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
- š°ļø Prior to the 1900s, geologists used relative dating to estimate the Earth's age, resulting in varied estimates from millions to billions of years.
- š The uncertainty in relative dating was due to the difficulty in determining the time represented by missing time unconformities.
- š The advent of radiometric dating provided a more accurate and detailed timeline of Earth's history when combined with relative dating methods.
- š Radiometric dating, or absolute dating, is based on the nuclear decay of radioactive nuclides to determine the exact age of rocks.
- š For radiometric dating to be effective, rocks must form closed systems with no exchange of atoms with the environment after formation.
- š The best candidates for radiometric dating are igneous rocks that have cooled quickly and not been reheated above the blocking temperature.
- š« Sedimentary rocks cannot be directly dated using absolute dating, but their constituent mineral grains, like zircon crystals, can be.
- ā°ļø Metamorphic rocks are challenging for radiometric dating due to the potential for hot fluids to alter parent and daughter isotopes during metamorphism.
- š Radiometric dating was first documented in 1907 by Dr. Bertram Boltwood, who discovered the decay of uranium into lead.
- š¬ 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.
- š 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
š 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.
š¬ 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
š”Relative dating
š”Radiometric dating
š”Unconformities
š”Nuclear decay
š”Isotopes
š”Half-life
š”Blocking temperature
š”Zircon crystals
š”Metamorphic rocks
š”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 thatĀ prior to the early 1900s, geologists usedĀ Ā
relative dating to estimate the age of theĀ Earth. These estimates varied dramatically,Ā Ā
ranging from millions to billions of years.Ā The main reason for this uncertainty is thatĀ Ā
there is no accurate way to determine how muchĀ missing time unconformities represent, but onceĀ Ā
radiometric dating was employed, the combinationĀ of the two dating methods produced a very detailedĀ Ā
and accurate timeline of Earthās history. Radiometric dating, or absolute dating,Ā Ā
is a method of extracting exact ages from rocksĀ that utilizes the concept of nuclear decay, whereĀ Ā
radioactive nuclides emit a high energy particleĀ to become a nuclide of some other element.Ā Ā
In order for radiometric dating to be applicable,Ā certain prerequisite criteria must be met. RocksĀ Ā
must form and then become closed systems whereĀ there is no exchange of atoms between the rockĀ Ā
and its environment. The best rocks with whichĀ to use radiometric dating are igneous rocks thatĀ Ā
cooled quicky and have not been reheated above theĀ blocking temperature, which is the temperature atĀ Ā
which parent and daughter isotopes can be lostĀ to the environment. Sedimentary rocks cannot beĀ Ā
dated using absolute dating, but their constituentĀ mineral grains can be. For example, in a previousĀ Ā
tutorial we mentioned the Jack Hills Conglomerate,Ā which was deposited about 3 billion years ago,Ā Ā
but it contains the oldest terrestrialĀ material found on Earth, in the form ofĀ Ā
4.4-billion-year-old zircon crystals. MetamorphicĀ rocks are difficult to date with radiometricĀ Ā
dating because the process of metamorphism canĀ involve hot fluids, which can add or remove parentĀ Ā
and daughter isotopes that are used to date theĀ rock. But for many rocks it is highly reliable,Ā Ā
so letās learn more about how this works. Radiometric dating was first documented in 1907Ā Ā
by Dr. Bertram Boltwood after he discovered thatĀ uranium decays into lead, and that the uranium toĀ Ā
lead ratio present in a rock would vary based onĀ the rockās age. Weāve discussed nuclear decay inĀ Ā
some detail over in the general chemistryĀ series, so check out this tutorial if youĀ Ā
need a thorough refresher on nuclear stability,Ā nuclear reactions, and applications. Otherwise,Ā Ā
letās reiterate the main concepts andĀ contextualize them. The utility of radiometricĀ Ā
dating is based on the existence of isotopes,Ā or atoms of the same element with differingĀ Ā
numbers of neutrons. Carbon, for example, hasĀ three naturally-occurring isotopes: carbon-12,Ā Ā
carbon-13, and carbon-14. Of these, only carbon-14Ā is radioactive, due to an unfavorable proton toĀ Ā
neutron ratio. It will therefore break down intoĀ nitrogen-14 via beta decay, or the emission ofĀ Ā
an electron which causes a neutron to becomeĀ a proton, thereby transmuting the nuclide. AllĀ Ā
radioactive isotopes decay at a specific rate thatĀ is represented by the isotopeās decay constant,Ā Ā
or number of disintegrations per year, which canĀ be used to calculate its half-life, or the amountĀ Ā
of time it takes for half of the radioactiveĀ parent nuclide to decay into the daughter nuclide.Ā Ā
The consistency of this half-life is extremelyĀ reliable, and does not depend on any aspect ofĀ Ā
the environment. It will be the same everywhereĀ in the universe, and under any set of conditions.Ā Ā
