Consistent Radiometric dates
by Joe Meert
Created Jan 2000
Updated January 6, 2004 (links fixed, added)

    One of the main objections to radiometric dating (on the part of young earth creationists) is that radiometric ages do not agree with each other or that contamination renders ages meaningless.   In fact, the claim is partially true.  Early mass spectrometers were not as sensitive as machines today and the methods for separating, cleaning and analysis were less sophisticated.  Although ye-creationists like Snelling talk about contamination of isotopic systems as if it were a foreign concept to modern geology, most geochronologists routinely check for possible contamination using a variety of methods.   In addition, geologists recognized that rocks could be contaminated with excess daughter or parent or loss of parent/daughter that would also affect the age as determined by radiometric methods.  Creationists have seized upon these discoveries and held them forth as evidence that radiometric dating is inaccurate.  But is this the case?   Simply put each radiometric system is based on the assumption that each system has a different half-life (derived from the decay 'constant' which is simply the length of time it takes for 1/2 of the radioactive parent to decay to a stable daughter).  In addition to variable half-lives, each mineral will 'close' at different temperatures (closure, is simply defined as the point where no daughter/parent is lost or gained*).   There are a number of different methods that geologists use to check for loss/gain and these are incorporated into most analyses (isochron methods, stepwise degassing etc).  If radiometric decay rates are not constant and rocks behave as open systems, it would be the exception, rather than the rule, for ages to agree with one another.    Here are a few examples in the recent literature of radiometric age determinations on the same rocks (using different isotopic methods^a).

 The ages of the Fen Complex (A,B) are on two separate dikes within the Fen Complex.  Not only are their ages similar, but the direction of magnetization in the rocks is also identical and indicates that Oslo, Norway was located at about 30 degrees south at the time.  This is an important consideration.  In order to refute the ages, ye-creationists must not only explain how three different isotopic systems (with different decay constants^a and chemical behavior) all gave the same age and the same magnetic direction.   It is also not trivial that the magnetic direction in these rocks indicates that Norway has moved northward following the emplacement of these rocks.

  The Madagascar results are equally intriguing since they are from two regions on the island.  These basalts (and gabbros) are thought by conventional geologists to have formed as Madagascar moved over the Marion hotspot during the Cretaceous.  The basalts overlie continental sandstones containing Mesozoic fossils and are overlain by limestones with Cretaceous-age fauna.  The first two ages are from southern Madagascar and the bottom two are from the northern part of Madagascar.    According to paleomagnetic data from these rocks (Torsvik et al., 1998), northern Madagascar passed over the hotspot before southern Madagascar in perfect agreement with the geochronologic data. Furthermore, these ages all fall within a time period when the Earth was in a long period of no magnetic reversal (called the Cretaceous Long Normal).  Indeed, if the ages are correct, then the paleomagnetic data should all be of a single polarity (and normal).  That is exactly what Torsvik et al. (1998) found.  Geochemical data (Ashwal, personal communication) indicate that these rocks all originated from the same source.    Once again, ye-creationists are faced with the daunting task of explaining why two isotopic systems gave the same age and why the progression of ages is consistent with the paleomagnetic and geochemical data.  

     Closure temperature* of isotopic systems also provides a check of radiometric dates.  In slowly-cooled igneous bodies such as granites, different minerals become closed systems at different temperatures (McDougall and Harrison, 1999).   This is due, at least in part, to the fact that the minerals crystallize at different temperatures.  The temperatures at which minerals close is easily verifiable through experimentation and this has been conducted numerous times including the famous experiments of Bowen.   Therefore, if a body has indeed cooled slowly then the radiometric dates from that rock should demonstrate such a cooling trend.   The Carion pluton in central Madagascar is approximately 20 kilometers in diameter and is the subject of an ongoing paleomagnetic study (Meert et al., 2001).  The rocks have been dated using both the U-Pb system and the ⁴⁰Ar/³⁹Ar system on a variety of minerals.  The following table outlines the closure temperatures of the various minerals used in the Carion study along with their ages.  Details regarding closure temperature studies can be found in McDougall and Harrison (1999).

