Radiometric dating of igneous rocks
- 8.4 Isotopic Dating Methods
- How are igneous and sedimentary rocks dated?
- How do you determine the age of an igneous rock?
- How are sedimentary rocks related to the radiometric time scale?
- How is the age of a rock determined by radiometric dating?
- How do igneous and sedimentary rocks form?
- How do scientists date igneous rocks?
- Can sedimentary rocks be dated directly?
- What is the temperature at which igneous rocks are formed?
8.4 Isotopic Dating Methods
The presupposition of long ages is an icon and foundational to the evolutionary model. Nearly every textbook and media journal teaches that the earth is billions of years old. Using radioactive dating, scientists have determined that the Earth is about 4. The earth is now regarded as between 4. The primary dating method scientists use for determining the age of the earth is. Proponents of publicize radioisotope dating as a reliable and consistent method for obtaining absolute ages of rocks and the age of the earth.
This apparent consistency in textbooks and the media has convinced many Christians to accept an old earth 4. What Is Radioisotope Dating? Radioisotope dating also referred to as radiometric dating is the process of estimating the age of rocks from the decay of their radioactive elements.
There are certain kinds of atoms in nature that are unstable and spontaneously change decay into other kinds of atoms. For example, uranium will radioactively decay through a series of steps until it becomes the stable element lead. Likewise, potassium decays into the element argon. The original element is referred to as the parent element in these cases uranium and potassiumand the end result is called the daughter element lead and argon.
The Importance of Radioisotope Dating The straightforward reading of Scripture reveals that the days of creation Genesis 1 were literal days and that the earth is just thousands of years old and not billions.
There appears to be a fundamental conflict between the Bible and the reported ages given by radioisotope dating. However, rather than accept the biblical account of creation, many Christians have the radioisotope dates of radiometric dating of igneous rocks of years and attempted to fit long ages into the Bible. The implications of doing this are profound and affect many parts of the. How Radioisotope Dating Works Radioisotope dating is commonly used to date igneous rocks.
These are rocks which form when hot, molten material cools and solidifies. Types of radiometric dating of igneous rocks rocks include granite and basalt lava. These types of rocks are comprised of particles from many preexisting rocks which were transported mostly by water and redeposited somewhere else.
Types of sedimentary rocks include sandstone, shale, and limestone. Uranium to lead decay sequence Uranium-238 Thorium-234 Protactinium-234 Uranium-234 Thorium-230 Radium-226 Radon-222 Polonium-218 Lead-214 Bismuth-214 Polonium-214 Lead-210 Bismuth-210 Polonium-210 Lead-206 stable Uranium-238 238U is an isotope of uranium.
Isotopes are varieties of an element that have the same number of protons but a different number of neutrons within the nucleus. For example, carbon-14 14C is a particular isotope. All carbon atoms have 6 protons but can vary in the number of neutrons. Extra neutrons often lead to instability, or radioactivity. Likewise, all isotopes varieties of uranium have 92 protons.
It is unstable and will radioactively decay radiometric dating of igneous rocks into 234Th thorium-234 and finally into 206Pb lead-206. Sometimes a radioactive decay will cause an atom to lose 2 protons and 2 neutrons called alpha decay. For example, the decay of radiometric dating of igneous rocks into 234Th is an alpha decay process. In this case the atomic mass changes 238 to 234. Atomic mass is the heaviness of an atom when compared to hydrogen, which is assigned the value of one.
Another type of decay is called beta decay. In beta decay, either an electron is lost and a neutron is converted into a proton beta minus decay or an electron is added and a proton is converted into a neutron beta plus decay. In beta decay the total atomic mass does not change significantly. The decay of 234Th into 234Pa protactinium-234 radiometric dating of igneous rocks an example of beta decay. The radioisotope dating clock starts when a rock cools.
During the molten state it is assumed that the intense heat will force any gaseous daughter elements like argon to escape. Once the rock cools it is assumed that no more atoms can escape and any daughter element found in a rock will be the result of radioactive decay. The dating process then requires measuring how much daughter element is in a rock sample and knowing the decay rate i.
