Methods of Dating the Earth Part 2: Absolute Dating (Radiometric Dating)
Summary
TLDRThis tutorial delves into the evolution of dating methods used by geologists to determine Earth's age. Initially, relative dating provided wide-ranging estimates, from millions to billions of years. The advent of radiometric dating, leveraging nuclear decay, revolutionized the field by offering precise age determinations. Key criteria for successful radiometric dating include rocks forming closed systems post-formation. The script explains the process using potassium-argon and uranium-lead dating, highlighting the importance of isotopes and their decay constants for calculating half-lives. It also introduces isochron dating, which doesn't require the assumption of no initial daughter isotopes. The narrative underscores the reliability of these methods and their profound impact on our understanding of Earth's history.
Takeaways
- 🕰️ Prior to the 1900s, geologists used relative dating to estimate the Earth's age, resulting in varied estimates from millions to billions of years.
- 🔍 The uncertainty in relative dating was due to the difficulty in determining the time represented by missing time unconformities.
- 📈 The advent of radiometric dating provided a more accurate and detailed timeline of Earth's history when combined with relative dating methods.
- 🌋 Radiometric dating, or absolute dating, is based on the nuclear decay of radioactive nuclides to determine the exact age of rocks.
- 💠 For radiometric dating to be effective, rocks must form closed systems with no exchange of atoms with the environment after formation.
- 🌋 The best candidates for radiometric dating are igneous rocks that have cooled quickly and not been reheated above the blocking temperature.
- 🚫 Sedimentary rocks cannot be directly dated using absolute dating, but their constituent mineral grains, like zircon crystals, can be.
- ⛰️ Metamorphic rocks are challenging for radiometric dating due to the potential for hot fluids to alter parent and daughter isotopes during metamorphism.
- 📚 Radiometric dating was first documented in 1907 by Dr. Bertram Boltwood, who discovered the decay of uranium into lead.
- 🔬 The process of radiometric dating involves selecting a suitable rock and isotope system, ensuring the rock's minerals have not been altered, and analyzing them with a mass spectrometer.
- 📈 Isochron dating is a method that does not require the assumption of no initial daughter isotopes, making it useful for certain types of rocks.
Q & A
What was the main issue with relative dating before the advent of radiometric dating?
-The main issue with relative dating was the dramatic variation in estimates of Earth's age, ranging from millions to billions of years, due to the inaccuracy in determining the time represented by missing time unconformities.
How does radiometric dating, or absolute dating, provide more accurate age estimates compared to relative dating?
-Radiometric dating provides exact ages from rocks by utilizing the concept of nuclear decay, where radioactive nuclides emit a high-energy particle to become a nuclide of another element. This method allows for a detailed and accurate timeline of Earth's history.
What are the prerequisite criteria for rocks to be suitable for radiometric dating?
-For radiometric dating, rocks must form and then become closed systems with no exchange of atoms between the rock and its environment. The best candidates are igneous rocks that cooled quickly and have not been reheated above the blocking temperature.
Why are sedimentary rocks not suitable for direct radiometric dating, and what alternative is used instead?
-Sedimentary rocks are not suitable for direct radiometric dating because they cannot be dated using absolute dating methods. However, their constituent mineral grains, such as zircon crystals, can be dated.
How does metamorphism affect the reliability of radiometric dating of metamorphic rocks?
-Metamorphism can involve hot fluids that can add or remove parent and daughter isotopes, making radiometric dating of metamorphic rocks difficult and less reliable.
Who first documented radiometric dating, and what was the key discovery that enabled this method?
-Dr. Bertram Boltwood first documented radiometric dating in 1907 after discovering that uranium decays into lead, and that the uranium to lead ratio in a rock varies based on the rock's age.
What is the significance of isotopes in the context of radiometric dating?
-Isotopes, or atoms of the same element with differing numbers of neutrons, are crucial in radiometric dating because they decay at a specific rate, represented by the isotope's decay constant, which can be used to calculate its half-life.
Why is the half-life of a radioactive isotope considered extremely reliable in radiometric dating?