    Note that the ages of the minerals yields a cooling-curve that is consistent with the experimentally-derived closure temperatures of the isotopic systems.   Had decay rates not been constant, then we might expect to see a gross discordance of mineral ages in this study.  Instead, we see a very nice cooling curve for this magma.  The story doesn't end there however!  This study also included a look at the paleoposition of Madagascar at the time this rock cooled.  This is done through the study of paleomagnetism.  Madagascar was thought to be a part of a larger supercontinent called Gondwana during this time period.  A reference curve for Gondwana has been developed that basically traces the paleoposition of Gondwana during the time interval from 550-475 Ma (Meert et al., 2001a, 2001b)..  If Madagascar was indeed a part of this supercontinent, then the paleomagnetic directions for Madagascar should be identical to the directions from other continents that make up Gondwana.  Since magnetic minerals in the Carion rocks lock in their directions at temperatures between 550-450 C (in this study), then the age of magnetization is about 508 +/- 11 Ma.  The position of Madagascar should match up with other 510 Ma directions from Gondwana---and they do!.   Here, as above, we have many independent verifications for the age of the Carion pluton that are internally self-consistent.  This presents a serious problem for those who might advocate random decay rates for the different isotopic systems.

    None of these examples are isolated, such examples of internally consistent decay rates that are supported by other types of studies are found throughout the geological literature.  Young Earth Creationists have little support for their contention that radiometric ages are unreliable.   Creationists have also proposed that radioactive decay was faster in the past, but this has some serious consequences for the notion of a young earth.  You can read about that in Roasting Adam.  If you are interested in problems with global flood chronology used by young earth creationists you can view my website at Noah's Flood.

Cheers

Joe Meert

^aTable of Decay constants and commonly used isotopes

Other Essays by the Author:

1. Roasting Adam
2. Paleosols and the Geologic Column
3. Intelligent Design or Constant Tinkering
4. Northrups unsupportable flood chronology.
5. The Depth of the Oceans and Runaway Subduction.
6. Creationist FAQ's and Discussions
7. RATE: ICR's RATE Project Flaws

Reference List:

1. Torsvik et al., 1998, Late Cretaceous magmatism in Madagascar: paleomagnetic evidence for a stationary Marion hotspot, EPSL (64), 221-232.

2. Meert et al., 1998, Tectonic significance of the Fen Province, S. Norway: Constraints from geochronology and paleomagnetism, J. Geol., 106, 553-564.

3. Storey et al., 1995, Timing of hotspot related volcanism and breakup of Madagascar and India, Science, 267, 852-855.

4. Dahlgren, 1994, Late Proterozoic and Carboniferous ultramafic magmatism of carbonatitic affinity in southern Norway, Lithos, 31, 141-154.

5. Dahlgren, S., 1987, The satellitic intrusions in the Fen carbonatite-alkaline rock province, Telemark, southeastern Norway. distribution, emplacement, compositional characteristics and         petrology., Thesis University of Oslo, 298 pp.

6. Verschure et al., 1983, dating explosive volcanism perforating the Precambrian basement in southern Norway, Norg. Geol. Under., 380, 35-49.

7. Andersen and Taylor, 1988, Pb isotope geochemistry of the Fen carbonatite complex, SE Norway, age and petrogenetic implications, Geochim, cosmochim acta, 53, 1067-1076.

8. McDougall, I. and Harrison, T.M., 1999, Geogchronology and thermochronology by the ⁴⁰Ar/³⁹Ar method, Oxford University Press, Oxford, 269 pp.

9. Meert, J.G. et al., 2001a, Paleomagnetism, geochronology and tectonic implications of the Cambrian-age Carion granite, central Madagascar, Tectonophysics, 340, 1-21.

10. Meert, J.G. et al., 2001b, Slow-cooling of a late-syn tectonic pluton: constraints from laser ⁴⁰Ar/³⁹Ar feldspar modelling, Gondwana Research, 4:3, 541-550.

11. Andersen and Sundvoll, 1986, Strontium and neodymium composition of an early tinguanite in the Fen Complex, Nor. Geol. Unders. Bull., 409, 29-34.

*Closure temperature is more technically defined in Dodson as the temperature of the system at the time given by its apparent age. 

** The original published ages on these samples were 539 +/- 14 Ma by Andersen and Taylor (1988). These whole rock carbonatite samples younger ages depends heavily upon two points from pyrochlore samples.  Dahlgren (1994) noted that the pyrochlores are metamictic and spongy suggesting U-Pb mobility.  The 573 +/- 60 Ma age is determined when the pyrochlore samples are removed from the analysis.  The original Rb-Sr age from Andersen and Sundvoll (1986) was 550 +/- 7 Ma using K-spar, nepheline, biotite and one whole rock phonolite sample.  Dahlgren (1994) notes that the biotite fracion from this phonolite are poorly preserved (see also Dahlgren, 1987) and therefore Dahlgren performed a regression without the biotites to obtain the reported age above.  Dahlgren (1994) notes the importance of examining each mineral used in a particular analysis in order to assure the most reliable age estimates.