effects of secular changes in 14C production?

A

Suess effect
deVries effect
bomb pulse

111
Q

What is the deVries effect?

A

fluctuations in 14C due to fluctuations in the intensity of solar wind and cosmic waves and Earth’s protective magnetic sphere
upy downy pattern

112
Q

What is the hazard of assuming a 14C activity of 13.56?

A

Calculated age will not reflect the true age of the sample
If the sample started with a higher initial activity the age will be overestimated
If the sample started with lower initial activity the age will be underestimated

113
Q

What is the Suess effect?

A

A marked decrease in the concentration of 14C in the atmosphere from 1850 to 1950
Caused by increased burning of fossil fuels which releases CO2 with no 14C
Dilutes 14C in the atmosphere

114
Q

What is the bomb pulse?

A

A sudden increase in the concentration of 14C in the atmosphere peaking around 1965 from nuclear bomb testing generating neutrons

115
Q

What is a benefit of the bomb pulse in dating?

A

can use the relatively rapid decay of excess Bomb pulse 14C in the atmosphere as another chronometer ie human tissues as an estimate year of birth and death

116
Q

How are human tissues dated?

A

Hair samples have 14C which reflect atmosphere at time of death
Enamel which is not replaced used to estimate time of birth

117
Q

How do we calibrate the 14C timescale?

A

tree rings, corals

118
Q

What are example of radiocarbon applications?

A

recent geological and archaeological samples
ie. wood samples, charcoal from early inhabited caves, dentine and tooth enamel, seeds and grains, peat, paper and cloth, bone, shells

ES2001 (9 decks)

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