• Radiometric Dating – Part II Isochrons

    We dealt with the issues around radiocarbon dating here.  That leaves a bunch more to cover.

    • That the atmosphere has had the same (all instead of Carbon 14) concentration in the past as now.  Who knows?  This is a false assumption. Done
    • That the production of parent isotopes has been constant.  Again, who knows?  This is a false assumption.
    • That the original ratio and amounts of the isotope in the fossil or rock is known.  This is based on current processes and assumes the rate of absorption was the same in the past.  This is a false assumption.
    • That the decay rate is constant.  Many things are known to affect decay rates.  This is a false assumption. Done
    • That no contamination has occurred.  This is a false assumption.
    • That no daughter (stable) element was originally in the fossil.  This is a false assumption.
    • That the decay rate was determined accurately.  Decay rates are constantly being modified and criticized.  This is a false assumption.  Done
    • That the only loss of the isotope is due to the decay process.  This is a false assumption.

    Let’s deal with the rest.  We can do that, because these are not assumptions.  These are known issues with radiometric dating and have been dealt with ages ago.

    The first radiometric dating of rocks was done in 1907.  These weren’t very good because a lot of knowledge about radioactivity really wasn’t known then.  But even then, 26 rock samples yielded dates from 250 million years to 1.3 billion years of age.

    The concept of isotopes was discovered in 1913 and it wasn’t for another 17 years before scientists understood that neutrons were the reason for the difference in isotope mass and what caused certain atomic nuclei to be unstable.

    Also in the 30s, geologists finally accepted the use of physics in their field and begun to accept that the age of the Earth was actually a measurable concept.

    What about those issues above?  Well, there are two methods of using radiometric dating that deal with those issues.

    The first is exceedingly simple.  Use multiple testing methods on the same sample.  If there is a huge problem with one kind of dating or even all of them, then the resulting dates should be all over the place.  But that’s not what we see.

    1. St. Severin (ordinary chondrite)
    1. Pb-Pb isochron 4.543 ± 0.019 GY
    2. Sm-Nd isochron 4.55  ± 0.33 GY
    3. Rb-Sr isochron 4.51  ± 0.15 GY
    4. Re-Os isochron 4.68  ± 0.15 GY
    2. Juvinas (basaltic achondrite)
    1. Pb-Pb isochron 4.556 ± 0.012 GY
    2. Pb-Pb isochron 4.540 ± 0.001 GY
    3. Sm-Nd isochron 4.56  ± 0.08 GY
    4. Rb-Sr isochron 4.50  ± 0.07 GY
    3. Allende (carbonaceous chondrite)
    1. Pb-Pb isochron 4.553 ± 0.004 GY
    2. Ar-Ar age spectrum 4.52  ± 0.02 GY
    3. Ar-Ar age spectrum 4.55  ± 0.03 GY
    4. Ar-Ar age spectrum 4.56  ± 0.05 GY

    from Dalrymple, Brent G. (2004). Ancient Earth, Ancient Skies: The Age of the Earth and Its Cosmic Surroundings. Stanford University Press. pp. 147, 169.

    These are radiometric dates from meteorites.  The first uses lead/lead dating, samarium/neodymium dating, rubidium/strontium dating, and rhenium/osmium dating.  Four totally different radiometric methods (and all are isochrons).

    If you sit down with some paper and plot the ranges, you’ll see that all the measured dates overlap between  4.53 GY (billion years) and 4.562 GY.   Because of the different natures of each of these radioactive decay sequences, it’s unlikely that random chance has resulted in 4 totally different dating methods all cover the same range.

    The second method is the use of isochrons.  Here’s the talk.origins FAQ on isochrons.  I’ll try to summarize.

    Basically an isochron is a

    : an imaginary line or a line on a chart connecting points at which an event occurs simultaneously or which represents the same time or time difference (M-W online)

    Isochron dating’s only assumption is that we have no idea what the ratios of parent material, and both radioactive and stable daughter material is.  If this method works, then it neatly dispenses with all of the remaining issues listed by the creationists.

    The scientist takes a sample of rock and then examines two main ratios.  The ratio of the parent isotope and a radiometric daughter isotope and the ratio of the parent isotope to a stable daughter isotope.  Perhaps I should explain.

    When most radioactive elements decay, they do not immediately form a stable atom.  They generally go through a series of other radioactive isotopes before becoming stable.  All of these products are called daughters.  Here’s the sequence for Thorium-232.

