Geochronology Flashcards
1
Q
What are way to determine relative age?
A
Sedimentary packages younging upwards
Fossil assemblages
Cross cutting relationships - intrusions
2
Q
How do you determine absolute age?
A
Geochronology
3
Q
What is geochronology?
A
Means of measuring the ages of rocks based on the laws of radioactivity
4
Q
How is radioactivity used for geochronology?
A
Long lived radioactive isotopes which decay to stable isotopes as a function of time only
5
Q
What do protons control?
A
Control the number of electrons and shape of electron cloud
6
Q
What does the electron cloud control?
A
Chemistry of an element
7
Q
How many elements are there?
A
118 elements (92 naturally occuring)
8
Q
How many isotopes are there?
A
> 3000 known
254 stable isotopes
9
Q
What does the chart of the nuclides show?
A
Stable and unstable isotopes plotted on graph of neutron vs proton numbers
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)
11
Q
What does the ratio of protons and neutrons control?
A
Determines the stability of a nucleus
Neutrons dilute instability
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
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
14
Q
What are the three types of radioactive decay?
A
Alpha
Beta
Gamma radiation
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
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
17
Q
What happens in beta plus decay?
A
Nucleus is too proton rich
Proton turns into a neutron + positron (e+)
Atomic number - 1q
18
Q
What happens in electron capture?
A
Nucleus absorbs a nearby electron shell
-ve charge balances +ve charge = neutron
Atomic number -1
19
Q
What is an alpha particle?
A
4 He
2
20
Q
When does alpha decay occur?
A
In heavy isotopes
Two protons two neutrons are ejected by from nucleus
Mass -4, proton -2
21
Q
What are the conditions of radioactive decay?
A
Unpredictable and spontaneous
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
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
24
Q
How do you rearrange the decay equation for t?
A
= -ln(P/P0) / y
25
What is half life?
Time it takes for half the number of radioactive isotopes in a sample to decay
T1/2 = ln(2)/ y
26
What does a larger half life mean?
Smaller decay constant
Slower decay
27
What happens after 8 half life’s?
Radioactive isotope is essentially extinct
28
What is the discovery outcrop, narrower gneiss terrane?
Alluvial fan/delta deposits dated at 3.6Ga
Contains 4.4Ga zircons
29
What are the current views on the hadean era (environment
) based on zircon evidence?
Faint young sun counteracted by thicker atmosphere
Solidified mantle insulates surface
Liquid water at surface
Limited bombardment?
30
How do we date zircons?
U-Pb dating
31
Why are sedimentary rock difficult to date?
Made up of older rocks
Need to date the cement which is very hard
Can date ash beds ie Permian Triassic extinction
32
What are the quantities of U and Pb in nature?
Trace elements
0.01-20ppm in terrestrial rocks
33
What are the three naturally occurring isotopes of Uranium and what do they decay to?
238U to stable 206Pb
235U to stable 207Pb
234U formed by 238U (rare, short lived)
34
What is the 238U->206Pb decay chain?
218U -> 206Pb + 8a + 6B- + Q
35
What is Q?
Energy of decay
36
What is the 235U->207Pb decay chain?
235U->207Pb + 7a + 4B- + Q
37
How do you calculate the number of original isotopes in a sample?
P0 = P + D
Where P=number of isotopes left
Where D=number of daughter isotopes
Or
P0 = P/e^-lamba x t
38
How do you find the number of daughter isotopes present in a system?
D* = P(e^gammap.t - 1)
39
What makes up the measurement of an isotope?
Initial isotope + radiogenic isotope (daughter)
D = D0 + D*
Measured = initial daughter + isotopic decay daughter
40
Where does initial daughter isotope come from?
From they melt, not generated by decay
41
How do we get around the problem of the initial daughter?
Use systems with high P/D ratios (therefore negligible D0)
Employ the isochron method
42
What are the characteristics of zircons?
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
43
Where are zircons found?
In most rock types
Common accessory mineral in felsic igneous rocks
Forms during metamorphism acculumates in sediments
44
Why is there U in zircons?
Uranium ion (U4+) readily substitutes for Zr4+ in mineral structure
45
Why do zircons not contain Pb?
Too large and incorrect charge to fit in mineral structure
46
What is the benefit of having no Pb in zircons?
Simplifies the decay equation )no initial PB
47
How do you date a granite using zircon U-Pb?
Collect granite
Smash the granite up
Find suitable zircon in the rock
Analyse the zircons U and Pb isotopes with a mass spectrometer
48
What is the benefit in geochronology of uranium having two decay chains?
Separate decay chains = independent calculates
Acts as a check of data quality = concordant vs discordant
49
Give examples of a system.
Single mineral grain
Single rock
Granite pluton
Metamorphic terrane
Whole planet
50
What is an implicit assumption in the decay equation?
Isotopes are only varying through radioactive decay
51
How can we assume a closed system?
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
What is the problem with open systems?
Invalidate the geochronological age
Adds/removes daughter/parent isotopes
53
What are possible reasons for zircon discordancy?
Result of Pb loss
- heating due to metamorphic event
- fluid interaction with source rock
- radiation due to zircon crystals
54
What is the best case scenario for dating the crystallisation age of granite?
Analyse large number of individual zircon grains from the granite
Each grain has concordant ages
Each grain gives the same age
55
What are the 4 assumptions of geochronology?
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
What does cogenetic mean?
Formed by the same event
57
What is Wetherill’s Concordia?
On a plot of 206Pb/238U against 207Pb/235U, the curve of all points yielding concordant dates
58
What are points that do not plot on the Concordia called?
Discordant
59
How do we find the ages of discordant zircons?