The decay rate is measured in terms of half-life. Half-life is defined as the length of time it takes half of the remaining atoms of a radioactive parent element to decay.
Half-lives as measured today are very accurate, even the extremely slow half-lives. That is, billion-year half-lives can be measured statistically in just hours of time. The following table is a sample of different element half-lives.
Parent Daughter Half-life Polonium-218 Lead-214 3 minutes Thorium-234 Protactinium-234 24 days Carbon-14 Nitrogen-14 5,730 years Potassium-40 Argon-40 1. Dating methods must also rely on another kind of radiometric dating of igneous rocks called historical science. Historical radiometric dating of igneous rocks cannot be observed. Determining the conditions present when a rock first formed can only be studied through historical science.
Determining how the environment might have affected a rock also falls under radiometric dating of igneous rocks science. Neither condition is directly observable. We can use scientific techniques in the present, combined with assumptions about historical events, to estimate the age.
Therefore, there are several assumptions that must be made in radioisotope dating. The Hourglass Illustration Radioisotope dating can be better understood using an illustration with an hourglass.
If we walk into a room and observe an hourglass with sand at the top and sand at the bottom, we could calculate how long the hourglass has been running. By estimating how fast the sand is falling and measuring the amount of sand at the bottom, we could calculate how much time has elapsed since the hourglass was turned over. All our calculations could be correct observational sciencebut the result could be wrong.
This is because we failed to take into account some critical assumptions. Unlike the open-system nature of a rock, this is not possible for a sealed hourglass.
Since we did not observe the initial conditions when the hourglass time started, we must make assumptions. All three of these assumptions can affect our time calculations. If scientists fail to consider each of these three critical assumptions, then radioisotope dating can give incorrect ages. The Facts We know that radioisotope dating does not always work because we can test it on rocks of known age. In 1997, a team of eight research scientists known as the RATE group Radioisotopes and the Age of The Earth set out to investigate the assumptions commonly made in standard radioisotope dating practices also referred to as single-sample radioisotope dating.
Their findings were significant and directly impact the evolutionary dates of millions of years. A rock sample from the newly formed 1986 lava dome from Mount St. Helens was dated using Potassium-Argon dating. The newly formed rock gave ages for the different minerals in it of between 0. These dates show that significant argon daughter element was present when the rock solidified assumption 1 is false. Eleven samples were taken from solidified lava and dated. These rocks are known to have formed from eruptions in 1949, 1954, and 1975.
The rock samples were sent to a respected commercial laboratory Geochron Laboratories in Cambridge, Massachusetts. Because these rocks are known to be less than 70 years old, it is apparent that assumption 1 is again false. When radioisotope dating fails to give accurate dates on rocks of known age, why should we trust it for rocks of unknown age? In each case the ages of the rocks were greatly inflated. Isochron Dating There is another form of dating called isochron dating, which involves analyzing four or more samples from the same rock unit.
This form of dating attempts to eliminate one of the assumptions in single-sample radioisotope dating by using ratios and graphs rather than radiometric dating of igneous rocks atoms present. It does not depend on the initial concentration of the daughter element being zero.
The isochron dating technique is thought to be infallible because it supposedly eliminates the assumptions about starting conditions. However, this method has different assumptions about starting conditions and can give incorrect dates. If single-sample and isochron dating methods are objective and reliable they should agree. However, they frequently do not. When a rock is dated by more than one method it may yield very different ages.
For example, the RATE group obtained radioisotope dates from ten different locations. To omit any potential bias, the rock samples were analyzed by several commercial radiometric dating of igneous rocks. In each radiometric dating of igneous rocks, the isochron dates differed substantially from the single-sample radioisotope dates. In some cases the range was more than 500 million years.
If different methods yield different ages and there are variations with the same method, how can scientists know for sure the age of any rock or the age of the earth?
In one specific case, samples were taken from the Cardenas Basalt, which is among the oldest strata in the eastern Grand Canyon. Next, samples from the western Canyon basalt lava flows, which are among the youngest formations in the canyon, were analyzed.