-The half-life of a radioactive isotope is extremely reliable because it does not depend on any aspect of the environment and remains constant everywhere in the universe under any conditions.
What is the importance of isotopic ratios in determining the age of a geologic material?
-Isotopic ratios are important because they allow comparison with naturally occurring ratios in matter that is being formed and freely exchanged with its environment, enabling the determination of how many half-lives have elapsed and thus the age of the object.
How does isochron dating differ from other forms of radiometric dating?
-Isochron dating does not require the assumption that the material being analyzed did not contain any isotopes of the daughter element at the time it was crystallized, unlike other forms of radiometric dating.
Can you explain the process of determining the age of a rock using the potassium-argon dating system as described in the script?
-To determine the age of a rock using the potassium-argon system, geologists analyze the amount of radiogenic argon in the rock using a mass spectrometer. They use a formula involving the current amount of radiogenic argon, the decay constant, and the fraction of potassium-40 that decays via electron capture to calculate the rock's age.
Outlines
📚 Introduction to Radiometric Dating
This paragraph introduces the concept of radiometric dating, also known as absolute dating, which provides precise age estimates for rocks by measuring the decay of radioactive isotopes. Prior to the advent of radiometric dating, geologists relied on relative dating methods that led to a wide range of age estimates for the Earth. The paragraph explains that radiometric dating requires rocks to be closed systems with no exchange of atoms, making igneous rocks ideal for this method. It also discusses the limitations of dating sedimentary and metamorphic rocks, and highlights the historical significance of radiometric dating's development in 1907 by Dr. Bertram Boltwood. The paragraph further delves into the principles of nuclear decay, half-life, and the importance of isotopes in dating processes.
🔬 Radiometric Dating Process and Isochron Dating
The second paragraph elaborates on the process of radiometric dating, emphasizing the selection of suitable rocks and isotope systems. It uses the potassium-argon system as an example to explain how geologists analyze rocks for age determination. The paragraph details the steps involved in the analysis, including the use of a mass spectrometer to measure radiogenic argon and the application of a formula to calculate the rock's age. Additionally, it introduces isochron dating, a method that does not require the assumption of no initial daughter isotopes, and explains how isochron graphs are used to determine the age of rocks by plotting the ratios of relevant isotopes. The paragraph concludes by reflecting on the profound impact of these dating methods on our understanding of the Earth's age and the power of scientific inquiry.
Mindmap
Keywords
💡Relative dating
💡Radiometric dating
💡Unconformities
💡Nuclear decay
💡Isotopes
💡Half-life
💡Blocking temperature
💡Zircon crystals
💡Metamorphic rocks
💡Isochron dating
Highlights
Before the early 1900s, geologists relied on relative dating methods, resulting in varied age estimates for the Earth.
The uncertainty in relative dating was due to the inability to accurately determine the time represented by unconformities.
Radiometric dating, introduced later, combined with relative dating to produce a detailed and accurate timeline of Earth's history.
Radiometric dating, also known as absolute dating, uses nuclear decay to determine the exact age of rocks.
For radiometric dating to be effective, rocks must form closed systems with no exchange of atoms with the environment.
Igneous rocks that cooled quickly and have not been reheated above the blocking temperature are ideal for radiometric dating.
Sedimentary rocks cannot be directly dated using absolute dating, but their constituent mineral grains can be.
Metamorphic rocks are challenging for radiometric dating due to the potential for hot fluids to alter parent and daughter isotopes.
Radiometric dating was first documented in 1907 by Dr. Bertram Boltwood, who discovered uranium decays into lead.
The uranium to lead ratio in a rock can be used to determine its age, based on the concept of nuclear decay.
Radiometric dating is based on the existence of isotopes, which are atoms of the same element with different numbers of neutrons.
Carbon-14 is a radioactive isotope of carbon that decays into nitrogen-14, and is used for dating materials up to 50,000 years old.
The decay constant of an isotope, representing its rate of decay, can be used to calculate its half-life.