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
How do you identify what metamorphic event a zircon is dating?
Use mineral association and zircon chemistry
61
What is the cause of a younger rim in zircons?
Result of new melt or metamorphism
62
Why do you have to be careful when dissolving whole zircons?
Different zones will have different ages and full dissolving will result in a mean value of age
63
What is orogenesis?
Process of mountain building
64
What causes orogenesis?
Thickening of crust associated with the melting of crustal rocks that are buried and heated up past their solidus
65
What do melts in orogenesis form?
Broad band of granite plutons
66
What are the concentrations of Rb and Sr in nature?
Rb 1-150ppm in igneous rocks
Sr 1-450ppm in igneous rocks
67
What are the properties of rubidium?
Alkali metal
Rb1+ oxidation state, substitutes for K in minerals
68
What are the properties of Strontium?
Alkali earth metal
Sr2+ oxidation state
Substitutes for Ca in minerals
69
What is the formula for olivine?
(Mg/Fe)SiO4
70
What allows other minerals to substitute into a mineral structure?
Ionic radius and charge similar to the major element they are replacing
71
When does Rb/Sr increase?
Increases over the course of fractional crystallisation
72
What are the naturally occurring isotopes of rubidium?
85Rb
87Rb - radioactive, long lived
73
What is the decay equation for 87Rb?
87Rb = 87Sr + B- + Q
74
What makes a decay equation more accurate?
Measuring the ratio of isotopes (by dividing all values by stable isotope of the daughter element with no radiogenic input)
75
What is the benefit of measuring the isotope ratio in Rb/Sr?
More accurate, removes the elemental variation
76
What is the normalised decay equation?
D/D’ = (D/D’)0 + P/D’(e^lamda.t - 1)
D’ = stable isotope
77
What are the quantities of Rb and Sr when a melt forms?
Contains 87Rb and 87Sr inherited from its source, the 87Sr that did not form by decay of 87Rb
78
How do you overcome the issue of the measure of 87Sr/86Sr being a combination of both initial and radiogenic Sr?
Use systems with high P/D ratios (negligible D0)
Employ the isochron method
79
When do you use the isochron method?
When you use the whole rock and mineral samples when D0=/0
80
How do you solve the decay equation with two unknowns?
Solve simultaneously by using a suite of samples from a system with the same age and initial daughter ratio
81
What are the four assumptions of geochronology for isochronology?
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
What is the normalised decay equation in terms of straight line equation?
D/D’ = (D/D’)0 + P/D’ (e^lamba.t -1)
y = c + x m
83
Using the straight line equation how do you find the initial daughter isotope D0?
Find c, derived from the graph line
84
How do you date a granite pluton using Rb-Sr?
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
What are the requirements for successful isochron dating?
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
What are you dating with Rb/Sr in different types of rocks?
Igneous = time of crystallisation
Sedimentary = time of deposition vs age of sediment provenance
Metamorphic = time of crystallisation vs time of recrystallisation
87
What does it mean if the plots on an isochron aren’t in a straight line?
The systems hasn’t remained closed?
88
What does a closed system depend on?
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
What is closure temperature?
The temperature below which the isotopes of interest become fixed in specific minerals in a rock
90
What is the closure temperature of the Rb/Sr system?
300-500 degrees C
Comparable to medium to high grade metamorphic conditions
91
What is affected during a metamorphic event in a Rb/Sr?
Each rock Sr isotope composition os homogenised
The pluton is not homogenised
92
Using Rb/Sr system on a isochron, how do you tell the age of crystallisation of a rock?
Based on the whole rock sample
93
Based on an isochron diagram, how do you identify the age of metamorphism?
1 whole rock and all its grains
94
What are the three types of short lived radionuclides?
those generated constantly by decay of more long-lived systems
extinct radionuclides
cosmogenic radionuclides
95
How do extinct radionuclides form?
formed via primordial processes - stellar and supernova nucleosynthesis
short half lives compared to the age of our solar system so are now 'extinct'
96
How are cosmogenic radionuclides formed?
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
What is the energy of cosmic rays and solar winds?
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
what are the isotopes of Carbon?
12C
13C
Both Stable
14C unstable
99
what is the formation of radiocarbon?
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
what is the radiocarbon cycle?
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
what is the decay of radiocarbon?
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
to what extent is 14C measurable?
in a sample ~ 8 times the half life = 45,000yrs
with accelerator mass spectrometer ~60,000 - 75,000 yrs
103
what is the value we use for carbon 14 halflife?
libby = 5568yrs
104
what is the decay constant used for 14C?
Libbys constant = 1.244x10^-4
105
How are radiocarbon ages reported?
In years before 1950
106
What are the three assumptions of calculating radiocarbon ages?
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
What is the activity of 14C?
13.56 dpm/g of C
disintegrations per min
108
What is the reason of using the Libby values instead of true values?
so data is comparable
109
What causes variation in atmospheric 14C?
natural and anthropogenic forcing
110
What are the effects of secular changes in 14C production?
Suess effect
deVries effect
bomb pulse
111
What is the deVries effect?
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
What is the hazard of assuming a 14C activity of 13.56?
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
What is the Suess effect?
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
What is the bomb pulse?
A sudden increase in the concentration of 14C in the atmosphere peaking around 1965 from nuclear bomb testing generating neutrons
115
What is a benefit of the bomb pulse in dating?
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
How are human tissues dated?
Hair samples have 14C which reflect atmosphere at time of death
Enamel which is not replaced used to estimate time of birth
117
How do we calibrate the 14C timescale?
tree rings, corals
118
What are example of radiocarbon applications?
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
