Uranium-Lead (U-Pb) Radioisotope Dating Method Problems

First Problem: Common Lead

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Abstract

Uranium-lead (hereafter U-Pb) radioisotope dating is now the preferred absolute dating method among geochronologists (geologists whose field of research is in dating earth materials: rocks, rock layers, fossils, etc.). But there are several problems with this particular radiometric dating method. Geologist Dr. Andrew Snelling has written a three-part series in our Answers Research Journal which identifies three main issues (each with several associated problems of their own) with the U-Pb dating method. This article will summarize the points from the first technical paper of the series.

The Primary Faulty Assumption

Radioisotope dating of minerals, rocks, and meteorites is perhaps the most-claimed “incontestable” proof for the alleged old age of the earth and the solar system. The declared absolute ages provided by the radioisotope dating methods provide an aura of certainty to the claimed millions and billions of years for the formation of the earth’s rocks. Consequently, the scientific community and the general public around the world appear convinced of the earth’s claimed great antiquity.

One of the (many) assumptions in radiometric dating, and specifically for U-Pb dating, is that most of the three lead (Pb) isotopes we see on earth (206Pb, 207Pb and 208Pb)—which today are produced by radiometric decay of Uranium (U), Thorium (Th), Actinium (Ac) and several other elements with radioactive isotopes—were derived in the past only from radiometric decay of these elements. This is a completely arbitrary and unprovable assumption presupposing a naturalistic evolutionary history for the universe. However, in the biblical creation worldview, God would have created all the isotopes of Pb, including both non-radiogenic Pb isotopes and the Pb isotopes, which today result from radioactive decay of U, Th, Ac, and other elements. 204Pb is the main non-radiogenic isotope of lead and is often referred to as common or initial Pb, but common or initial lead can also contain all the other stable isotopes of Pb, including 206Pb, 207Pb, and 208Pb.

Creationist Research on Radiometric Dating

Accurate radioisotope age determinations require that the decay constants (half-lives) of the parent radionuclides be precisely known and constant. However, as Dr. Snelling has written about in other articles, Uranium decay constants aren’t accurately known due to wide decay differences in the U isotope ratio in minerals and rocks which had been assumed to be constant by conventional geologists.1 Additionally, the creationist 1997–2005 RATE (Radioisotopes and the Age of The Earth) project successfully pointed out some of the pitfalls in the radioisotope dating methods, and especially in demonstrating that radioisotope decay rates have not always been constant at today’s measured rates, but have had a period of accelerated nuclear decay (during the global flood of Noah’s day).

secular publications show that any data points which fall outside the isochron are discarded as “contamination” without proving that they really are due to contamination

Accurate determinations, however, depend not only on accurate determinations of the decay constants of the respective parent radioisotopes but on the reliability of the other two assumptions these supposed absolute dating methods rely on. Those are the “parent element atoms only” or “known amount of daughter element atoms” starting conditions and the a priori assumption of no contamination of closed systems. Both assumptions are unprovable because no observers were there in the past to observe the starting conditions and that there was no subsequent contamination. Yet secular geochronologists claim they can be circumvented via the isochron technique because it is claimed to be independent of the starting conditions and sensitive to revealing any contamination. However, even the secular publications show that any data points which fall outside the isochron are discarded as “contamination” without proving that they really are due to contamination. Such discarding of data points is completely arbitrary and therefore is not good science.

Lead and Zircons

Lead (Pb) is widely distributed throughout the earth, occurring not only as the radiometric decay daughter of U and Th, but also forming its own minerals apart from any U and Th. Therefore, the isotopic composition of Pb varies between wide limits, from highly “radiogenic Pb” in supposedly old U- or Th-bearing minerals to the “common Pb” in galena (PbS) and other minerals. Pb is also a trace element in many other rocks.

Zircon (ZrSiO4) is a common mineral, especially in granites and sandstones. It is usually claimed that such zircon grains make excellent U-Pb geochronometers. This is because it is claimed that when they crystallize, they do not incorporate Pb atoms into their crystal lattice structure. Pb2+ is regarded as being excluded from being admitted into zircon crystals because of its large ionic radius (1.32 Å) and its low charge (2+). Therefore, zircons are supposed to contain very little initial Pb at their time of formation and have high U/Pb ratios. Thus, it is presumed that all the Pb measured in them today has been added by radio decay of parent U and Th atoms since the grains crystallized.

