Download Radiometric Dating - mercerislandschools.org

Radiometric Dating
Relative dating techniques are based on __________________ principles can be
used to differentiate the relative age rock units and landforms. Relative dating
techniques by themselves cannot be used to assign independent __________________
ages.
Numerical or Radiometric dating allows geologist to assign actual ages to a deposit or
landform. Many of the methods are based on the ratio of radioactive
________________ and their respective decay products measured in a material to be
dated.
In many cases relative dating techniques are used in tandem with numerical dating
techniques to constrain the age of a landform or deposit. This is called
“_____________________.”
Unit A (225 m.y. old)
Unit B (how old is it?)
Unit C (275 m.y. old)
Bracketing
Dating of
volcanic ash
Certain layers in rock, particularly
volcanic deposits like ash or lava
flows, are more useful for
radiometric dating techniques
than others. For example, a
volcanic ash layer:
• Represent a very __________
period of time in geologic
history
• Might be _____________ in an
area
• Contains radioactive elements
that are _____________ for
radiometric dating.
Isotopes
Fig. 3–3
• Isotopes = 2 atoms of the same
element (same atomic #) that
have different numbers of
neutrons, and therefore
different _______________.
• Most elements have one
isotope that is prevalent.
• Many elements have isotopes
that are radioactive.
• All isotopes with the same
atomic mass (number of
_____________) are the same
element
• The number given before or
after the element symbol is the
atomic ______________
Examples:
H, D, T (see fig 3-3)
Carbon 12 & Carbon 14 (12C & 14C)
U-235 & U-238
Stable vs. Radioactive isotopes
• The atomic
_____________
(number of
protons)
determines what
element it is (i.e.
Carbon will
always have 6
protons)
• Different elements
may have the
same mass,
depending on the
number of
_____________.
Radioactivity
Radioactive decay refers to
the process in which an
atomic nucleus of an
unstable atom loses energy
by emitting
_______________ particles.
The radioactive parent atom
is converted to
_______________ atom
which can be stable, or
radioactive (subject to
further decay).
Radioactive decay occurs
via three decay processes:
Alpha Decay, Beta
Emission, Electron Capture.
For the radioactive decay process to be useful for finding the age of a rock
or fossil, the daughter isotope must only be able to be created through the
___________________ of the parent isotope.
Otherwise, there would be no way to tell how much daughter was
produced from the original radioactive parent isotope.
Age Equation
D = Do + N(eλt-1)
D= number of daughter atoms
N = number of existing parent
atoms
Do = number of initial daughter
atoms
e = exponential function
λ = decay constant
t = time
• Radioactive isotopes will decay spontaneously to form daughter atoms.
Radioactive decay is _________________________________. Different
radioactive elements will have different decay rates, but the decay curve will
be the same (purple curve).
• As radioactive parent atoms decay, daughter atoms are correspondingly
created (blue curve). If you can measure the ____________ of parent to
daughter atoms in a isotope system, you can solve for the age (T).
Radioactive Decay Processes
Simple Decay: represents a
decay process where a
radioactive isotope will decay to a
___________ radiogenic
daughter atom. For example,
radiocarbon (14C) will always
decay to nitrogen (14N).
Branched Decay: represents a
decay process where the
radioactive isotope can decay to
_________________radiogenic
daughter atom. For example 40K
can decay to either 40Ca (88.2% of
the time) or 40Ar (11.8% of the time).
Radioactive Decay
Unstable isotopes undergo radioactive decay to form
other isotopes, & in the process energy is emitted in
three forms:
alpha particle: __ protons + __ neutrons (He nucleus)
beta particle: fast–moving, ________________
gamma rays: high–energy electromagnetic
______________
Alpha decay __________________ the atomic
number and mass of the element, since a 2 P and 2
N are emitted.
Beta electron decay (ß-) does not ____________ the
mass number. The atomic number ______________
because the electron is produced by the decay of a
neutron which results in a ______________.
Half-Life: The amount of time that
it takes half of a radioactive sample
to decay to something else.
Determining amount of
radioactive isotope remaining
1 half-life
2 half- lives
3 half-lives
4 half-lives
½ (1/21)
¼ (1/22)
1/
3)
(1/2
8
1/
4)
(1/2
16
Formula: N = No (0.5)t
t= number of half-lives
N = amount left No = original amount
http://www.colorado.edu/physics/2000/isotopes/radioactive_decay3.html
Radioactive Decay Processes
A ___________________involves the
radioactive decay of intermediate
radioactive daughter atoms that
eventually decay to stable daughter
such as the decay of 238U to 206Pb.
