Friday, January 24, 2014

Question from a Reader: Radioactive Dating, Part 4

← Read Part 1 for an explanation of how radiometric dating works.
← Read Part 2 for the first two critical assumptions made in radiometric dating.
← Read Part 3 for the third critical assumption, constant decay rate.

The RATE Project

In 1997, a group of six creationist scientists from the Institute of Creation Research (ICR) began an eight-year research project, sponsored by the Creation Research Society (CRS), to investigate the possibility of accelerated radioactive decay.  The project was called RATE, which stands for Radioisotopes and the Age of the Earth.  Their results were presented in 2005 at a conference entitled "Thousands ... Not Billions" and are available to read in their entirety on ICR's RATE page.  The team found several lines of evidence that appear to support accelerated decay, but the results have been met with quite a bit of controversy and criticism.

A cross-section of a polonium radiohalo.
Image source.
Radiohalos, also known as pleochroic halos, are microscopic, spherical, darkened regions of certain minerals, such as zircon and biotite.  Grains of heavy radioactive elements within the minerals form the halos as they decay, giving off particles that disrupt the surrounding crystal structure.  Each decay particle has a certain amount of energy, so it always travels a certain distance through the crystal before stopping, hence the spherical shape.  The energy of the decay particle is defined by the isotope that released it, meaning that, in theory, the radius of the halo can identify the isotope that formed it.  By way of analogy, imagine a firearms expert (or reckless teenager) standing in a spherical bubble in the middle of a giant cube of ballistic gel.  He fires thousands of bullets from a particular pistol in random directions.  He then does the same with a high-powered rifle.  The result would be two spheres of bullets embedded in the gel—a smaller, inner sphere from the pistol, and a larger outer sphere from the rifle.

Some creationists claim that these halos demonstrate that the rocks formed instantly, equating them to bubbles being frozen in water or a grenade explosion frozen in time, but this is highly misleading.  Halos are not miniature explosions captured in time, but they are built up over time by radioactive decay.  The darkened areas are not the decay particles themselves but the path of destruction they left behind.

Specifically, creationists emphasize the radiohalos formed by polonium.  When uranium-238 decays, it becomes thorium, which decays to radium, which decays to radon, which decays to polonium (this is an abbreviated version of the series; see here for the full chain).  Polonium has a relatively short half-life (138 days for Po-210, 164 microseconds for Po-214, and about 3 minutes for Po-218) compared to uranium (245,500 years for U-234 and 4.47 billion years for U-238), so it is interesting that polonium radiohalos are found without any evidence of uranium decay.  Originally, this was presented as evidence that the granites in which they were found were created instantly from nothing rather than from slowly cooling magma.  This view was popularized by Robert Gentry's book, Creation's Tiny Mystery, printed in 1986.  However, subsequent studies have found similar polonium halos in rocks that most creationist geologists interpret as having formed during the Flood (Paleozoic and Mesozoic rocks).

The RATE project expanded on a previously proposed explanation for the polonium halos.  The scientists noted that in order to form enough polonium to make the polonium radiohalos, a significant amount of uranium would have to have decayed, which would take billions of years at today's rates.  Yet, because of the short half-life of polonium, the necessary amounts of polonium would never build up, and it is extremely unlikely that each newly-formed polonium atom was somehow transported to a particular spot before it decayed.  It seems, then, that the uranium must have been decaying at an extremely fast rate (or the polonium delayed in decaying, but either way, the rates are inconsistent).  Andrew Snelling, a geologist associated with the RATE team, proposed that decay rates accelerated during the Flood, and as the rocks were cooling from magma, hot fluids transported chunks of new polonium (and other isotopes) along the cracks in the minerals, depositing them in different locations within the crystals.  Once the rock cooled below 150°C, the crystal structure became rigid enough to be affected by the radioactivity, thus recording the decay as radiohalos.

It took me some time to understand this argument, and I'm still not sure that I grasp all of the implications.  Nevertheless, there appears to be little response from the old-earth community.  The leading criticism seems to focus on the claim that most of the polonium rings were actually formed by radon-222, the parent isotope of polonium-218, because of their similar decay energies.  I find this claim somewhat dubious, as other isotopes in the sequence have similar differences, yet they apparently can be distinguished.  Also, radon-222 has a half-life of just under four days, which does not help the critic's case.  Therefore, this appears to be the leading evidence of accelerated nuclear decay, but I still find it to be uncomfortably tenuous.