Getting the Lead Out

It is acknowledged in the secular literature that for isotope dilution thermal ionization mass spectrometer (ID-TIMS)2 analyses common or initial Pb can be an issue with age determination of zircons, but that there are several methods to minimize or account for such initial Pb. Common lead in zircons is claimed to be primarily in inclusions, present as surface contamination, or introduced during chemical processing. To address these problems, minimization (reducing contamination) of laboratory blanks3 has remained the single most important requirement for high-precision U-Pb analyses. Most laboratories now claim to have reduced analytical blanks to below 5 picograms (a picogram is defined as one-trillionth of a gram, or 10-12 grams), and some to less than 1 picogram, of Pb. Additionally, clear, crack- and inclusion-free zircons separated from volcanic rocks are used and are claimed to have little to no indigenous common Pb. Lastly, common Pb in laboratories can come from airborne particulates, labware, and reagents, and the contributions from these sources can change over time. So, most laboratories have incorporated into their procedures a realistically large uncertainty “error margin” for their blanks. It is often claimed that in an ID-TIMS zircon U-Pb analysis, the most crucial parameter is the ratio of radiogenic to common Pb, often indirectly expressed by the measured 206Pb/204Pb ratio. The secular literature then advocates that labs develop tight protocols to establish the sensitivity of a data set to estimate the assumed initial common Pb component. Then they recommend the data be filtered through a range of geologically reasonable initial Pb isotope ratios. As Dr. Snelling pointed out, “they of course mean assumption driven selection of suitable standards that produce the desired results based on their evolutionary geology model, because there are no objective absolute standards for initial Pb isotope ratios.”4

SIMS (secondary ion mass spectrometry)5 technology for zircon geochronology is the method used to supposedly measure common Pb in zircons, even though the 206Pb and 207Pb atoms of the common or initial Pb are identical to the 206Pb and 207Pb atoms produced by radiometric decay. The most direct method is by measuring non-radiogenic 204Pb that is unique to common Pb. Then once the other Pb isotopes are determined, they can be subtracted from the analysis. Then assuming the ratio of total (204Pb/206Pb) has always been a constant throughout time, you can calculate the 206Pb isotopic composition of the common Pb. Although 204Pb provides the most direct measure of common Pb, researchers realized that the low relative abundance of 204Pb can make that correction procedure imprecise, so they suggested a more precise estimate of the isotopic composition of the common Pb can sometimes be made from the 208Pb/206Pb and the measured Th/U (Thorium/Uranium) ratios. But this measurement relies on the unprovable assumptions that neither the Th/U nor radiogenic 208Pb/206Pb ratio has changed throughout the zircon’s history, except by decay, and that the zircon’s age is known.

It could be argued that when such selections are made with the predetermined assumption that the isotopically concordant portions of the signals must yield the “correct” age of the sample being analyzed, that this is not good science, but rather is agenda-driven dogma

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)6 to date zircon is claimed not to usually require large common Pb corrections. But in many cases, they selectively integrate only the most isotopically concordant data points, thereby hugely (and artificially) reducing the incidence of analyses they believed were affected by common Pb and Pb loss. However, it could be argued that when such selections are made with the predetermined assumption that the isotopically concordant portions of the signals must yield the “correct” age of the sample being analyzed, that this is not good science, but rather is agenda-driven dogma.

Quick Clarification and the Problem Is Still Not Solved

Dr. Snelling included a brief disclaimer about two terms. “Common Pb and initial Pb are terms sometimes used synonymously, but they are not necessarily the same. Common Pb can be defined as the isotopic composition of the Pb in the rocks in a region that had a common origin in a mantle or crustal reservoir from which they were extracted. On the other hand, initial Pb would be the isotopic composition of the Pb that initially was in a mineral or rock when it formed, that is, the Pb it inherited. Often the common and initial Pb in a mineral or rock may be the same. However, sometimes the formation of a mineral or rock may involve fractionation,7 extraction, and/or partitioning processes8 that may not transfer all the common Pb in the source to the mineral or rock when it forms, so that the isotopic composition of the inherited initial Pb may be different to that of the common Pb in the source from which it was formed.”9

Assuming Old Ages to Prove Old Ages

Additionally, mass spectrometers are designed to primarily measure isotopic ratios, not absolute quantities of individual isotopes. And while this is usually not made evident in secular literature, sometimes that fact creeps out. McLean, Bowring, and Gehrels (2016) admitted: that researchers are “interested in the relative abundances of isotopes present, usually expressed as ratios, and rarely require or have information on their absolute abundance to the same precision.”10 As Dr. Snelling then explained, the absolute quantity of 204Pb in samples cannot be measured with certainty. Plus, to directly measure the absolute quantity of 204Pb with sufficient accuracy is muddied by isobaric interference from the 204Hg (Mercury) signal, particularly in LA-ICP-MS procedures. Dr. Snelling then recounted the root of the problem with U-Pb dating, in regard to Mercury interference and overall.

