"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.