    File:Decay chain(4n,Thorium series).PNG
    by Tosaka source:Wikimedia commons (http://en.wikipedia.org/wiki/File:Decay_chain(4n,Thorium_series).PNG)

    In this diagram, the label on the arrows shows the type of decay.  The time shows is the half-life of that isotope.  And it is stacked so that the position shows the atomic number of the isotope.

    The stable daughter in the thorium series is lead-208 (all paths end up with lead).  Any other isotope would work for the unstable daughter isotope in an isochron dating test.

    The other thing that isochron dating requires is that multiple samples be tested from the same source material.  This is because, when these ratios are determined, they are plotted on a graph.  The slope of the line determines the age of material.

    Since two points can make a line, we must have more than two points to effectively determine the age.  The closer all these points are to a straight line, the more accurate the date is.

    The isochron method doesn’t need to determine the amount of parent material.  There is a mathematical equality between the two ratios determined during the dating process.  Because of this, the issues of unknown amounts of parent and daughter materials doesn’t matter.  This is especially true if you can make multiple tests with multiple radioactive isotopes.

    A final note.  Geologists who work with radiodating methods are well versed in determining if a work could possibly have had leeching of the daughter material out of it.  This is especially important with potassium argon dating techniques as the daughter material is a gas and can easily escape.

    Let me know if I wasn’t clear about anything.  Allergy medications sometimes… influence my writing ability.


    Category: ChemistryCreationismPhysicsScience


    Article by: Smilodon's Retreat

    • Eric Collier

      Are the ratios we are measuring the ratios between the radioactive daughter element and the non-radioactive isotope of the same element? Where did the non-radioactive isotope come from? Is it also produced by the decay of the parent element or is it something just naturally present in the sample material?

      • SmilodonsRetreat

        Hi Eric. I’ll answer the questions directly, then attempt to explain why all those pieces are important.

        The first question is yes. If you look at the thorium decay series above, lead appears as both a radioactive isotope (lead-212) and a stable isotope (lead-208). So that would be an excellent choice.

        The non-radioactive isotope came from the decay of the parent, in this case thorium.

        Your last question is the kicker and why isochrons are useful. If the stable isotope only comes from the parent isotope (thorium), then the ratio of lead-208/thorium-232 and lead-212/thorium-232 when taken from several points in the same rock will form a straight line (an isochron).

        If those comparisons of several samples don’t form a straight line, then no isochron is present. Something happened. Maybe lead-208 was already present. Maybe some lead-208 escaped from the rock (somehow). Regardless, no date can be determined from that rock and it’s useless for dating. This kind of thing is very common with decay series that have gaseous daughters (like radon or helium).

        Does that help or am I clear as mud?

    • Denver Greene

      I’m having the most confusion with this paragraph.
      “The isochron method doesn’t need to determine the amount of parent material. There is a mathematical equality between the two ratios determined during the dating process. Because of this, the issues of unknown amounts of parent and daughter materials doesn’t matter”

      It still seems that the Parent, Daughter, and Daughter Isotope are being measured. How does the mathematical equality make it so that the initial levels of those three things not matter?

      • SmilodonsRetreat

        I was going to do a blog post in reply, but I’d be using these images anyway. http://www.talkorigins.org/faqs/isochron-dating.html#isowrong

        Basically, there is a chain of daughter atoms. The parent becomes one daughter atom (D1), then another daughter atom, then another, then another, then another, etc until it becomes a stable atom (D2). The Ds are the isotopes we will use for our study. D1can be any isotope in the chain.

        The isochron is a graph. The x xis is the ratio of Parent to the first daughter isotope. The Y-axis is the ratio of the first daughter to the second daughter (the stable one). If there’s no parent isotope, then the line will intersect the origin.

        If the data points on the plot are colinear, and the line has a positive slope, it shows an extremely strong correlation between:

        The amount of P in each sample, and

        The extent to which it is enriched in D, relative to Di.

        This is a necessary and expected consequence, if the additional D is a product of the decay of P in a closed system over time. It is not easily explained, in the general case, in any other way.

        from the link…

        As P decays, each atom will result in one atom of D1. So D1 is the important bit.

      • shalamabobbi

        They matter, but you don’t need to know them to employ the technique. The initial daughter falls out in the plot as the y-axis intercept value.