This is not a trivial matter, because it is assumed that all the measured 204Pb, the only stable Pb isotope not derived by radioactive decay from a precursor radioisotope, is the most significant component of the common or initial Pb isotopic composition. Yet measuring the absolute amounts of 204Pb in samples is the only way those amounts can be known without recourse to assumptions. Every one of the other methods to determine the common or initial Pb isotopic composition mentioned above involves using the measured Pb isotopic ratios and assumptions. Ratios are simply that. The only way to determine an absolute amount of 204Pb from them is to make assumptions about the past history of the Pb isotopes in the samples, especially a deep time history for the earth and its origin, as well as for a deep time history for the samples being dated (for example, the Pb-evolution models). Yet the U-Pb radioisotope ages derived using those assumptions are then used to construct that deep time history. So, the outcome is model dependent, and the model chosen will be dependent on one’s worldview.11

Dr. Snelling then listed a few more assumptions built into the entire paradigm of U-Pb dating. That primordial lead has a particular isotopic composition, based on the assumption that all the solar system’s components (planets, asteroids, meteorites) were formed out of the solar nebula, and that the isotopic composition of the earth’s primordial Pb would closely match the Pb in meteoritic troilite, the iron sulfide mineral (FeS). It was assumed that the meteorites are fragments of larger bodies (mostly asteroids) that formed along with the earth early in the history of the solar system. It is postulated that these early asteroids and meteorites formed troilite virtually free of U and Th. So, the large concentration of Pb in troilite (specifically for dating purposes—the Canyon Diablo iron meteorite’s troilite) has then been deemed the primordial Pb amount, which is assumed to have remained constant since it crystallized supposedly 4.56 BY ago (yet another assumption).

Furthermore, the formation of igneous and metamorphosed rocks involves melting, crystallization, and/or recrystallization of minerals, which could cause the distributions of the various elements and their isotopes to be altered. So, there is no guarantee that all the atoms of U, Th, and Pb isotopes in the source rocks will be transferred into the new rocks that form (or are metamorphosed) and their constituent minerals. It is basically impossible to quantify the amount of isotopic mixing, extraction, and fractionation in mantle and crustal isotopic reservoirs at each stage, which then impacts the assumed Pb isotopic evolution in the next stage.

The Biblical Model

Radically different from the secular billions-of-years evolutionary history of the earth is the biblically derived YEC timescale of ~6,000 years. Consequently, there is no allowance in the biblical text for cosmic or geologic evolution over billions of years, nor of those time lengths for the radiometric decay of U and Th (and other elements) to lead. As Dr. Snelling explained, the biblical creation model has several entirely scientifically reasonable postulations, which would then have significant effects on radiometrically-derived dates.

  • It is scientifically reasonable for biblical Christians to believe that when God brought the earth into existence at the beginning by supernatural creation, he gave the earth an initial Pb isotopic composition, which included all four stable Pb isotopes.
  • Of those four stable Pb isotopes, none of those Pb atoms had been derived by radioactive decay from U or Th (and other element’s) isotopes.
  • Since that initial or primordial Pb isotopic endowment was not that of the Canyon Diablo iron meteorite’s troilite, the starting point in Pb isotopic evolution models on the divinely created earth could be very different to that assumed by evolutionists.
  • Some mixing and redistribution of Pb isotopes may have occurred during Day 3 of creation week in the “Great Upheaval” where the dry land was formed from under the waters.
  • The crust and the mantle underwent major melting and isotopic mixing at the time of the Flood, and there quite probably was a period of accelerated nuclear decay during this time.
  • If accelerated nuclear decay did occur, then ~600 million years’ worth of daughter Pb isotopes and portions thereof were sequentially added to crustal minerals and rocks during the Flood year, which would greatly skew Pb isotopic compositions leading to secular interpretations of vastly inflated apparent ages.

Dr. Snelling then bluntly brought out the point in his conclusion. The earth’s deep time evolutionary history has only been assumed, not proven.

