How Old is the Earth: Radiometric Dating

How Old is the Earth

sm-nd dating

Radiometric dating is commonly used on igneous rocks lava , and on some sedimentary minerals. For example, lavas taken from the ocean bottom off the island [sic] of Hawaii on a submarine extension of the east rift zone of Kilauea volcano gave an age of 22 million years, but the actual flow happened less than years ago. Slusher and Morris 92 advanced this argument in an attempt to show that the K-Ar method is unreliable, but the argument is a red herring. Instead, I describe briefly only the three principal methods. In fact, some sources say that Sr and Ar have similar mobilities in rock, and Ar is very mobile. One example is the rocks from the Kaupelehu Flow, Hualalai Volcano in Hawaii which was known to have erupted in Rb and Sr are quite different elements and are incorporated into the various minerals in varying proportions according to the composition and structure of the minerals.

A Response to “Scientific” Creationism

We will also get a distribution of averaged values for samples in each period. If these dates are correct, this calls the Biblical account of a recent creation of life into question. Huxley , attacked Thomson's calculations, suggesting they appeared precise in themselves but were based on faulty assumptions. My experience is that whenever I look into an evidence for evolution or now the reliability of radiometric dating on the geologic column, it blows up on me, too. Thus, rubidium and strontium in minerals tend to be inversely correlated; minerals high in rubidium are generally low in strontium and vice versa. If a rock gives a too old date, one says there is excess argon.

The Earth is assumed to have an initial Pb composition identical with that of chondritic meteorites. The most primitive Pb isotopic composition measured is that of Canyon Diablo meteorite. The Geochron is the isochron that includes meteorites and the Earth. The modern-day basalt field overlaps with Geochron but extends to right, indicating progressive gain of U relative to Pb.

This method is not very accurate because the Pb isotopic compositions along the Geochron are changing only slowly with time and it is unlikely that the Pb within the galena had isotopic compositions that exactly matched that of the Geochron i.

OIB or plume basalts have very radiogenic Pb, consistent with their derivation from recycled oceanic crust. U is thought to be preferentially incorporated into the oceanic crust during hydrothermal alteration on the ocean floor and during subduction.

Both U and Pb have somewhat similar chemistries so these elements are not strongly fractionated from each other during magmatic or hydrothermal processes unless a high-U accessory mineral, such as zircon Zr2SiO4 or baddeleyite ZrO2 , is crystallising. These elements are also generally mobile during hydrothermal metamorphism and alteration. The rate of change of the abundance of the daughter isotope Pb is low because U has largely decayed away. Furthermore, only one Pb isotope Pb is unaffected by radiogenic decay, so it is not possible to precisely correct Pb isotope ratios for instrumentally induced mass fractionation.

Mistakes do occur but they are usually caught by the various checks employed in the well-designed experiment. The Rb-Sr method is based on the radioactivity of 87 Rb, which undergoes simple beta decay to 87 Sr with a half-life of Rubidium is a major constituent of very few minerals, but the chemistry of rubidium is similar to that of potassium and sodium, both of which do form many common minerals, and so rubidium occurs as a trace element in most rocks.

Because of the very long half-life of 87 Rb, Rb-Sr dating is used mostly on rocks older than about 50 to million years. This method is very useful on rocks with complex histories because the daughter product, strontium, does not escape from minerals nearly so easily as does argon.

As a result, a sample can obey the closed-system requirements for Rb-Sr dating over a wider range of geologic conditions than can a sample for K-Ar dating. Unlike argon, which escapes easily and entirely from most molten rocks, strontium is present as a trace element in most minerals when they form. For this reason, simple Rb-Sr ages can be calculated only for those minerals that are high in rubidium and contain a negligible amount of initial strontium.

In such minerals, the calculated age is insensitive to the initial strontium amount and composition. For most rocks, however, initial strontium is present in significant amounts, so dating is done by the isochron method, which completely eliminates the problem of initial strontium. In the Rb-Sr isochron method, several three or more minerals from the same rock, or several cogenetic rocks with different rubidium and strontium contents, are analyzed and the data plotted on an isochron diagram Figure 2.

The 87 Rb and 87 Sr contents are normalized to the amount of 86 Sr, which is not a radiogenic daughter product.

