of adequate precision and accuracy to answer the question asked. . A date is a number calculated using measured isotopic ratios and the. The overall reliability of radiometric dating was addressed in some detail We scientists who measure isotope ages do not rely entirely on the. the date. It relates only to the accuracy of the measuring equipment in the laboratory. The dates calculated are based on the isotopic composition of the rock.
The broader ecological and sedimentary conditions under which dolomitization within ratite shells occurs are unclear, but the shells tested by Jain et al. Open image in new window Fig. Some measure of chemical degradation to the outer surface of the OES would need to occur to expose the inner layers to diagenesis.
However, given sufficiently acidic soil conditions and ample exposure to moisture, there is nothing about the chemical structure of OES that would preclude dolomitization of calcites.
Other diagenetic effects can occur in OES. Vogel et al. AMS ages on OES were significantly older in all cases relative to the pottery ages, and in most cases, implausibly old by thousands or tens of thousands of years Janz et al. Therefore, it is critical for all researchers to consider the potential for postdepositional dating accuracy Isotope in OES located in limestone basement systems or areas with low or variable soil pH conditions, which may reflect high concentrations of surface carbonates Fig.
Generally, high pH regions would have a higher potential for inducing dolomitization in ostrich eggshell and carbon reservoir effects in mollusk shells compared to low pH regions. High pH areas will typically have higher potential for bone diagenesis. Also shown are coastal upwelling zones delineated by the National Oceanic Atmospheric Administration Mollusks In addition to diagenetic effects, metabolic pathways of living organisms must be considered when obtaining radiocarbon ages.
Organisms that create chitin [C8H13O5N] n -based exoskeletons, such as beetles, or calcium carbonate CaCO3 -based shells, such as dating accuracy Isotope mollusks, can metabolically recycle 14C-depleted carbon from bedrock as they synthesize bicarbonate for skeletal production Rick et al.
Vast tracts of the Sahara and southern Africa are particularly susceptible to reservoir effects, because the bedrock is primarily made of limestone and shale. On the other hand, in a study of potential reservoir carbon in ostracodes from Lake Dating accuracy Isotope, Halfman et al. Paired freshwater mollusk and optically stimulated luminescence OSL ages from Lake Turkana also determined no detectable reservoir effect Forman et al.
Coastal zones dating accuracy Isotope many portions of Africa are subject to upwelling of marine currents Fig. Upwelling brings organic nutrients and inorganic carbon from the deep ocean floor to the near surface environment where marine mollusks live dating accuracy Isotope feed. Dating accuracy Isotope, they use those minerals to construct their shells. Marine mollusks were important sources of food for past people living in coastal regions, and therefore have great potential to provide a chronology of past human activities.
However, if marine mollusks are to be used for radiocarbon dating accuracy Isotope, specific marine reservoir corrections should be made through online databases such as OxCal and CALIB.
Marine reservoir corrections are currently constructed from regional databases, which augment the global marine reservoir correction architecture originally presented in Reimer et al. Geographic gaps in the effectiveness of the marine reservoir correction persist Hua et al. Old Wood Africa is subject to similar wood scarcities as the American Southwest due to the large proportion of the continent that falls within arid and semiarid climatic zones Fig.
To complicate matters, many of the now hyperarid regions of Africa were dating accuracy Isotope woodland environments deMenocal et al. Normalized and averaged raster map of precipitation and temperature raw data acquired from University of East Anglia on a scale of 0 to 1. Zero represents the coolest and dating accuracy Isotope average aggregated conditions between the years andwhile dating accuracy Isotope.
The interpretation is that the charcoal formed from a natural forest fire, which smoldered the roots of the trees in situ, turning them to charcoal. Erosion later exposed the charcoal, which was opportunistically used by ironsmiths for forging. The old wood phenomenon can also apply to wood that has not been curated or recycled, but derives from a long-lived species. The study showed that single baobab trees can yield radiocarbon ages separated by hundreds of years, depending on the aspect of the tree that is sampled.
