Dating_MethodsLecture

Report
Dating Techniques
• Four Categories
–
–
–
–
Radio-isotope methods
Paleomagnetic methods
Organic/inorganic chemical methods
Biological methods
• Relative dating:
– Chronological succession (e.g., dendrochronology).
– Synchronous events (e.g. volcanic ash).
• Absolute dating:
– Recognition of time-dependent processes (e.g.,
radioactivity).
Radio-isotopic Method
• Based on disintegration of unstable nuclei
– Negatron decay (n
p+ + b- + energy)
– Positron decay (p+ n + b+ + energy)
– Alpha decay
(AX
A-4Y
+ He)
Radioactivity-Concepts
• Half-life (t1/2 ):
N= N0/2
• Mean life:
t=1/l
• Activity: # radioactive disintegrations/sec (dps)
• Specific activity:
dps/wt. or dps/vol
• Units: Becquerel (Bq) =1 dps
• Decay Rates:
Ln (No/N) = lt
t = t*Ln (No/N)
To be a useful for dating, radioisotopes must:
•
•
•
•
•
be measurable
have known rate of decay
have appropriate t1/2
have known initial concentrations
be a connection between event and
radioisotope
Radioactivity-based Dating
• Quantity of the radio-isotope relative to its
initial level (e.g., 14C).
• Equilibrium /non-equilibrium chain of
radioactive decay (e.g., U-series).
• Physical changes on sample materials
caused by local radioactive process (e.g.,
fission track).
Radiocarbon Dating
• 12C: 42*1012; 13C: 47*1010; 14C: 62 tons
• t1/2 = 5730 yr
• l= 1.0209*10-4/yr
• Formed in the atmosphere:
14N
+ 1n
14C
• Decay:
14C
14N
+ b-
+ 1H
W.F. Libby’s discovery of
radiocarbon
• S. Korff’s discovery:
cosmic rays generate
~2 neutrons/cm2sec
• 14C formed through
nuclear reaction.
• 14C readily oxidizes
with O2 to form 14CO2
• Libby’s t1/2 = 5568 yr.
Conventional Radiocarbon
Dating
•
•
•
•
Current t1/2 = 5730±40 yr
t=8033*Ln(Asample/Astandard), where A:activity.
Oxalic acid is the standard (prepared in 1950).
Dates reported back in time relative to 1950
(radiocarbon yr BP).
• Astandard in 1950 = 0.227 Bq/g
• Astandard in 2000 = 0.225 Bq/g
Conventional Radiocarbon dating
• Activity of 14C needs to be “normalized” to
the abundance of carbon:
• D14C: “normalized value”
 D14C(‰) = d14C –2(d13C+25)(1+d13C/103)
 d14C(‰) = (1-Asample/Astandard)*103
• Radiocarbon age = 8033*ln(1+ D14C/103)
Conventional Radiocarbon dating
• Precision has increased
• Radiocarbon
disintegration is a random
process.
• If date is 5000±100:
• 68% chance is 4900-5100
• 99% chance is 4700-5300
Radiocarbon dating-Problems
Radiocarbon dating-Corrections
• Radiocarbon can be
corrected by using
tree-ring chronology.
• Radiocarbon dates can
then be converted into
“Calendar years” (cal
yr).
Radiocarbon dating-Problems
• Two assumptions:
– Constant cosmic ray intensity.
– Constant size of exchangeable carbon reservoir.
• Deviation relative to dendrochronology due to:
– Variable 14C production rates.
– Changes in the radiocarbon reservoirs and rates of
carbon transfer between them.
– Changes in total amount of CO2 in atmosphere,
hydrosphere, and atmosphere.
Deviation of the initial radiocarbon activity.
Bomb-radiocarbon
Nuclear testing significantly increased D14C
Bomb 14C can be used as a tracer
Radiocarbon dating-conclusion
• Precise and fairly accurate (with adequate
corrections).
• Useful for the past ~50,000 yr.
• Widespread presence of C-bearing
substrates.
• Relatively small sample size (specially for
AMS dates).
• Contamination needs to be negligible.
Other Radio-isotopes
• K-Ar
– 40K simultaneously decays to 40Ca and 40Ar(gas)
– t1/2=1.3*109 yr (useful for rocks >500 kyr
– Amount of 40Ar is time-dependent
– Problems:
• Assumes that no 40Ar enters or leaves the system
• Limited to samples containing K
• U-series
Other radio-isotopes
• Uranium series
– 236U and 238U decay to 226Ra and 230Th
– U is included in carbonate lattice (e.g., corals)
– Age determined on the abundance of decay
products
– Problems:
• Assumes a closed system
• Assumes known initial conditions.
Thermo-luminescence (TL)
• TL is light emitted from a crystal when it is
heated.
• TL signal depends on # e- trapped in the crystal.
• Trapped e- originate from radioactive decay of
surrounding minerals.
• TL signal is proportional to time and intensity.
• Useful between 100 yr and 106 yr
TL-Applications
• Archaeological artifacts
– Heating (>500oC) re-sets TL signal to zero
– Used for dating pottery and baked sediments
• Sediments
– Exposure to sunlight re-sets the “clock”
– Used for dating loess, sand dunes, river sand.
TL-Problems
• Different response to ionization
– # lattice defects
– saturation
• Incomplete re-setting
• Water can absorb radiation
• Unknown amount of ionization
Fission-Track Dating
• 238U can decay by spontaneous fission
• Small “tracks” are created on crystals
(zircon, apatite, titanite) and volcanic glass.
• Track density is proportional to U-content
and to time since the crystal formed.
• Useful for dating volcanic rocks (>200 kyr)
• Problem: tracks can “heal” over time

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