The half-life of an isotope is consistent and does not depend on environmental conditions.
Each radioisotope has a unique time period over which it is useful for dating, related to its half-life.
Potassium-argon dating is used for rocks with potassium-containing minerals, and uranium-lead dating for the oldest materials, like zircon grains.
An important assumption in radiometric dating is that the material analyzed did not contain any isotopes of the daughter element at the time of crystallization.
Isochron dating does not require the assumption of no initial daughter isotopes, offering an alternative approach.
The process of radiometric dating involves choosing a suitable rock and isotope system, ensuring the rock's minerals have not been altered, and analyzing with a mass spectrometer.
The age of a rock can be calculated using the formula involving the current amount of radiogenic argon, the decay constant, and the fraction of potassium-40 that decays.
Other isotope pairs like rubidium-strontium are used for dating, where the age is determined by analyzing the relevant isotopes and plotting them to create an isochron.
The isochron method allows for the determination of the age of a rock by the slope of the line and the y-intercept, which gives the ratio of isotopes at the time of crystallization.
The ability to date geological features and the Earth itself is a testament to the power of scientific inquiry and the dedication of scientists.
Transcripts
In the previous tutorial we learned that prior to the early 1900s, geologists used
relative dating to estimate the age of the Earth. These estimates varied dramatically,
ranging from millions to billions of years. The main reason for this uncertainty is that
there is no accurate way to determine how much missing time unconformities represent, but once
radiometric dating was employed, the combination of the two dating methods produced a very detailed
and accurate timeline of Earth’s history. Radiometric dating, or absolute dating,
is a method of extracting exact ages from rocks that utilizes the concept of nuclear decay, where
radioactive nuclides emit a high energy particle to become a nuclide of some other element.
In order for radiometric dating to be applicable, certain prerequisite criteria must be met. Rocks
must form and then become closed systems where there is no exchange of atoms between the rock
and its environment. The best rocks with which to use radiometric dating are igneous rocks that
cooled quicky and have not been reheated above the blocking temperature, which is the temperature at
which parent and daughter isotopes can be lost to the environment. Sedimentary rocks cannot be
dated using absolute dating, but their constituent mineral grains can be. For example, in a previous
tutorial we mentioned the Jack Hills Conglomerate, which was deposited about 3 billion years ago,
but it contains the oldest terrestrial material found on Earth, in the form of
4.4-billion-year-old zircon crystals. Metamorphic rocks are difficult to date with radiometric
dating because the process of metamorphism can involve hot fluids, which can add or remove parent
and daughter isotopes that are used to date the rock. But for many rocks it is highly reliable,
so let’s learn more about how this works. Radiometric dating was first documented in 1907
by Dr. Bertram Boltwood after he discovered that uranium decays into lead, and that the uranium to
lead ratio present in a rock would vary based on the rock’s age. We’ve discussed nuclear decay in
some detail over in the general chemistry series, so check out this tutorial if you
need a thorough refresher on nuclear stability, nuclear reactions, and applications. Otherwise,
let’s reiterate the main concepts and contextualize them. The utility of radiometric
dating is based on the existence of isotopes, or atoms of the same element with differing
numbers of neutrons. Carbon, for example, has three naturally-occurring isotopes: carbon-12,
carbon-13, and carbon-14. Of these, only carbon-14 is radioactive, due to an unfavorable proton to
neutron ratio. It will therefore break down into nitrogen-14 via beta decay, or the emission of
an electron which causes a neutron to become a proton, thereby transmuting the nuclide. All
radioactive isotopes decay at a specific rate that is represented by the isotope’s decay constant,
or number of disintegrations per year, which can be used to calculate its half-life, or the amount
of time it takes for half of the radioactive parent nuclide to decay into the daughter nuclide.
The consistency of this half-life is extremely reliable, and does not depend on any aspect of
the environment. It will be the same everywhere in the universe, and under any set of conditions.