Special Artifacts A related issue pertains to the curation of jewelry made out of OES beads and cowrie shells, and wooden artifacts like headrests. These artifacts can be passed down intergenerationally and traded as prestige items across time and space. Hodder identifies this phenomenon in the Lake Baringo area of Kenya. In his ethnographic account, artifacts move in prescribed dating accuracy Isotope, whereas some artifacts are created and never move out of their home area.
Wooden artifacts were also highly sought after in the West African trans-Saharan exchange network, transported hundreds of kilometers to their end destinations, and highly prized and conserved as a result Lawal ; Stoller In the end, it becomes difficult for archaeologists to interpret whether the bits of preserved organic remains are those from a stool that has been conserved for multiple generations, or were buried a short time after the accumulation of carbon ceased in the organism from which the artifact was derived.
Bone Diagenesis As outlined above, following dating accuracy Isotope of an organism, the assumption of radiocarbon dating is that 14C decays in half-lives while 12C and 13C remain at constant dating accuracy Isotope. However, in hot and humid environments, which are common in Africa Fig.
In addition to the OES, bone apatite is particularly susceptible to postmortem carbon molecule exchange, and is no longer widely used for dating purposes without clear evidence for a lack of diagenetic effects. However, leaching of carbon and microbial attack on collagen can also be acute in tropical ecosystems particularly if the artifact is not rapidly buried Jans There is nothing to exclude postdepositional diagenetic changes in plant macrofossils as well, particularly on the outer portions of the stalk or seed.
Many of these effects are now controlled by laboratories that assay carbon isotopes by performing alternating acid and alkali washes to remove diagenetic contaminates such as groundwater carbonates and sedimentary carbon; ultrafiltration is also used to concentrate the high-molecular weight material Ambrose ; Brock et al. The best way to isolate diagenetic effects on bone is by conducting experimental archaeology on samples whose taphonomy is well understood.
A study conducted on 58 skeletal samples from a single cemetery near Khartoum, Sudan found significantly dating accuracy Isotope bone diagenesis from individuals buried during wetter climatic conditions than during drier conditions Dal Sasso et al. Fourier transform infrared spectroscopy FTIR or scanning electron microscopy SEM are excellent tools to determine the degree of diagenesis on bone before undertaking 14C dating or other stable carbon isotope analyses e.
What's more, in observed supernova events that we observe in telescopes today, most of which occurred many millions of years ago, the patterns of light and radiation are completely consistent with the half-lives of radioactive isotopes that we measure today [ Isaakpg. As another item of evidence, researchers studying a natural nuclear reactor in Africa have concluded that a certain key physical constant "alpha" has not changed measurably in hundreds of millions of years [ Barrowpg.
Finally, researchers have just completed a study of the proton-electron with Biak up Hook girl in ratio approximately Thus scientists are on very solid ground in asserting that rates of radioactivity have been constant over geologic time.
The issue of the "uniformitarian" assumption is discussed in significantly greater detail at Uniformitarian. Responses to specific creationist claims Wiens' online article, mentioned above, is an excellent resource for countering claims of creationists on the reliability of geologic dating.
In an appendix to this article, Wiens addresses and responds to a number of specific creationist criticisms. Here is a condensed summary of these items, quoted from Dating accuracy Isotope article [ Wiens ]: Radiometric dating is based on index fossils whose dates were assigned long before radioactivity was discovered. This is not at all true, though it is implied by some young-earth literature. Radiometric dating is based on the half-lives of the radioactive isotopes.
These half-lives have been measured over the last years. They are not calibrated by fossils. No one has measured the decay rates directly; we only know them from inference. Decay dating accuracy Isotope have been directly measured over the last years. In some cases a batch of dating accuracy Isotope pure parent material is weighed and then set aside for a long time and then the resulting daughter material is weighed.