The intercept of this line with the ordinate represents the isotopic composition of the initial strontium. From then on, as each atom of 87 Rb decays to 87 Sr, the points will follow the paths 3 shown by the arrows. The intercept of the line on the ordinate gives the isotopic composition of the initial strontium present when the rock formed. Note that the intercepts of lines a-b-c and a'-b'-c' are identical, so the initial strontium isotopic composition can be determined from this intercept regardless of the age of the rock.

Note that the Rb-Sr isochron method requires no knowledge or assumptions about either the isotopic composition or the amount of the initial daughter isotope — in fact, these are learned from the method.

The rocks or minerals must have remained systems closed to rubidium and strontium since their formation; if this condition is not true, then the data will not plot on an isochron. Also, if either the initial isotopic composition of strontium is not uniform or the samples analyzed are not cogenetic, then the data will not fall on a straight line. As the reader can easily see, the Rb-Sr isochron method is elegantly self-checking. If the requirements of the method have been violated, the data clearly show it.

An example of a Rb-Sr isochron is shown in Figure 3 , which includes analyses of five separate phases from the meteorite Juvinas 3. The data form an isochron indicating an age for Juvinas of 4. This meteorite has also been dated by the Sm-Nd isochron method, which works like the Rb-Sr isochron method, at 4. The U-Pb method relies on the decays of U and U. These two parent isotopes undergo series decay involving several intermediate radioactive daughter isotopes before the stable daughter product, lead Table 1 , is reached.

If necessary, a correction can be made for the initial lead in these systems using Pb as an index. If these three age calculations agree, then the age represents the true age of the rock. Lead, however, is a volatile element, and so lead loss is commonly a problem.

As a result, simple U-Pb ages are often discordant. The U-Pb concordia-discordia method circumvents the problem of lead loss in discordant systems and provides an internal check on reliability. This method involves the U and U decays and is used in such minerals as zircon, a common accessory mineral in igneous rocks, that contains uranium but no or negligible initial lead.

This latter requirement can be checked, if necessary, by checking for the presence of Pb, which would indicate the presence and amount of initial lead.

The location of the point on concordia depends only on the age of the sample. If at some later date say, 2. At any time after the episodic lead loss say, 1. This chord is called discordia. If we now consider what would happen to several different samples, say different zircons, from the same rock, each of which lost differing amounts of lead during the episode, we find that at any time after the lead loss, say today, all of the points for these samples will lie on discordia.

The upper intercept of discordia with concordia gives the original age of the rock, or 3. There are several hypotheses for the interpretation of the lower intercept, but the most common interpretation is that it indicates the age of the event that caused the lead loss, or 1 billion years in Figure 4.

Note that this method is not only self-checking, but it also works on open systems. What about uranium loss? Uranium is so refractory that its loss does not seem to be a problem. If uranium were lost, however, the concordia-discordia plot would indicate that also.

The U-Pb concordia-discordia method is one of the most powerful and reliable dating methods available. It is especially resistant to heating and metamorphic events and thus is extremely useful in rocks with complex histories.

Quite often this method is used in conjunction with the K-Ar and the Rb-Sr isochron methods to unravel the history of metamorphic rocks, because each of these methods responds differently to metamorphism and heating.

For example, the U-Pb discordia age might give the age of initial formation of the rock, whereas the K-Ar method, which is especially sensitive to argon loss by heating, might give the age of the latest heating event.

An example of a U-Pb discordia age is shown in Figure 5. This example shows an age of 3. The K-Ar ages on rocks and minerals from this area in southwestern Minnesota also record this 1. This argument is specious and akin to concluding that all wristwatches do not work because you happen to find one that does not keep accurate time. Like any complex procedure, radiometric dating does not work all the time under all circumstances.

Each technique works only under a particular set of geologic conditions and occasionally a method is inadvertently misapplied. There are, to be sure, inconsistencies, errors, and results that are poorly understood, but these are very few in comparison with the vast body of consistent and sensible results that clearly indicate that the methods do work and that the results, properly applied and carefully evaluated, can be trusted. A few examples will demonstrate that their criticisms are without merit.

The creationist author J. He claims that these examples cast serious doubt on the validity of radiometric dating. The use of radiometric dating in Geology involves a very selective acceptance of data. Discrepant dates, attributed to open systems, may instead be evidence against the validity of radiometric dating. However, close examination of his examples, a few of which are listed in Table 2 , shows that he misrepresents both the data and their meaning.