And because the isotopes of a given element alsoĀ have a very reliable natural abundance, which weĀ Ā
use to determine atomic mass, it is a relativelyĀ straightforward matter to compare some isotopicĀ Ā
ratio in a particular geologic material with theĀ naturally occurring ratio in matter which is beingĀ Ā
formed and freely exchanged with its environment,Ā like the way that carbon-14 is produced by cosmicĀ Ā
rays from the sun and maintains a nearly constantĀ concentration in the atmosphere. This comparisonĀ Ā
allows us to determine how many half-lives haveĀ elapsed, and therefore the age of the object.Ā
Each radioisotope has a unique time period overĀ which it is useful for dating, which is relatedĀ Ā
to its half-life. Namely, the parent must haveĀ decayed enough to produce a measurable amountĀ Ā
of the daughter isotope, but not so much that theĀ parent has almost totally disintegrated. Here areĀ Ā
a few of the most commonly used parent-daughterĀ pairs and the ages over which they are useful.Ā Ā
Carbon-14 and nitrogen-14, 300 to 50,000 years.Ā Potassium-40 and argon-40, 100,000 to 4.3 billionĀ Ā
years. Uranium-238 and lead-206, 1 million to 4.5Ā billion years. Potassium-argon dating is used toĀ Ā
date rock with potassium-containing minerals, suchĀ as biotite and potassium feldspar. Uranium-leadĀ Ā
dating is often used to date the oldest materialsĀ on Earth, primarily because these old materialsĀ Ā
are almost exclusively zircon grains, which takeĀ up small amounts of uranium, but do not take upĀ Ā
lead, meaning that all the lead which can be foundĀ in zircon accumulates by the decay of uranium.Ā Ā
This is an important assumption that must be madeĀ in order to do some types of radiometric dating,Ā Ā
which is that the material being analyzed did notĀ contain any isotopes of the daughter element atĀ Ā
the time it was crystallized. However,Ā isochron dating does not require this.Ā
Letās now go over how the radiometric datingĀ process works. First, a suitable rock and anĀ Ā
isotope system must be chosen. Letās sayĀ our hypothetical rock contains biotite asĀ Ā
its only source of potassium, so we will use theĀ potassium-argon system. Geologists must then checkĀ Ā
the rockās minerals with a microscope to ensureĀ that the rock has not been hydrothermally alteredĀ Ā
in any way. The sample is then analyzed by a massĀ spectrometer to determine the amount of radiogenicĀ Ā
argon, which can then be used to calculate the ageĀ using the following formula, where 40Art is theĀ Ā
current amount of radiogenic argon, 40Art0 is theĀ amount of argon-40 at the time of crystallization,Ā Ā
40Kt0 is the amount of potassium-40 at the timeĀ of crystallization, Ī» is the decay constant,Ā Ā
which is 5.543 x 10-10, and Ī»_e/Ī» is the fractionĀ of potassium-40 that decays via electron capture,Ā Ā
which is 0.1048, as potassium-40 can alsoĀ decay to calcium-40 through beta emission.Ā Ā
Next, because argon is not taken upĀ into biotite during crystallization,Ā Ā
we can assume that 40Art0 is zero. We canĀ also calculate 40Kt0 simply by knowing theĀ Ā
amount of potassium in biotite, whichĀ is 4.49 x 10-4 moles potassium per gramĀ Ā
biotite and multiplying it by the fraction ofĀ potassium 40 in nature, which is 1.19 x 10-4.Ā Ā
After rearranging the equation we arrive at this,Ā and we can now solve for t. A rock that containsĀ Ā
5 x 10-10 moles of argon per gram of biotiteĀ would have a radiogenic age of 154 million years.Ā
There are other isotope pairs, such asĀ rubidium-strontium and uranium-lead, where age isĀ Ā
determined by analyzing the relevant isotopes ofĀ each mineral of a rock and graphing them. In theĀ Ā
case of rubidium-strontium, rubidium-87 breaksĀ down into strontium-87 through negatron decay,Ā Ā
another way of saying beta emission. Here,Ā rubidium-87 is the parent isotope, strontium-87Ā Ā
is the daughter isotope, and strontium-86 is theĀ non-radiogenic isotope of the daughter element.Ā Ā
Making a graph with the strontium-87 to 86 ratioĀ on the y-axis and the rubidium-87 to strontium-86Ā Ā
ratio on the x-axis, and plotting the valuesĀ for the various minerals contained in the rock,Ā Ā
creates a straight line called an isochron.Ā Steeper slopes indicate older samples whereĀ Ā
more decay has occurred over a long periodĀ of time. The exact age can be determinedĀ Ā
by dividing the slope of the isochronĀ by the decay constant. Furthermore,Ā Ā
the y-intercept of the isochron gives the ratio ofĀ strontium 87 to 86 at the time of crystallization.Ā
So that covers some basic information regardingĀ relative and absolute dating methods, which weĀ Ā
can use to determine the age of geologicĀ features, and even the Earth itself. ItĀ Ā
is quite astounding that we are able to probeĀ nature to such a sophisticated degree that weĀ Ā
are able to get answers to questions that haveĀ profound philosophical impact, such as the ageĀ Ā
of the world we live on, but this is simply aĀ testament to the power of scientific inquiry,Ā Ā
and the efforts of those who dedicate themselvesĀ to expanding the breadth of scientific knowledge.
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