Samenvatting
Summary Sedimentology and Stratigraphy chapter 21: Dating an correlation techniques
- Instelling
- Universiteit Utrecht (UU)
Samenvatting Sedimentology and Stratigraphy Gary Nichols Chapter 21: Dating and correlation techniques
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Chapter 21: Dating and correlation techniques“Radiometric dating techniques have been available to geologists since the discovery of radioactivity
in the early part of the 20th century and they have provided an absolute scale of millions and billions
of years for events in Earth history. Several different radioactive decay series are used for dating
igneous rocks. Dating a grain in a sandstone bed usually provides the age when the mineral originally
formed in, say, a granite, and does not provide much information about the age of the sedimentary
rock. The magnetic and chemical properties of rocks can be used to carry out magnetostratigraphic
and chemo-stratigraphic correlation, techniques that are mainly used in combination with other
methods. In practice the whole process of dating and correlating rocks relies on the integration of
information from a number of different sources and techniques. Dating in the Quaternary is a
specialist area using a different range of techniques, including carbon-14 dating, which can be used to
date organic material formed in the past few tens of thousands of years.”
21.1 Dating and correlation techniques
Lithology does not provide a temporal framework, only indicated which rock units are younger and
which are older. Biostratigraphy correlates strata on both local and region scales.
Radiometric dating provided a framework that is measured in years. New developments make it
possible to make measurements which higher precision on smaller samples of material.
Magnetostratigraphy and chemo-stratigraphy are also techniques that have benefited from
technological advances, with more sensitive magnetometers and chemical analysis being carried out
to higher precision.
21.2 Radiometric dating
Uses the decay of isotopes of elements present in minerals as a measure of the age of the rock:
Rate of decay must be known
Proportion of different isotopes present when the material formed has to be assumed
Proportion of different isotopes present today must be measured
Determining age of igneous rocks, including volcanic units that occur within sedimentary strata. Also,
usable on authigenic minerals in some sedimentary rocks. Radiometric dating of minerals in
metamorphic rocks usually indicates the age of the metamorphism.
21.2.1 Radioactive decay series
A number of elements have unstable isotopes (forms of the element that have different atomic
masses). These change by radioactive decay to an isotope of a different element. Radioactive half-life
= time needed for half of the original isotope (parent) to decay to the new isotope (daughter). Half-
lifes of tens of thousands or thousands of millions of years are interesting. By measuring the
proportion between parent and daughter, periods of geological time can be calculated.
Information needed:
Half life (l?) decay constant
Amount of parent and daughter present in the rock when it formed (N 0)
Present proportions (N)
N= N0 e-lt
, 1 N
t= ∗ln 0 +1
l N ( )
It is assumed that when a mineral crystallizes out of a magma, only parent isotopes are present.
Assumed that only the parent is present when a mineral is precipitated chemically in sediment or
forms by solid-state recrystallisation in a metamorphic rock. The radiometric clock starts as the
mineral is formed.
All daughters formed as a result of decay. Not always true, they can be added or lost due to
weathering, diageneses and metamorphism. Try to ensure that it happened in a closed system, by
using only unweathered and unaltered material. The choice of method depends on the abundance of
the elements and by its age.
21.2.2 Practical radiometric dating
Large rocks to eliminate inhomogeneities in the rock.
Rocks are crushed and thoroughly mixed.
If a specific mineral is to be dated, these are separated by
o Using heavy liquids, some float others sink
o Magnetic separation
The mineral concentrate may then be dissolved for isotopic or elemental analysis, expect for
argon isotope analysis, in which case the mineral grains are heated in a vacuum and the
composition of the argon gas driven off is measured directly.
Mass spectrometer: measures the concentration of different isotopes. The sample is heated in a
vacuum to ionize the isotopes. These particles are accelerated by a potential difference. Part-way
along the tube a magnetic field induced by an electromagnet deflects the charged particles. The
amount of deflection depends on their mass. Detectors on the end will record the number of charged
particles of a particular atomic mass and provide a ratio of the isotopes present in the sample.
21.2.2 Potassium-argon and argon-argon dating
Most widely used, because it can be used on potassium-rich authigenic mineral glauconite and
volcanic rocks that contain potassium in minerals such as some feldspars and micas.
40
❑ K ( proton becomes neutron=electroncapture )→ 40
❑ Ar
Other product is 40-Ca. half-life of this decay is 11.93 billion years. Potassium is very common in the
crust, and therefore easily measured. However 40-K potassium is only 0.012% and most of this decays
to 40-Ca, only 11% becomes 40-Ar. Argon is an inert gas and the isotopes of very small quantities of
argon can be measured by a mass spectrometer by driving the gas out of the minerals.
K-Ar is widely used, with 1 major problem, daughter isotope can escape by diffusion because it is a
gas. Therefor the amount of Ar is commonly less than the total amount produced. This results in an
underestimation of the age of the rock.
This problem can be overcome by using Ar-Ar method.
1. Irradiation of the sample by neutron bombardment to form 39-Ar from 39-K occurring in the
rock.
The ratio 39-K to 40-K is known constant so if the amount of 39-Ar produced from 39-K can
be measured, this provides an indirect method of calculating the 40-K present.
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