The two ages from gulf coast localities Table 2 are from a report by Evernden and others These are K-Ar data obtained on glauconite, a potassium-bearing clay mineral that forms in some marine sediment. Woodmorappe fails to mention, however, that these data were obtained as part of a controlled experiment to test, on samples of known age, the applicability of the K-Ar method to glauconite and to illite, another clay mineral. He also neglects to mention that most of the 89 K-Ar ages reported in their study agree very well with the expected ages.

Evernden and others 43 found that these clay minerals are extremely susceptible to argon loss when heated even slightly, such as occurs when sedimentary rocks are deeply buried.

As a result, glauconite is used for dating only with extreme caution. The ages from the Coast Range batholith in Alaska Table 2 are referenced by Woodmorappe to a report by Lanphere and others Whereas Lanphere and his colleagues referred to these two K-Ar ages of and million years, the ages are actually from another report and were obtained from samples collected at two localities in Canada, not Alaska.

There is nothing wrong with these ages; they are consistent with the known geologic relations and represent the crystallization ages of the Canadian samples.

The Liberian example Table 2 is from a report by Dalrymple and others These authors studied dikes of basalt that intruded Precambrian crystalline basement rocks and Mesozoic sedimentary rocks in western Liberia. The dikes cutting the Precambrian basement gave K-Ar ages ranging from to million years Woodmorappe erroneously lists this higher age as million years , whereas those cutting the Mesozoic sedimentary rocks gave K-Ar ages of from to million years.

Woodmorappe does not mention that the experiments in this study were designed such that the anomalous results were evident, the cause of the anomalous results was discovered, and the crystallization ages of the Liberian dikes were unambiguously determined. The Liberian study is, in fact, an excellent example of how geochronologists design experiments so that the results can be checked and verified.

The final example listed in Table 2 is a supposed 34 billion-year Rb-Sr isochron age on diabase of the Pahrump Group from Panamint Valley, California, and is referenced to a book by Faure and Powell Again, Woodmorappe badly misrepresents the facts. The data do not fall on any straight line and do not, therefore, form an isochron. The original data are from a report by Wasserburg and others , who plotted the data as shown but did not draw a billion-year isochron on the diagram.

As discussed above, one feature of the Rb-Sr isochron diagram is that, to a great extent, it is self-diagnostic. The scatter of the data in Figure 6 shows clearly that the sample has been an open system to 87 Sr and perhaps to other isotopes as well and that no meaningful Rb-Sr age can be calculated from these data.

This conclusion was clearly stated by both Wasserburg and others and by Faure and Powell There are two things wrong with this argument. First, the lead data that Kofahl and Segraves 77 cite, which come from a report by Oversby , are common lead measurements done primarily to obtain information on the genesis of the Reunion lavas and secondarily to estimate when the parent magma the lava was derived from was separated from primitive mantle material. These data cannot be used to calculate the age of the lava flows and no knowledgeable scientist would attempt to do so.

We can only speculate on where Kofahl and Segraves obtained their numbers. The data Morris 92 refers to were published by Evernden and others 44 , but include samples from different islands that formed at different times!

The age of 3. The approximate age of , years was the mean of the results from four samples from the Island of Hawaii, which is much younger than Kauai. Many of the rocks seem to have inherited Ar 40 from the magma from which the rocks were derived.

Volcanic rocks erupted into the ocean definitely inherit Ar 40 and helium and thus when these are dated by the K 40 -Ar 40 clock, old ages are obtained for very recent flows. For example, lavas taken from the ocean bottom off the island [sic] of Hawaii on a submarine extension of the east rift zone of Kilauea volcano gave an age of 22 million years, but the actual flow happened less than years ago.

Slusher and Morris 92 advanced this argument in an attempt to show that the K-Ar method is unreliable, but the argument is a red herring. Two studies independently discovered that the glassy margins of submarine pillow basalts, so named because lava extruded under water forms globular shapes resembling pillows, trap 40 Ar dissolved in the melt before it can escape 36 , This effect is most serious in the rims of the pillows and increases in severity with water depth.

The excess 40 Ar content approaches zero toward pillow interiors, which cool more slowly and allow the 40 Ar to escape, and in water depths of less than about meters because of the lessening of hydrostatic pressure. The purpose of these two studies was to determine, in a controlled experiment with samples of known age, the suitability of submarine pillow basalts for dating, because it was suspected that such samples might be unreliable.