In many cases it is easier to detect radioactive decays by the energy burst that each decay gives off. For this a batch of the pure parent material is carefully weighed and then put in front of a Geiger counter or gamma-ray detector.
These instruments count the number of decays over a long time. If the half-lives are billions of years, it is impossible to determine them from measuring over just dating accuracy Isotope few years or decades.
The example given in the section [in Wiens' article] titled, "The Radiometric Clocks" shows that an accurate determination of the half-life is easily achieved by direct counting of decays over a decade or shorter. This is because: Additionally, lavas of historically known ages have been correctly dated even using methods with long half-lives. The decay rates are poorly known, so the dates are inaccurate. Most of the decay rates used for dating rocks are known to within two percent.
Such small uncertainties are no reason to dismiss radiometric dating accuracy Isotope. Whether a rock is million years or million years old does not make a great deal of difference. To date a rock one must know the original amount of the parent element. But there is no way to measure how much parent element was originally there. It is very easy dating accuracy Isotope calculate the original parent abundance, but that information is not needed to date the rock.
All of the dating schemes work from knowing the present abundances of the parent and daughter isotopes. There is little or no way to tell how much of the decay product, that is, the daughter isotope, was originally in the rock, leading to dating accuracy Isotope old ages.
A good part of [Wiens' article] is devoted to explaining how one can tell how much of a given element or isotope was originally present. Usually it involves using more than one sample from dating accuracy Isotope given rock. It is done by dating accuracy Isotope the dating accuracy Isotope of parent and daughter isotopes relative to a stable isotope for samples with different relative amounts of the parent isotope.
From this one can determine how much of the daughter isotope would be present if there had been no parent isotope. This is the same as the initial amount it would not change if there were no parent isotope to decay. Figures 4 and 5 [in Wiens' article], and the accompanying explanation, tell how this is done most of the time. There are only a few different dating methods. There are actually many more methods out there.
Well over forty different radiometric dating methods are in use, and dating accuracy Isotope number of non-radiogenic methods not even mentioned here.
A young-Earth research group reported that they sent a rock erupted in from Mount Saint Helens volcano to a dating lab and got back a potassium-argon age of several million years. This shows we should not trust radiometric dating. Radiometric dating dating accuracy Isotope largely done on rock that has formed from solidified lava. Lava properly called magma before it erupts fills large underground chambers dating accuracy Isotope magma chambers.
Most people are not aware of the many processes that take place in lava before it erupts and as it solidifies, processes that can have a tremendous influence on daughter to parent ratios. Such processes can cause the daughter product to be enriched relative to the parent, which would make the rock look older, or cause the parent to be enriched relative to the daughter, which would make the rock look younger.
This calls the whole radiometric dating accuracy Isotope scheme into serious question. Geologists assert that older dates are found deeper down in dating accuracy Isotope geologic column, which they take as evidence that radiometric dating is giving true ages, since it is apparent that rocks that are deeper must be older.
But even if it is true that older radiometric dates are found lower down in the geologic column, which is open to question, this can potentially be explained by processes occurring in magma chambers which cause the lava erupting earlier to appear older than the lava erupting later. Dating accuracy Isotope erupting earlier would come from the top of the magma chamber, and lava erupting later would come from lower down.
A number of processes could cause the parent substance dating accuracy Isotope be depleted at the top of the magma chamber, or the daughter product dating accuracy Isotope be enriched, both of which would cause the lava erupting earlier to appear very old according to radiometric dating, and lava erupting later to appear younger.
Mechanisms that can alter daughter-to-parent ratios What happens when magma solidifies and melts and its implications for radiometric dating The following quote from The Earth: The general idea is that many different minerals are formed, which differ from one another in composition, even though they come from the same magma.
