Methods of Dating the Earth Part 2: Absolute Dating (Radiometric Dating)

Summary

This 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

00:00

📚 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.

05:05

🔬 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.

Keywords

💡Relative dating

Relative dating is a technique used by geologists to determine the age of rocks and the order of geological events without necessarily determining their absolute age. It's based on the observation that in a sequence of undisturbed sedimentary rocks, the oldest rocks are at the bottom and the youngest are at the top. This concept is crucial in the video as it sets the stage for understanding the limitations prior to the advent of radiometric dating. For example, the script mentions that before the early 1900s, geologists used relative dating, which led to varied and imprecise estimates of Earth's age.

💡Radiometric dating

Radiometric dating, also known as absolute dating, is a method of determining the age of rocks and minerals based on the decay of naturally occurring radioactive isotopes. It provides a more precise age estimate compared to relative dating. The video emphasizes the significance of radiometric dating by explaining how it revolutionized the understanding of Earth's history by offering a detailed and accurate timeline. The script describes how radiometric dating works and its prerequisites, such as the rock being a closed system with no exchange of atoms.

💡Unconformities

Unconformities are gaps in the geological record where there is evidence of a period of time missing from the rock layers. They represent periods during which no deposition occurred or during which the existing rock layers were eroded away. The video script highlights the challenge that unconformities pose to relative dating, as they make it difficult to determine the exact amount of time represented by the missing intervals, contributing to the uncertainty in age estimates.

💡Nuclear decay

Nuclear decay is a process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. This concept is fundamental to radiometric dating, as it relies on the predictable decay of radioactive isotopes. The video script uses the example of uranium decaying into lead to illustrate how radiometric dating can determine the age of rocks by measuring the ratio of parent to daughter isotopes.

💡Isotopes

Isotopes are variants of a particular chemical element which differ in neutron number, and hence in nucleon number. The video script explains that isotopes are crucial for radiometric dating because they allow scientists to measure the decay of a radioactive parent isotope into a stable daughter isotope. The script mentions carbon-14, potassium-40, and uranium-238 as examples of radioactive isotopes used in dating.

💡Half-life

The half-life of a radioactive isotope is the time it takes for half of the atoms of the isotope to decay. This concept is central to radiometric dating because it allows scientists to calculate the age of a rock sample by measuring the ratio of parent to daughter isotopes and knowing the half-life of the parent isotope. The video script explains that the half-life is a consistent value that does not depend on external conditions, which makes it a reliable tool for dating.

💡Blocking temperature

The blocking temperature is the temperature at which isotopes can be lost from a mineral due to diffusion, effectively resetting the radiometric clock. This concept is important in the video as it explains why certain rocks, like those that have been reheated above their blocking temperature, are not suitable for radiometric dating. The script mentions that igneous rocks that cooled quickly and have not been reheated above this temperature are ideal for radiometric dating.

💡Zircon crystals

Zircon crystals are a type of mineral that can be dated using radiometric methods and are known for containing some of the oldest material on Earth. The video script refers to zircon crystals found in the Jack Hills Conglomerate, which are 4.4 billion years old, to illustrate the potential of radiometric dating in determining the age of Earth's oldest materials.

💡Metamorphic rocks

Metamorphic rocks are formed from the transformation of existing rock types, often due to heat, pressure, or hot fluids. The video script discusses the challenges of dating metamorphic rocks using radiometric methods because the metamorphic process can alter the isotopic composition of the rock, potentially adding or removing parent and daughter isotopes.

💡Isochron dating

Isochron dating is a method used in radiometric dating that plots the ratios of parent and daughter isotopes to determine the age of a rock. Unlike some other dating methods, isochron dating does not assume that the rock initially contained no daughter isotopes. The video script explains that isochron dating is useful for certain types of rocks and provides a way to verify the age of rocks without making initial assumptions about their isotopic composition.

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.