Such studies are not unusual because each different type of mineral and rock has to be tested carefully before it can be used for any radiometric dating technique. In the case of the submarine pillow basalts, the results clearly indicated that these rocks are unsuitable for dating, and so they are not generally used for this purpose except in special circumstances and unless there is some independent way of verifying the results.

The citation for this statement is to a report by Turner Turner, however, made no such comment about excess argon in lunar rocks, and there are no data in his report on which such a conclusion could be based.

The statement by Rofahl and Segraves 77 is simply unjustifiable. Volcanic rocks produced by lava flows which occurred in Hawaii in the years were dated by the potassium-argon method. Excess argon produced apparent ages ranging from million to 2. Similar modern rocks formed in near Hualalai, Hawaii, were found to give potassium-argon ages ranging from million years to 3 billion years.

Kofahl and Segraves 77 and Morris 92 cite a study by Funkhouser and Naughton 51 on xenolithic inclusions in the flow from Hualalai Volcano on the Island of Hawaii. The flow is unusual because it carries very abundant inclusions of rocks foreign to the lava. These inclusions, called xenoliths meaning foreign rocks , consist primarily of olivine, a pale-green iron-magnesium silicate mineral. They come from deep within the mantle and were carried upward to the surface by the lava.

In the field, they look like large raisins in a pudding and even occur in beds piled one on top of the other, glued together by the lava.

The study by Funkhouser and Naughton 51 was on the xenoliths, not on the lava. The xenoliths, which vary in composition and range in size from single mineral grains to rocks as big as basketballs, do, indeed, carry excess argon in large amounts. Quite simply, xenoliths are one of the types of rocks that cannot be dated by the K-Ar technique.

Funkhouser and Naughton were able to determine that the excess gas resides primarily in fluid bubbles in the minerals of the xenoliths, where it cannot escape upon reaching the surface. Studies such as the one by Funkhouser and Naughton are routinely done to ascertain which materials are suitable for dating and which are not, and to determine the cause of sometimes strange results. They are part of a continuing effort to learn.

Two extensive K-Ar studies on historical lava flows from around the world 31 , 79 showed that excess argon is not a serious problem for dating lava flows. In nearly every case, the measured K-Ar age was zero, as expected if excess argon is uncommon.

An exception is the lava from the Hualalai flow, which is so badly contaminated by the xenoliths that it is impossible to obtain a completely inclusion-free sample. There is really no valid way of determining what the initial amounts of Sr 87 in rocks were. As discussed above in the section on Rb-Sr dating the simplest form of Rb-Sr dating i.

Such samples are rare, and so nearly all modern Rb-Sr dating is done by the isochron method. The beauty of the Rb-Sr isochron method is that knowledge of the initial Sr isotopic composition is not necessary — it is one of the results obtained.

A second advantage of the isochron method is that it contains internal checks on reliability. Look again at the isochron for the meteorite Juvinas Figure 3. The data are straightforward albeit technically complex measurements that fall on a straight line, indicating that the meteorite has obeyed the closed-system requirement. The decay constants used in the calculations were the same as those in use throughout the world in The age of 4.

There is far too much Ar 40 in the earth for more than a small fraction of it to have been formed by radioactive decay of K This is true even if the earth were really 4.

In the atmosphere of the earth, Ar 40 constitutes This is around times the amount that would be generated by radioactive decay over the hypothetical 4. Certainly this is not produced by an influx from outer space. Thus it would seem that a large amount of Ar 40 was present in the beginning.

Since geochronologists assume that errors due to presence of initial Ar 40 are small, their results are highly questionable. This statement contains several serious errors. First, there is not more 40 Ar in the atmosphere than can be accounted for by radioactive decay of 40 K over 4.

An amount of 40 Ar equivalent to all the 40 Ar now in the atmosphere could be generated in 4. Current estimates of the composition of the Earth indicate that the crust contains about 1. The 40 Ar content of the atmosphere is well known and is 6. Thus, the Earth and the atmosphere now contain about equal amounts of 40 Ar, and the total could be generated if the Earth contained only ppm potassium and released half of its 40 Ar to the atmosphere.

Second, there have been sufficient tests to show that during their formation in the crust, igneous and metamorphic rocks nearly always release their entrapped 40 Ar, thus resetting the K-Ar clock. In addition, scientists typically design their experiments so that anomalous results, such as might be caused by the rare case of initial 40 Ar, are readily apparent. The study of the Liberian diabase dikes, discussed above, is a good example of this practice. First, if it is assumed that there is a uniform distribution of Sr 87 in the rock, then it is assumed that there is also a uniform distribution of Rb It only requires that the Sr isotopic composition , i.