The mineral makeup of an igneous rock is ultimately determined by the chemical composition of the magma from which it crystallized. Such a large variety of igneous rocks exists that it is logical to assume an equally large variety of magmas must also exist. However, geologists have found that various eruptive stages of the same volcano often extrude lavas exhibiting somewhat different mineral compositions, particularly if an extensive period of dating accuracy Isotope separated the eruptions.
Evidence of this type led them to look into the possibility that a single magma might produce rocks of varying mineral content. A pioneering investigation into the crystallization of magma was carried out by N.
Bowen in the first quarter of this century. Bowen discovered that as magma cools in the laboratory, certain minerals crystallize first. At successively lower temperature, other minerals begin to crystallize as shown in Figure 3.
As the crystallization process continues, the composition of the melt liquid portion of a magma, excluding any solid material continually changes. For example, at the stage when about 50 percent of the magma has solidified, the melt will be greatly depleted in iron, magnesium, and calcium, because these elements are found in the earliest formed minerals. But at the same time, it will be enriched in the elements contained in the later forming minerals, namely sodium and potassium.
Further, the silicon content of the melt becomes enriched toward the latter stages of crystallization. Bowen also demonstrated that if a mineral remained in the melt after it had crystallized, it would react with the remaining melt and produce the next mineral in dating accuracy Isotope sequence shown in Figure 3.
For this reason, this arrangement of minerals became known as Bowen's reaction series. On the upper left branch of this reaction series, olivine, the first mineral to form, Ml] react with the remaining melt to become pyroxene. This reaction will continue until the last mineral in the series, biotite mica, is formed.
This dating accuracy Isotope branch is called a discontinuous reaction series because each mineral has a different crystalline structure. Recall that olivine is composed of a single tetrahedra and that the other minerals in this sequence are composed of single chains, double chains, and sheet structures, respectively. Ordinarily, these reactions are not complete so that various amounts of each of these dating accuracy Isotope may exist at any given time.
The right branch of the reaction series is a continuum in which the earliest formed calcium-rich feldspar crystals react with the sodium ions contained in the melt to become progressively more sodium rich. Oftentimes the rate of cooling occurs rapidly enough to prohibit the complete transformation of calcium-rich feldspar into sodium-rich feldspar. In these instances, the feldspar crystals will have calcium-rich interiors surrounded by zones that are progressively richer in sodium.
During the last stage of crystallization, after most of the magma has solidified, the remaining melt will form the minerals quartz, muscovite mica, and potassium feldspar. Although these minerals crystallize in the order shown, this sequence is not a true reaction series. Bowen demonstrated that minerals crystallize from magma in a systematic fashion. But how does Bowen's reaction series account for the great diversity of igneous rocks? It appears that at one or more stages in the crystallization process, a separation of the solid and liquid components of a magma frequently occurs.
This can happen, dating accuracy Isotope example, if the earlier formed minerals are heavier than the liquid portion and settle to the bottom of the magma chamber as shown in Figure 3. This settling is thought to occur frequently with the dark silicates, such as olivine. When the remaining melt crystallizes, either in place or in a new location if it migrates out of the chamber, it will form a rock with a chemical composition much different from the original magma Figure 3.
In many instances the melt which has migrated from the dating accuracy Isotope magma chamber will undergo further segregation.
As crystallization progresses in the " new" magma, the solid particles may accumulate into rocklike masses surrounded by pockets of the still molten material. It is very likely that some of this melt will be squeezed from the mixture into the cracks which develop in the surrounding rock.
This process will generate an igneous rock of yet another composition. The process involving the segregation of minerals by differential crystallization an separation is called fractional crystallization.
At any stage in the crystallization process the melt might be separated from the dating accuracy Isotope portion of the magma. Consequently, fractional dating accuracy Isotope can produce igneous rocks having a wide range of compositions. Bowen successfully demonstrated that through fractional crystallization one magma can generate several different igneous rocks. However, more recent work has indicated that this process cannot account for the relative quantities of the various rock types dating accuracy Isotope to exist.
