February 15, 2016 | David F. Coppedge

"Fundamental Assumption" in Zircon Dating Called into Question

A commonly-used dating method has been threatened by new findings that undermine assumptions.

Crystals of zircon often contain uranium and have been used for a long time to date rocks into the millions-of-years range. The assumption has been that the parent (U) and daughter (lead, Pb) remain locked in the tight crystal lattices of zircon, so that mineralogists can accurately measure ratios of the elements resulting from radioactive decay. That assumption has been called into question by a new paper just published in Nature Communications. First, the impact:

Our findings have important implications for the use of zircon as a geochronometer, and highlight the importance of deformation on trace element redistribution in minerals and engineering materials.

Now, the reasons for the concern:

Trace elements diffuse negligible distances through the pristine crystal lattice in minerals: this is a fundamental assumption when using them to decipher geological processes. For example, the reliable use of the mineral zircon (ZrSiO4) as a U-Th-Pb geochronometer and trace element monitor requires minimal radiogenic isotope and trace element mobility. Here, using atom probe tomography, we document the effects of crystal–plastic deformation on atomic-scale elemental distributions in zircon revealing sub-micrometre-scale mechanisms of trace element mobility. Dislocations that move through the lattice accumulate U and other trace elements. Pipe diffusion along dislocation arrays connected to a chemical or structural sink results in continuous removal of selected elements (for example, Pb), even after deformation has ceased. However, in disconnected dislocations, trace elements remain locked.

This means that parent and daughter elements in the radioactive decay chain are not locked into the crystal: they can move.

Our results demonstrate the importance of deformation processes and microstructures on the localized trace element concentrations and continuous redistribution from the nanometre to micrometre scale in the mineral zircon. Dislocation movement through the zircon lattice can effectively sweep up and concentrate solute atoms at geological strain rates. Dislocation arrays can act as fast pathways for the diffusion of incompatible elements such as Pb across distances of >10 μm if they are connected to a chemical or structural sink. Hence, nominally immobile elements can become locally extremely mobile. Not only does our study confirm recent speculation that an understanding of the deformation microstructures within zircon grains is a necessity for subsequent, robust geochronological analyses but it also sheds light on potential pit-falls when utilizing element concentrations and ratios for geological studies. Our results have far-reaching implications for the interpretation of local elemental variations in not only deformed minerals but also a range of engineering materials.

The authors do not provide any specific examples of rock dates being misinterpreted either as older or younger, or by how much the error could be. They only show that a “fundamental assumption” in the dating method is not true; the elements can move quickly and become “extremely mobile.” For this reason, they warn, “when interpreting local elemental and isotopic variations in both deforming and deformed crystalline materials, a thorough characterization of deformation-related dislocation structures is essential.

This is not the first time zircons have been called into question as geochronometers. See “Geological Theories Are Not Set in Stone” (1/07/16), “Major Scientific Revolutions Are Still Possible” (11/24/15), “How Rocks Can Look Older Than They Are” (4/08/15), “The Trouble with Zircons” (3/25/13), “Uranium-Lead Dating Fraught With Discordance” (1/08/13), and “Discovery Upsets Geological Dating” (11/17/11).

We do not know the degree of impact this paper will have on interpretations of rock ages other than the authors’ warnings that the implications could be “far-reaching”. Creation geologists may wish to dig into the details of this open-access paper and offer comments below. Perhaps the consequences will be minimal; perhaps not. It depends on how it affects standard methods of measuring elemental ratios.

Even if the impact of these findings is low, there will still be problems with other assumptions. Creationists may remember the findings of ICR’s RATE project that showed unexpected helium retention in zircons under high heat in deep wells (see explanation by D. Russell Humphreys at ICR). This new paper appears to present a possible significant challenge to another leading assumption about zircon dating that would reinforce the RATE finding. For if lead can diffuse around in the crystal, how much more the slippery, lightweight noble gas helium?

If nothing else, this paper points out that long-age dates are not “set in stone” like some kind of sealed time capsule. You have to make assumptions to interpret a measurement, and assumptions are subject to change. So what other dating methods will have their assumptions questioned in the future? Be careful when scientists offer “proof” of long ages.

 

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Comments

  • Jon Saboe says:

    Wise to not jump in with elation as if this totally DIS-proves all radiometric dating.

    What is most important to remember is that the different forms of radiometric dating — when applied to events of KNOWN dates — is always millions or tens of millions of years off.

  • drhumph says:

    Your article mainly questions uranium-to-lead dating in zircons, but at the end it asks a question about the creationist helium-leak dating method in zircons, part of the Radioisotopes and the Age of the Earth (RATE) research initiative. If you go to the source technical article in Nature Communications at

    http://www.nature.com/ncomms/2016/160212/ncomms10490/full/ncomms10490.html ,

    you’ll find that the article raises the possibility of only submicron (less than one micron) movements of uranium and lead atoms in zircons that show some evidence of having been slightly deformed in their early history, when they were very hot. (At lesser temperatures, zircons are very hard and difficult to deform.) Typically the zircons are dozens to hundreds of hundreds of microns in size (RATE’s were about 50-60 microns long), and the usual method of uranium-lead dating looks at the total amount of each isotope in the whole crystal. So sub-micron movements of those atoms within the crystal wouldn’t significantly affect those older U/Pb methods.

    However, a newer technique based on a device called a sensitive high-resolution ion microprobe (SHRIMP) has been coming into use. It looks at submicron pockets of uranium and lead in the zircons to try to figure out what happened to the zircon at various times in its history. This article is raising the possibility that lead atoms could move fractions of a micron away from the uranium atoms that produced them, and vice versa. That, of course, could affect the interpretation of SHRIMP data. But it wouldn’t affect the whole-crystal U/Pb dates for RATE’s zircons, about 1.5 Ga.

    Now we get to the question in your article:

    “This new paper appears to present a possible significant challenge to another leading assumption about zircon dating that would reinforce the RATE finding. For if lead can diffuse around in the crystal, how much more the slippery, lightweight noble gas helium?”

    Yes, of course, the helium atoms might move around more easily in a zircon that had suffered some deformation. However, that would not affect RATE’s helium-leak dates at all. That’s because we measured the helium leak rates (diffusion coefficients) on the zircons as they are today, and at the temperatures they experienced at present in the borehole. Essentially all we did was divide the amount of helium they had lost by their present leak rates to get 6,000 (+/- 2000) years. So even if RATE’s zircons were presently leakier than normal (I don’t think they were) because of past deformation, so what?

    — Russ Humphreys
    RATE helium diffusion project

    • Editor says:

      Thank you for the comment, Dr. Humphreys. I was hoping the paper would generate some discussion about whether it was a matter of consequence for interpreting dates or not. The paper itself did not say; it was just questioning an assumption used in the method. Your clarification is valuable; you are among the best to know. Thanks again.

  • Buho says:

    Thank you for your comments, Dr. Humphreys. I’ve been following the zircon helium debate for a long time.

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