Therefore, without being able to unequivocally distinguish the daughter Pb atoms produced by in situ U and Th decay from the initial Pb atoms in a mineral or rock, it is impossible to determine their absolute U-Pb radioisotope ages. All the unprovable assumptions ultimately depend on an assumed deep time history. Its rejection is recognized as fatal to the earth’s claimed age of billions of years. There is thus no impediment to accepting and using the Bible’s account of Creation and the Flood as a reliable framework for unravelling the history of the earth and the Pb isotopes found in its minerals and rocks.12

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Footnotes

  1. For just one example among many, see Andrew A. Snelling, “Determination of the Decay Constants and Half-Lives of Uranium-238 (238U) and Uranium-235 (235U), and the Implications for U-Pb and Pb-Pb Radioisotope Dating Methodologies,” Answers Research Journal 10 (January 18, 2017): 1–38, https://answersingenesis.org/geology/radiometric-dating/determination-decay-constants-half-lives-uranium/.
  2. Defined as “the addition of an isotope tracer to a dissolved sample to make a homogeneous isotopic mixture, and the measurement of isotopic composition of the mixture using a thermal ionization mass spectrometer,” Randall R. Parrish and Stephen R. Noble, “Zircon U-Th-Pb Geochronology by Isotope Dilution—Thermal Ionization Mass Spectrometry (ID-TIMS),” Reviews in Mineralogy and Geochemistry 53, no. 1 (2003): 183, https://doi.org/10.2113/0530183.
  3. Please see laboratory “equipment” and “method” blank descriptions at this site for more information. Douglas E. Raynie, “The Vital Role of Blanks in Sample Preparation,” LCGC North America 36, no. 8 (August 1, 2018): 494–497, http://www.chromatographyonline.com/vital-role-blanks-sample-preparation.
  4. Andrew A. Snelling, “Problems with the U-Pb Radioisotope Dating Methods—1. Common Pb,” Answers Research Journal 10 (2017): 142, https://answersingenesis.org/geology/radiometric-dating/problems-radioisotope-dating-u-pb/.
  5. “Secondary ion mass spectrometry is based on sputtering a few atomic layers from the surface of a sample using a primary ion beam and analyzing the emitted secondary ions, distinguished by their mass-to-charge ratio, and ejected from a sample with a mass spectrometer” (Claire Chenu, Cornelia Rumpel, and Johannes Lehmann, “Methods for Studying Soil Organic Matter: Nature, Dynamics, Spatial Accessibility, and Interactions with Minerals,” Soil Microbiology, Ecology and Biochemistry, Fourth Edition, ed. Eldor A. Paul (San Diego, Ca., Academic Press 2015): 406).
  6. “In a typical LA-ICP-MS trace element determination of geological material, a circular laser beam with a diameter ranging from 15 to 100 µm is focused on the area of interest, ablating this part of the sample. The ablated material is then transported to the ICP-MS where it is analysed” (Maurizio Petrelli, Kathrin Laeger, and Diego Perugini, “High Spatial Resolution Trace Element Determination of Geological Samples by Laser Ablation Quadrupole Plasma Mass Spectrometry: Implications for Glass Analysis in Volcanic Products,” ARXIV.org, 2017, https://arxiv.org/ftp/arxiv/papers/1706/1706.10120.pdf).
  7. “Fractionation,” An Introduction to Geology, Salt Lake Community College, http://opengeology.org/textbook/glossary/fractionation/.
  8. “When a mantle rock begins to melt, the incompatible elements will be ejected preferentially from the solid and enter the liquid. This is because if these elements are present in minerals in the rock, they will not be in energetically favorable sites in the crystals. Thus, a low degree melt of a mantle rock will have high concentrations of incompatible elements. As melting proceeds the concentration of these incompatible elements will decrease because (1) there will be less of them to enter the melt, and (2) their concentrations will become more and more diluted as other elements enter the melt. Thus, incompatible element concentrations will decrease with increasing % melting” (Stephen A. Nelson, “Chemical Variation in Rock Suites,” Magmatic Differentiation, Tulane University (January 30, 2012), https://www.tulane.edu/~sanelson/eens212/magmadiff.htm).
  9. Snelling, “Problems with the U-Pb Radioisotope Dating Methods—1. Common Pb,” 153.
  10. N. M. McLean, J. F. Bowring, and G. Gehrels, “Algorithms and Software for U-Pb Geochronology by LAICPMS,” Geochemistry, Geophysics, Geosystems 17 no. 7, (2016): 2482, doi:10.1002/2015GC006097.
  11. Snelling, “Problems with the U-Pb Radioisotope Dating Methods—1. Common Pb,” 153.
  12. Ibid., 163.

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