Discordant Grand Canyon rock dates
In another study by Dr. Snelling, a series of rocks from the Brahma Schist, a series of metamorphic rocks from the bottom of the column of rocks seen at the Grand Canyon, were dated by professional labs using several different methods.  In the general literature, these rocks are considered to have formed 1.71 to 1.69 billion years ago from basalts that hardened from lava flows 1.75 to 1.74 billion years ago.  Twenty-seven samples were taken from the same outcrop of amphibolite, a type of metamorphic rock, and sent to a laboratory in Canada and in Australia.  Using potassium-argon (K-Ar) dating, the labs gave a range of dates from 405 million years to 2.57 billion years, which is obviously an unacceptable range for a technique that is supposed to give "absolute" ages.  Isochron dating (discussed in part 2) was used with three other methods, yielding dates of 1.24 billion years for rubidium-strontium (Rb-Sr), 1.66 billion years for samarium-neodymium (Sm-Nd), and 1.88 billion years for lead-lead (Pb-Pb).  Even accounting for the reported error margins, none of the age ranges overlapped at all, demonstrating the inconsistency present in the supposedly rigorous isochron dating method.

Though I have not found any specific rebuttals of this study, critics will likely refer to the complex histories of metamorphic rocks to account for the discordant dates.  A preliminary criticism of the proposed RATE research claimed that different decay chains are affected differently by heat, and that different minerals within the metamorphic rocks likely have different histories, with some having formed before metamorphism and some forming during the event.  I am not sure how Snelling's methods differ from the "standard" method of dating metamorphic rocks, so I can't comment much beyond this.

Critics also point out the number of cases in which the methods were consistent in their dates.  As I mentioned earlier, I could spend weeks just naming off the studies that purport to show consistency or inconsistency in the dates.  Curiously, it always seems to work out that old-earthers find concordant dates when they test multiple samples, while young-earthers find discordant dates when they do the same.  Maybe this just shows that radiometric dating is not as unbiased as many claim.

Helium Diffusion

Dr. Humphreys' graph, showing how the experimental data
(blue) compared with models based on young-earth
assumptions (red) and old-earth assumptions (pink).  Note
that the y-axis is logarithmic.
Image source.
One of my favorite studies to come out of the RATE project involves helium in zircon crystals.  We discussed previously how radioactive atoms give off particles when they decay (usually).  One of these particles, the alpha particle, consists of two protons and two neutrons, which is actually the same as the nucleus of a helium atom.  Therefore, when an element like uranium decays, it is producing helium in addition to its daughter isotope.

Helium atoms are extremely small—the smallest, in fact, by diameter of the electron cloud.  This makes them very "slippery," able to fit through the tiniest cracks.  That's why helium balloons seem to deflate so quickly.  So, when helium is produced by radioactive decay inside of a crystal, it will eventually find its way out.

In 1982, Robert Gentry (the same one who popularized radiohalos) published a report detailing the contents of zircons from a borehole in New Mexico.  The zircons had previously been dated at 1.5 billion years, yet Gentry found that over half of the helium produced by all of that decay was still in the crystals!  The exact significance of this was difficult to quantify, however, as there was little data pertaining to how quickly the helium could be expected to diffuse out of the crystals.  Then in 2000, Dr. Russell Humphreys of the RATE group created two models that would produce the measured helium levels: one that assumed that the rocks were 6,000 years old, and one that assumed the rocks were 1.5 billion years old.  The next year, RATE worked through an intermediary mining company to have a non-creationist scientist test the rate at which helium diffuses from zircons at various temperatures.  When the results returned, the data lined up perfectly with the 6,000 year model!  In theory, this result indicates that the rocks are only 6,000 years old, yet have undergone billions of years of radioactive decay, implying an accelerated decay rate in the past.

Needless to say, this study has been met with much criticism from the old-earth community, both theistic and non-theistic.  The charges vary, including insufficient sample size, fudged data, modification of previous works, bad math, and poor geologic considerations.  Dr. Humphreys countered to many of the claims in a 2005 reply to a critic, mostly justifying his interpretation of previous data and demonstrating that many of the supposed "errors" don't affect the result anyway.  The RATE team further reinforced their findings in 2011 at the Fifth International Conference on Creationism in Pittsburgh, PA with additional data and responses to critics, including counters to claims that past heating events would have skewed the helium levels.  In my opinion, RATE's argument and data present a very strong case for the young-earth position on radioactive decay.

I hope you enjoyed this series on radiometric dating!  Feel free to leave your own opinion in the comment section, and remember to email with your own questions!

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