T1/2 of U-238 = 4.5 billion years
T1/2 of U-235 = 703.8 million years
The decay constant and half-life (The half life is the time it takes one half of the
existing parent atoms to decay to daughter atoms) of a radionuclide are related
λ = decay constant
mathematically:
T1/2 = 0.693/λ
Generally, the greater the half-life of a radioactive isotope pair the greater the
_________________for dating purposes, but the less useful it is for more
___________ events.
Potassium-Argon dating can be used to date potassium-rich basalt flows that exceed
_____________ years in age. 40K will also decay to stable 40Ca by beta decay (but
that ratio is not used for dating purposes- Why?)
K-Ar Dating
40Ca
is the stable
and common form
of Calcium. It
would be
impossible to
determine how
much 40Ca was
produced from the
decay of 40K.
K-Ar dating was utilized to assign chronology of early hominid finds in northern
Ethiopia. Paleomagnetic data can be used to fill in gaps within the chronology.
______________________is used to date the age of the basaltic ocean floor.
Although the K content of the ocean basalts is relatively low, there is enough
potassium to yield meaningful ages.
In the 1950’s and 1960’s “magnetic stripes” were discovered by geophysicists
along the ocean floor. These stripes represent fluctuating polarity changes of the
Earth’s magnetic field preserved in the basaltic ocean crust. The reversals have
been dated using K-Ar dating, producing a paleomagnetic time scale.
K-Ar dating has been very important in
assigning ages to the timing of polarity
changes and establishing the paleomagnetic
time scale. The paleomagnetic time scale is
subdivided into “__________” which represent
intervals of time defined by a given magnetic
polarity.
The steep front of this Hawaiian basalt flow represents an ice-contact front. The
basalt flow abutted against glacial ice and its K-Ar age provides constraints on the
timing of glacial advance on the Big Island of Hawaii.
Up-thrown
Block
Down-dropped
Block
The range front fault (trace shown by dotted line) exposed along the eastern Sierra
Nevada cross-cuts a basaltic cinder cone that was dated using K-Ar dating. How
could this age be used to constrain the uplift rate along this fault? ______________
Radiocarbon Dating
• Radiocarbon is first produced in the
atmosphere by collisions of ________
with nitrogen atoms (Nitrogen has 7
protons and 7 neutrons in its nucleus).
• The neutron will knock out a _______
from the nitrogen atom’s nucleus.
How many protons will now be present
in the atom’s nucleus?
• _________
• How many neutrons? ________
• The carbon atom is now radioactive
14C (radiocarbon) with a half-life of
5730 years. It will decay back to 14N
via Beta decay.
• Organisms keep a constant level of
14C while they are alive, but once they
die, the level of 14C drops as it decays
to 14N.
Radiocarbon Dating
Radiocarbon Decay Activity
14C
decays to 14N as it emits a beta (β) particle (____________). “Modern” carbon
(1850 A.D.) has a decay activity of ~15 dpm/gm. The decay activity (Beta emission
rate) will decrease by 50% every half-life (5,730 years). Radiocarbon ages can be
determined for organic matter by directly counting β-emissions.
What will the approximate
decay activity be for a
sample that is 6000 years
old?
_____ dpm/gm
12,000 years old?
______ dpm/gm
After about _____________
years, there is not enough
radicarbon left to have
enough activity to measure
(see earlier chart).
dpm = disintegrations/minute
Organic samples are prepared for radiocarbon dating by combusting (paper or
wood) or dissolving (calcite in shells) the organics and producing CO2 gas (stored in
glass containers). The decay activity in the CO2 gas will be directly related to the
_____________________of the organic material.
Improvement in accelerator mass spectrometry (AMS) have allowed geochronologists
to directly measure the 14C to stable carbon(12C and 13C) ratio. The ratio of 14C to 12C
or 13C will be reduced by _______ after each half-life. AMS 14C dating requires a much
smaller organic sample (milligrams versus grams) than the beta-counting method.
Most 14C analyses today are measured using AMS.
Why do you think we do not measure the ratio of parent (14C) to daughter (14N) ratio to
determine a 14C age?
Radiocarbon dating is useful for assigning ages to sediment that may incorporate
organics during erosion and deposition, such as this log present in glacial till.
How is the glacial till age related to the age of the log incorporated into the till?
___________
Radiocarbon dating is useful for assigning ages to young (<50,000 years old)
volcanic ash layers, such as the Mazama (Crater Lake) tephra shown above.
Why can’t we simply date the 6800 year old Mazama ash using K-Ar dating?
______________________