Using the rubidium-strontium isochron dating method, an age of 1. The youngest rocks gave a billion year age the same as the oldest rocks! Are the dates given in textbooks and journals accurate and objective? When assumptions are taken into consideration and discordant disagreeing or unacceptable dates are not omitted, radioisotope dating often gives inconsistent and inflated ages.
Two Case Studies The RATE team selected two locations to collect rock samples to conduct analyses using multiple radioisotope dating methods. Both sites are understood by geologists to date from the Precambrian supposedly 541—4,600 million years ago.
All rock samples whole rock and separate minerals within the rock were analyzed using four radioisotope methods. These included the isotopes potassium-argon K-Arrubidium-strontium Rb-Srsamarium-neodymium Sm-Ndand lead-lead Pb-Pb. In order to avoid any bias, the dating procedures were contracted out radiometric dating of igneous rocks commercial laboratories located in Colorado, Massachusetts, and Ontario, Canada.
In order to have a level of confidence in dating, different radioisotope methods used to date a rock sample should closely coincide in age. When this occurs, the sample ages are said to be concordant. In contrast, if multiple results for a rock disagree with each other in age they are said to be discordant. Beartooth Mountains Sample Results Geologists believe the Bearthooth Mountains rock unit to contain some of the oldest rocks in the United States, with an estimated age of 2,790 million years.
The following table summarizes the RATE results. Dating Isotopes Millions of Years Type of Data whole rock or separate mineral within the rock Potassium-Argon single-sample 1,520 2,011 2,403 2,620 Quartz-plagioclase mineral Whole rock Biotite mineral Hornblende mineral Rubidium-Strontium isochron 2,515 2,790 5 minerals Previously published result based on 30 whole rock samples 1982 Samarium-Neodymium isochron 2,886 4 minerals Lead-Lead isochron 2,689 5 minerals The results show a significant scatter in the ages for the various minerals and also between the isotope methods.
In some cases, radiometric dating of igneous rocks whole rock age is greater than the age of the minerals, and for others, the reverse occurs. The potassium-argon mineral results vary between 1,520 and 2,620 million years a difference of 1,100 million years.
Bass Rapids Sill Sample Results The 11 Grand Canyon rock samples were also dated commercially using the most advanced radioisotope technology.
The generally accepted age for this formation is 1,070 million years. The RATE results are summarized in the following table. Dating Isotopes Millions of Years Type of data whole rock or separate mineral within the rock Potassium-Argon 841. Especially noteworthy is the multiple whole rocks potassium-argon isochron age of 841. Possible Explanations for the Discordance There are three possible explanations for the discordant isotope dates. There are ways to determine if this has occurred and can be eliminated as a possible explanation.
However, there is no evidence that lava cools and solidifies in the same place at such an incredibly slow pace. Therefore this explanation can be eliminated. The following section will show that this provides the best explanation for the discordant ages. New Studies New studies by the RATE group have provided evidence that radioactive decay supports a young radiometric dating of igneous rocks.
One of their studies involved radiometric dating of igneous rocks amount of helium found in granite rocks. Granite contains tiny zircon crystals, which contain radioactive uranium 238Uwhich decays into lead 206Pb. During this process, for each atom of radiometric dating of igneous rocks decaying into 206Pb, eight helium atoms are formed and migrate out of the zircons and granite rapidly. Within the zircon crystals, any helium atoms generated by nuclear decay in the distant past should have long ago migrated outward and escaped from these crystals.
One would expect the helium gas to eventually diffuse upward out of the ground and then disappear into the atmosphere. The decay of 238U into lead is a slow process half-life of 4. Since helium migrates out of rocks rapidly, there should be very little to no helium remaining in the zircon crystals.
Why is so much helium still in the granite? One likely explanation is that sometime in the past the radioactive decay rate was greatly accelerated. The decay rate was accelerated so much that helium was being produced faster than it could have escaped, causing an abundant amount of helium to remain in the granite.
The RATE group has gathered evidence that at some time in history nuclear decay was greatly accelerated. The experiments the RATE project commissioned have clearly confirmed the numerical predictions of our Creation model. The data and our analysis show that over a billion years worth of nuclear decay has occurred very recently, between 4000 and 8000 years ago. Confirmation of this accelerated nuclear decay having occurred is provided by adjacent uranium and polonium radiohalos that formed at the same time in the same biotite flakes in granites.