And because the isotopes of a given element also have a very reliable natural abundance, which we
use to determine atomic mass, it is a relatively straightforward matter to compare some isotopic
ratio in a particular geologic material with the naturally occurring ratio in matter which is being
formed and freely exchanged with its environment, like the way that carbon-14 is produced by cosmic
rays from the sun and maintains a nearly constant concentration in the atmosphere. This comparison
allows us to determine how many half-lives have elapsed, and therefore the age of the object.
Each radioisotope has a unique time period over which it is useful for dating, which is related
to its half-life. Namely, the parent must have decayed enough to produce a measurable amount
of the daughter isotope, but not so much that the parent has almost totally disintegrated. Here are
a few of the most commonly used parent-daughter pairs and the ages over which they are useful.
Carbon-14 and nitrogen-14, 300 to 50,000 years. Potassium-40 and argon-40, 100,000 to 4.3 billion
years. Uranium-238 and lead-206, 1 million to 4.5 billion years. Potassium-argon dating is used to
date rock with potassium-containing minerals, such as biotite and potassium feldspar. Uranium-lead
dating is often used to date the oldest materials on Earth, primarily because these old materials
are almost exclusively zircon grains, which take up small amounts of uranium, but do not take up
lead, meaning that all the lead which can be found in zircon accumulates by the decay of uranium.
This is an important assumption that must be made in order to do some types of radiometric dating,
which is that the material being analyzed did not contain any isotopes of the daughter element at
the time it was crystallized. However, isochron dating does not require this.
Let’s now go over how the radiometric dating process works. First, a suitable rock and an
isotope system must be chosen. Let’s say our hypothetical rock contains biotite as
its only source of potassium, so we will use the potassium-argon system. Geologists must then check
the rock’s minerals with a microscope to ensure that the rock has not been hydrothermally altered
in any way. The sample is then analyzed by a mass spectrometer to determine the amount of radiogenic
argon, which can then be used to calculate the age using the following formula, where 40Art is the
current amount of radiogenic argon, 40Art0 is the amount of argon-40 at the time of crystallization,
40Kt0 is the amount of potassium-40 at the time of crystallization, λ is the decay constant,
which is 5.543 x 10-10, and λ_e/λ is the fraction of potassium-40 that decays via electron capture,
which is 0.1048, as potassium-40 can also decay to calcium-40 through beta emission.
Next, because argon is not taken up into biotite during crystallization,
we can assume that 40Art0 is zero. We can also calculate 40Kt0 simply by knowing the
amount of potassium in biotite, which is 4.49 x 10-4 moles potassium per gram
biotite and multiplying it by the fraction of potassium 40 in nature, which is 1.19 x 10-4.
After rearranging the equation we arrive at this, and we can now solve for t. A rock that contains
5 x 10-10 moles of argon per gram of biotite would have a radiogenic age of 154 million years.
There are other isotope pairs, such as rubidium-strontium and uranium-lead, where age is
determined by analyzing the relevant isotopes of each mineral of a rock and graphing them. In the
case of rubidium-strontium, rubidium-87 breaks down into strontium-87 through negatron decay,
another way of saying beta emission. Here, rubidium-87 is the parent isotope, strontium-87
is the daughter isotope, and strontium-86 is the non-radiogenic isotope of the daughter element.
Making a graph with the strontium-87 to 86 ratio on the y-axis and the rubidium-87 to strontium-86
ratio on the x-axis, and plotting the values for the various minerals contained in the rock,
creates a straight line called an isochron. Steeper slopes indicate older samples where
more decay has occurred over a long period of time. The exact age can be determined
by dividing the slope of the isochron by the decay constant. Furthermore,
the y-intercept of the isochron gives the ratio of strontium 87 to 86 at the time of crystallization.
So that covers some basic information regarding relative and absolute dating methods, which we
can use to determine the age of geologic features, and even the Earth itself. It
is quite astounding that we are able to probe nature to such a sophisticated degree that we
are able to get answers to questions that have profound philosophical impact, such as the age
of the world we live on, but this is simply a testament to the power of scientific inquiry,
and the efforts of those who dedicate themselves to expanding the breadth of scientific knowledge.
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