Although more than one rock type can be generated from a single magma, apparently other mechanisms also exist to generate dating accuracy Isotope of quite varied dating accuracy Isotope compositions. We will examine some of these mechanisms at dating accuracy Isotope end of the next chapter. Illustration of how the earliest formed minerals can be separated from a magma by settling. The remaining dating accuracy Isotope could migrate to a number of different locations and, upon further crystallization, dating accuracy Isotope rocks having a composition much different from the parent magma.
So we see dating accuracy Isotope many varieties of minerals are produced from the same magma by the different processes of crystallization, dating accuracy Isotope these different minerals may have very different compositions. It is possible that the ratio of daughter to parent substances for radiometric dating could differ in the different minerals.
Clearly, it is important to have a good understanding of these processes in order to evaluate the reliability of radiometric dating. Another quotation about fractionation follows: Faure discusses fractional crystallization relating to U and Th in his book p. These values may be taken as an indication of the very low abundance of dating accuracy Isotope elements in the mantle and crust of the Earth. In the course of partial melting and fractional crystallization of magma, U dating accuracy Isotope Th are concentrated in the liquid phase and become incorporated into the more silica-rich products.
For that reason, igneous rocks of granitic composition are strongly enriched in U and Th compared to rocks of basaltic or ultramafic composition. Progressive geochemical differentiation of the upper mantle of the Earth has resulted in the concentration of U and Th into the rocks of the dating accuracy Isotope crust compared to those of the upper mantle.
The concentration of Pb is usually so much higher than U, that a 2- to 3-fold increase of U doesn't change the percent composition much e. We see that there are at least two kinds of magma, and U and Th get carried along in silica rich magma rather than in basaltic magma. This represents major fractionation. Of course, any process that tends to concentrate or deplete uranium or thorium relative to lead would have an influence on the radiometric ages computed by uranium-lead or thorium-lead dating.
Also, the fact that there are two dating accuracy Isotope of magma could mean that the various radiometric ages are obtained by mixing of these kinds of magma in different proportions, and do not represent true ages at all. Finally, we have a third quotation from Elaine Dating accuracy Isotope. Kennedy in Geoscience Reports, SpringNo.
Contamination and fractionation issues are frankly acknowledged by the geologic community. If this occurs, initial volcanic eruptions would have a preponderance of daughter products relative to the parent isotopes.
Such a distribution would give the appearance of age. As the magma chamber is depleted in daughter products, subsequent lava flows and ash beds would have younger dating accuracy Isotope. Such a scenario does not answer all of the questions or solve all of the problems that radiometric dating poses for those who believe the Genesis account of Creation and the Flood.
It does suggest at least one aspect of the problem that could be researched more thoroughly. Principles of Isotope Geology: John Wiley and Sons, Inc. It is interesting dating accuracy Isotope contamination and fractionation issues are frankly acknowledged by the geologic community. But they may not be so familiar to the readers of talk.
So we have two kinds of processes taking place. There are those processes taking place when lava solidifies and various minerals crystallize out at different times. There are also processes taking place within a magma chamber that can cause differences in the composition of the magma from the top to the bottom of the chamber, since one might expect the temperature at the top to be cooler.
Both kinds of processes can influence radiometric dates. In addition, the magma chamber would be expected dating accuracy Isotope be cooler all around its borders, both at the top and the bottom as well as in the horizontal extremities, and these effects must also dating accuracy Isotope taken into account.
For example, heavier substances will tend to sink to the bottom of a magma chamber. Also, substances with a higher melting dating accuracy Isotope will tend to crystallize out at the top of a magma chamber and fall, since it will be cooler at the top. These substances will then fall to the lower portion of the magma chamber, where it is hotter, and remelt.
This will make the composition of the magma different at the dating accuracy Isotope and bottom of the chamber. This could influence dating accuracy Isotope dates. This mechanism was suggested by Jon Covey and others. The solubility of various substances in the magma also could be a function of temperature, and have an influence on the composition of the magma at the top and dating accuracy Isotope of the magma chamber.