However, because the daughter polonium atoms are only short-lived for example, polonium-218 decays within 3 minutes, compared to 4. But in order to supply the needed polonium atoms to produce these polonium radiohalos within that timeframe, the nearby uranium atoms had to decay at an accelerated rate. The RATE group suggested that this accelerated decay took place during the Week or during the Flood. Accelerated decay of this magnitude would result in immense amounts of heat being generated in rocks.
Determining how this heat was dissipated presents a new and exciting opportunity for creation research. Conclusion The best way to learn about history and the age of the earth is to consult the history book of the universe—the.
Many scientists and theologians accept a straightforward reading of Scripture and agree that the earth is about 6,000 years old. Based on the measured helium retention, a statistical analysis gives an estimated age for the zircons of 6,000 ± 2,000 years. This age agrees with literal biblical history and is about 250,000 times shorter than the conventional age of 1.
The conclusion is that helium diffusion data strongly supports the radiometric dating of igneous rocks view of history. Johnson Biology: Visualizing Life, Holt, Rinehart, and Winston, Austin, Texas, 1998, p. McGeary, Physical Geology, McGraw Hill, New York, 2006, pp.
DeYoung,Master Books, Green Forest, Arkansas, 2005. Austin, Excess argon within mineral concentrates from the new dacite lava dome at Mount St Helens volcano, Creation Ex Nihilo Technical Journal 10 3 : 1996 pp. Snelling, Isochron discordances and the role of inheritance and mixing of radioisotopes in the mantle and crust, in Vardiman et al. Joseph, Missouri, 2005, pp. Austin, Do radioisotope clocks need repair?
Testing the assumptions of isochron dating using K-Ar, Rb-Sr, Sm-Nd, and Pb-Pb isotopes, in Vardiman et al. Hoesch, Radioisotopes in the diabase sill Upper Precambrian at Bass Rapids, Grand Canyon, Arizona: an application and test of the isochron dating methods, in R. Austin, in Vardiman et al. Humphreys, Young helium diffusion age of zircons supports accelerated nuclear decay, in Vardiman et al.
Snelling, Radiohalos in granites: evidence of accelerated nuclear decay, in Vardiman et al. DeYoung,2005, pp.
How are igneous and sedimentary rocks dated?
Most of the large igneous rock masses of the world have been dated in this manner. Most sedimentary rocks such as sandstone, limestone, and shale are related to the radiometric time scale by bracketing them within time zones that are determined by dating appropriately selected igneous rocks, as shown by a hypothetical example.
How do you determine the age of an igneous rock?
Thus an igneous or metamorphic rock or melt, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the closure temperature. The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature.
How are sedimentary rocks related to the radiometric time scale?
Most sedimentary rocks such as sandstone, limestone, and shale are related to the radiometric time scale by bracketing them within time zones that are determined by dating appropriately selected igneous rocks, as shown by a hypothetical example.
How is the age of a rock determined by radiometric dating?
The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature. This temperature varies for every mineral and isotopic system, so a system can be closed for one mineral but open for another.
How do igneous and sedimentary rocks form?
Igneous rocks form when molten rock (magma or lava) cools and solidifies. Sedimentary rocks originate when particles settle out of water or air, or by precipitation of minerals from water.
How do scientists date igneous rocks?
Scientists date igneous rock using elements that are slow to decay, such as uranium and potassium. By dating these surrounding layers, they can figure out the youngest and oldest that the fossil might be; this is known as “bracketing” the age of the sedimentary layer in which the fossils occur.
Can sedimentary rocks be dated directly?
ANSWER: Sedimentary rocks cannot be dated directly using radiometric dating, which is based on the idea that when rocks are in liquid form, their radiometric clock resets.
What is the temperature at which igneous rocks are formed?
Igneous rocks are formed as a result of the cooling and solidification of magma or lava. Temperatures of igneous rocks formation ranges from 650-800 o C (for felsic/rhyolitic magma) to 1000-1200 o C (for mafic/ basaltic magma). The maximum temperature of formation of sedimentary rocks is considered to be 200 o C.