Even though the various minerals will incorporate different amounts of Sr as they cool and form, the Sr isotopic composition will be the same because natural processes do not significantly fractionate isotopes with so little mass difference as 87 Sr and 86 Sr.

Second, Slusher has confused isotopes and elements. Rb and Sr are quite different elements and are incorporated into the various minerals in varying proportions according to the composition and structure of the minerals. There is no way to correct for this natural isotopic variation since there is no way to determine it. This renders the Rb 87 -Sr 87 series useless as a clock.

Slusher is wrong again. He has used an invalid analogy and come to an erroneous conclusion. Arndts and Overn 8 and Kramer and others 78 claim that Rb-Sr isochrons are the result of mixing, rather than of decay of 87 Rb over long periods:.

It is clear that mixing of pre-existent materials will yield a linear array of isotopic ratios. We need not assume that the isotopes, assumed to be daughter isotopes, were in fact produced in the rock by radioactive decay.

Thus the assumption of immense ages has not been proven. The straight lines, which seem to make radiometric dating meaningful, are easily assumed to be the result of simple mixing. This preliminary study of the recent evolutionary literature would suggest that there are many published Rb-Sr isochrons with allegedly measured ages of hundreds of millions of years which easily meet the criteria for mixing, and are therefore more cogently indicative of recent origin.

Kramer and others 78 and Arndts and Overn 8 have come to an incorrect conclusion because they have ignored several important facts about the geochemistry of Rb-Sr systems and the systematics of isochrons.

First, the chemical properties of rubidium and strontium are quite different, and thus their behavior in minerals is dissimilar. Both are trace elements and rarely form minerals of their own.

It is chemically similar to potassium and tends to substitute for that element in minerals in which potassium is a major constituent, such as potassium feldspar and the micas muscovite and biotite. It commonly substitutes for calcium in calcium minerals, such as the plagioclase feldspars. The chemical properties of rubidium and strontium are so dissimilar that minerals which readily accept rubidium into their crystal structure tend to exclude strontium and vice versa.

Thus, rubidium and strontium in minerals tend to be inversely correlated; minerals high in rubidium are generally low in strontium and vice versa. This relation, however, is a natural consequence of the chemical behavior of rubidium and strontium in minerals and of the decay of 87 Rb to 87 Sr over time, and has nothing to do with mixing.

Second, mixing is a mechanical process that is physically possible only in those rock systems where two or more components with different chemical and isotopic compositions are available for mixing. Examples include the mingling of waters from two streams, the mixing of sediment from two different source rocks, and the contamination of lava from the mantle by interactions with the crustal rocks through which it travels to the surface.

Mixing in such systems has been found 49 , 70 , but the Rb-Sr method is rarely used on these systems. The Rb-Sr isochron method is most commonly used on igneous rocks, which form by cooling from a liquid. Mineral composition and the sequence of mineral formation are governed by chemical laws and do not involve mixing.

In addition, a rock melt does not contain isotopic end members that can be mechanically mixed in different proportions into the various minerals as they form, nor could such end members be preserved if they were injected into a melt. Fourth, if isochrons were the result of mixing, approximately half of them should have negative slopes.

Images: sm-nd dating

sm-nd dating

Rocks known to have formed in historical times should not yield dates of millions of years. One sedimentary mineral of particular importance for K-Ar dating is glaucony. The partial pressure of argon should be largest deepest in the earth, and decrease towards the surface.

sm-nd dating

Helens K-Ar dating, and historic lava flows and their excess argon.

sm-nd dating

It is true that by using additional isotopes if sm-nd dating are sufficiently abundant and do not fractionateone can often detect mixings of multiple sm-nd dating. Excess argon produced apparent ages ranging from million to 2. In the late s, Nier published Pb s,-nd analyses on 21 samples of uranium ore from 14 localities in Africa, Europe, India, and North Sm-nd dating and calculated simple Ssm-nd ages for these samples. So one obtains a series of best dating site in usa 2015 crystallizing out of sm-nd dating lava. But the value is not really known. Heating and deformation of rocks can cause these atoms to migrate, and water percolating through the rocks can transport these substances and redeposit them. There were other estimates but the calculations were hotly disputed because they all were obviously flawed by uncertainties in datinf the initial assumptions and the data.