Finally, minerals that crystallize at the top of the chamber and fall may tend to incorporate other substances, and so these other substances will also tend to have a change in concentration from the top to the bottom of the magma chamber. There are quite a number of mechanisms in operation in a magma chamber.
I count at least three so far -- sorting by density, sorting by melting point, and sorting by how easily something is incorporated into minerals that form at the top of a magma chamber. Then you have to remember that sometimes one has repeated melting and solidification, introducing more complications. There dating accuracy Isotope also a fourth dating accuracy Isotope -- differences in solubilities.
How anyone can keep track of this all is a mystery to me, especially with the difficulties encountered in exploring magma chambers. These will be definite factors that will change relative concentrations of dating accuracy Isotope and daughter isotopes in some way, and call into question the reliability of radiometric dating.
In dating accuracy Isotope, I think this is a very telling argument against radiometric dating. Another possibility to keep in mind is that lead becomes gaseous at low temperatures, and would dating accuracy Isotope gaseous in magma if it were not for the dating accuracy Isotope pressures dating accuracy Isotope in the earth.
It also becomes very mobile when hot. These processes could influence the distribution of lead in magma chambers. Let me suggest how these processes could influence uranium-lead and thorium-lead dates: The following is a quote from The Earth: The magnesium and iron rich minerals come from the mantle subducted oceanic plateswhile granite comes from continental sediments crustal rock.
The mantle part solidifies first, and is rich in magnesium, iron, and calcium. Dating accuracy Isotope it is reasonable to expect that initially, the magma is rich in iron, magnesium, and calcium and poor in uranium, thorium, sodium, and potassium. Later on the magma is poor in iron, magnesium, and calcium and rich in uranium, thorium, sodium, and potassium. It doesn't say which class Vila Slut Velha in is in.
But lead is a metal, and to me it looks more likely that lead would concentrate along with the iron. If this is so, the magma would initially be poor in thorium and uranium and rich in lead, and as it cooled it would become rich in thorium and uranium and poor dating accuracy Isotope lead. Thus its radiometric age would tend to decrease rapidly with time, and lava emitted later would tend to look younger.
Another point is that of time. Suppose that the uranium does come to the top by whatever reason. Perhaps magma that is uranium rich tends to be lighter than other magma. Or maybe the uranium poor rocks crystallize out first and the remaining magma is enriched in uranium. Would this cause trouble for our explanation? Not necessarily. It depends how fast it happened. Some information from the book Uranium Geochemistry, Mineralogy, Geology provided by Jon Covey gives us evidence that fractionation processes are making radiometric dates much, much too old.
The half life of U is 4.
Thus radium is decaying 3 million times as fast as U At equilibrium, which should dating accuracy Isotope attained inyears for this decay series, we should expect to have 3 million times as much U as radium to equalize the amount of daughter produced. Cortini says geologists discovered that ten times more Ra than the equilibrium value was present in rocks from Dating accuracy Isotope. They found similar excess radium at Mount St.
Helens, Vulcanello, and Lipari and other volcanic sites. The only place where radioactive equilibrium of the U series exists in zero age lavas is in Hawiian rocks. We need to consider the implications of this for radiometric dating. How is this excess dating accuracy Isotope radium being produced? This radium cannot be the result of decay of uranium, since there is far too much of it.
Either it is the result of an unknown decay dating accuracy Isotope, or it is the result of fractionation which is greatly increasing the concentration of radium or greatly decreasing the concentration of uranium. Thus only a small fraction of the radium present in the lava at most 10 percent is the result of decay of the uranium in the lava. This is interesting because both radium and lead are daughter products of uranium. If similar dating accuracy Isotope processes are dating accuracy Isotope for lead, this would mean that only a small fraction of the lead is the result of decay from the parent uranium, implying that the U-Pb radiometric dates are much, much too old.
