1. RADIOACTIVE DATING (U-Pb)
NAME OF THE STUDENTS:- NANDINI(5509) AND SATVIK SHARMA(5531)
COURSE AND YEAR:- B.Sc.(H) GEOLOGY,1ST YEAR
CONCEPT OF STRATIGRAPHY ASSIGNMENT
SESSION:- 2022-2026
2. WHY WE DATE IN GEOLOGY ?
Dating, in geology, determining a chronology or calendar of events in the
history of Earth, using to a large degree the evidence of organic evolution
in the sedimentary rocks accumulated through geologic time in marine
and continental environments. To date past events, processes, formations,
and fossil organisms, geologists employ a variety of techniques. These
include some that establish a relative chronology in which occurrences can
be placed in the correct sequence relative to one another or to some
known succession of events. Radiometric dating and certain other
approaches are used to provide absolute chronologies in terms of years
before the present. The two approaches are often complementary, as
when a sequence of occurrences in one context can be correlated with an
absolute chronology elsewhere.
3. RADIOACTIVE DATING
Radiometric dating, radioactive dating or radioisotope dating is a
technique which is used to date materials such as rocks or carbon, in which
traces radioactive impurities were selectively incorporates when they are
formed. Means of determining the age of certain materials by reference to
the relative abundances of the parent isotope (which is radioactive) and
the daughter isotope (which may or may not be radioactive). If the decay
constant (the half-life or disintegration rate of the parent isotope) and the
concentration of the daughter isotope are known, it is possible to calculate
an age. Isotopic dating is the measurement of time using the decay of
radioactive isotopes and accumulation of decay products at a known rate.
With isotopic chronometers, we determine the time of the processes that
fractionate parent and daughter element.
4.
5. PRINCIPLES OF RADIOACTIVE DATING
All absolute isotopic ages are based on radioactive decay, a process whereby a
specific atom or isotope is converted into another specific atom or isotope at a
constant and known rate. Most elements exist in different atomic forms that are
identical in their chemical properties but differ in the number of neutral particles—
i.e., neutrons—in the nucleus. For a single element, these atoms are called isotopes.
Because isotopes differ in mass, their relative abundance can be determined if the
masses are separated in a mass spectrometer
Radioactive decay can be observed in the laboratory by either of two means: (1) a
radiation counter (e.g., a Geiger counter), which detects the number of high-energy
particles emitted by the disintegration of radioactive atoms in a sample of geologic
material, or (2) a mass spectrometer, which permits the identification of daughter
atoms formed by the decay process in a sample containing radioactive parent
atoms. The particles given off during the decay process are part of a profound
fundamental change in the nucleus. To compensate for the loss of mass (and
energy), the radioactive atom undergoes internal transformation and in most cases
simply becomes an atom of a different chemical element.
6. LAW OF RADIOACTIVE DECAY
The law of radioactive decay states that at any instant the rate of radioactive
disintegration is directly proportional to the number of nuclei present in it, at that
instance of time. Radioactive decay is the process by which an unstable atomic
nucleus loses its energy due to radiation. It is a random process at the level of
single atoms.
7. HALF LIFE
Half-life (symbol t1⁄2) is the time required for a quantity to reduce to half of its
initial value. The term is commonly used in nuclear physics to describe how quickly
unstable atoms undergo radioactive decay or how long stable atoms survive.
It is important to understand half lives as they help you to estimate whether a
sample of radioactive material is safe to handle. When a sample of radioactivity
falls below detaction limits, it is considered safe.
This happen after ten half lives
8. VARIOUS METHODS OF RADIOACTIVE
DATING
Techniques used to date materials such as rocks by observing the abundance of
naturally occurring radioactive isotope and it’s decay products.
ISOTOPIC DATING METHODS ;-
Uranium-Lead method
Potassium-Argon method
Rubidium-Strontium method
Carbon Dating method etc.
We will discuss about uranium-lead method in the most elaborated way in the
following slides.
9. URANIUM-LEAD METHOD OF DATING
ROCKS
Uranium-lead dating is one of the oldest and if done properly one of the most
accurate.
Uranium comes as two common isotopes, U235 and U238.
Both are unstable and radioactive, shedding nuclear particles in a cascade that
doesn’t stop until they becomes lead(Pb).
The two cascades are different- U235 becomes Pb207 (half life-704 million
years) and U238 becomes Pb206 (half life- 4.47 billion years).
10. Lead atoms created by uranium decay are trapped in the crystal and
build up in concentration with time; helping us in dating.
The favourite mineral among U-Pb dates is zircon(ZrSiO4) for several
good reason’.
Some zircons are obviously disturbed and can be ignored, while other
cases are harder to judge. In these cases, the Concordia diagram is a
valuable too;.
Uranium- lead dating works only for metamorphic and igneous rock.
11.
12. ADVANTAGES OF U-Pb DATING METHOD
Uranium–lead dating, abbreviated U–Pb dating, is one of the oldest and most
refined of the radiometric dating schemes. It can be used to date rocks that
formed and crystallised from about 1 million years to over 4.5 billion years
ago with routine precisions in the 0.1–1 percent range.
This method should also be applied only to minerals that remained in a closed
system with no loss or gain of the parent or daughter isotope. Uranium-Lead
(U-Pb) dating is the most reliable method for dating Quaternary sedimentary
carbonate and silica, and fossils particulary outside the range of radiocarbon.
By allowing the establishment of geological timescales, it provides a significant
source of information about the ages of fossils and the deduced rates of
evolutionary change
13. LIMITATIONS OF U-Pb DATING METHOD
The limitation of using U238 dating are that there must be enough of the
parent material in the rock originally to produce measurable results and the
rock cannot be a sedimentary rock.
Additionally, if there is not enough of U-238 when the rock formed, the
measurement will provide inconclusive and unssuportable data.
Sedimentary rocks can often contain U238 and Pb206 but the results of dating
of sedimentary rocks are problematic.
14. A RESEARCH PAPER
EXPLAINING DIFFICULTIES
OF DATING ZIRCON.
In a paper published this week in
science, Geochemist Roland Mundil of
the Berkeley Gechronology Center
(BGC) and his colleagues at BGC and UC
Berkeley report that U-Pb can be
extremely accurate- to within 250,000
years but only if the zircons from
volcanic ash used in the analysis are
specially treated. To date, zircons-
known to many as a semiprecious stone
and December’s birthstone have often
produced confusing and inaccurate
results.
15. CONCLUSION
Radiometric or radioisotopic dating methods are one of the most
effective tools in age determination.
They are even used to date fossils.
Radiometric dating methods other than the ones listed above are;
samarium 147 to neodylum-143, cosmogenic nuclide dating and so
on.
Uranium–lead dating, abbreviated U–Pb dating, is one of the oldest
and most refined of the radiometric dating schemes.