Cortini, in an article appearing in the Journal of Volcanology and Geothermal Research also suggests this possibility. He says: By analogy with the behaviour of Ra, Th and U it can be suggested that Pb, owing to its large mobility, was also fed to the magma by fluids. This can and must be tested. The open-system behaviour of Pb, if true, would have dramatic consequences On the other hand, even if such a process is not operating for lead, the extra radium will decay rapidly to lead, and so in either case we have much too much lead in the lava and radiometric dates that are much, much too ancient!
It is also a dating accuracy Isotope proof that some kind of drastic fractionation is taking place, or else an unknown process is responsible. He says this is inexplicable in a closed-system framework and certainly invalidates the Th dating method. And it dating accuracy Isotope also possible that something similar is happening in the U decay chain, invalidating U based radiometric dates as well. In fact, U and Th both have isotopes of radium in their decay chains with half lives of a dating accuracy Isotope or two, and 6.
Any process that is concentrating one isotope of radium will probably concentrate the others as well and invalidate these dating methods, too. Radium has a low melting point degrees K which may account for its concentration at the top of magma chambers.
What radiometric dating needs to do to show its reliability is to demonstrate that no such fractionation could take place. Can this be done? With so many unknowns I don't think so. How Uranium and Thorium are preferentially incorporated in various minerals I now give evidences that uranium and thorium are incorporated into some minerals more than others.
This is not necessarily a problem for radiometric dating, because it can be taken into account. But as we saw above, processes that take place within magma chambers involving crystallization could result in a different concentration of uranium and thorium at the top of a magma chamber than at the bottom.
How reliable is geologic dating?
This can happen because different minerals incorporate different amounts of uranium and thorium, and these different minerals also have different melting points and different densities. If minerals that crystallize at the top of a magma chamber and fall, tend to incorporate a lot of uranium, this will tend to deplete uranium at the top of the magma chamber, and make the magma there look older.
Concerning the distribution of parent and daughter isotopes in various substances, there are appreciable dating accuracy Isotope. Faure shows that in granite U is 4. Some process is causing the differences in the ratios of these magmatic rocks. Depending on their oxidation state, according to Faure, uranium minerals can be very soluble in dating accuracy Isotope while thorium compounds are, generally, dating accuracy Isotope insoluble.
These elements also show preferences for the minerals in which they are incorporated, so that they will tend to be "dissolved" dating accuracy Isotope certain mineral "solutions" preferentially to one another. More U is found in carbonate rocks, while Th has a very strong preference for granites in comparison.
I saw a reference that uranium reacts strongly, and is never found pure in nature. So the question is what the melting points of its oxides or salts would be, I suppose. I also saw a statement that uranium is abundant in the crust, but never found in high concentrations. To me this indicates a high melting point for its minerals, as those with a low melting point might be expected to concentrate in the magma remaining after others crystallized out.
Such a high melting point dating accuracy Isotope imply fractionation in the magma. Thorium is close to uranium in the periodic table, so it may have similar properties, and similar remarks may apply to it.
It turns out that uranium in magma is typically found dating accuracy Isotope the form of uranium dioxide, with a melting point of degrees centrigrade. This high melting point suggests that uranium would crystallize and fall to the bottom of magma chambers.
Geologists are aware of the problem of initial concentration of daughter elements, and attempt to take it into account. U-Pb dating attempts to get around the lack of information about initial daughter concentrations by the choice of minerals that are dated. For example, zircons are thought to accept little lead but much uranium. Thus dating accuracy Isotope assume that the lead in zircons resulted from radioactive decay. But I don't know how they can be sure how much lead zircons accept, and even they admit that zircons accept some lead.
Lead could easily reside in impurities and imperfections in the crystal structure. Also, John Woodmorappe's paper has some examples of anomalies involving zircons.