How Rocks Can Look Older Than They Are

Researchers find that the most common dating method can produce “spuriously old” dates.

A team from Europe took a closer look at how uranium-lead ages are determined, and found problems. One of the assumptions going into dating zircons (zirconium silicate crystals encasing uranium that decays to lead) is that the clock is “reset” when the parent rock under goes the high heat and pressure of metamorphism. This team found that nanosphere inclusions of extraneous metallic lead (Pb) can confuse the dating technique, making the rock look older than it is. Writing in PNAS, they say:

Zircon (ZrSiO₄) is the most commonly used geochronometer, preserving age and geochemical information through a wide range of geological processes. However, zircon U–Pb geochronology can be affected by redistribution of radiogenic Pb, which is incompatible in the crystal structure. This phenomenon is particularly common in zircon that has experienced ultra-high temperature metamorphism, where ion imaging has revealed submicrometer domains that are sufficiently heterogeneously distributed to severely perturb ages, in some cases yielding apparent Hadean (>4 Ga) ages from younger zircons.

The paper provides what they feel are safeguards to prevent erroneous dates. It appears, however, that this finding overthrows common assumptions used in the dating technique.

The reliability of the oldest zircon ages, which include some reversely discordant analyses (i.e., with U–Pb ages older than ²⁰⁷Pb/²⁰⁶Pb ages), has been questioned based on evidence from ion imaging for disturbance of the U–Pb system. This is important because ²⁰⁷Pb/²⁰⁶Pb ages are generally considered to be more robust than U–Pb ages for older zircons. However, if radiogenic Pb has been decoupled from its parent U and not locally incorporated into the crystal lattice during an ancient geological event, when radiogenic ²⁰⁷Pb/²⁰⁶Pb values are significantly higher than at present, reverse discordance and spuriously old ²⁰⁷Pb/²⁰⁶Pb age estimates may result.

In other words, the more lead in the crystal (“decoupled from its parent U”), the more a scientist might infer that it is billions of years old, when some of that lead got mixed in when a younger rock underwent metamorphism. Science Daily has a photograph of nanospheres of metallic lead embedded inside a zircon under the headline, “True ages of rocks might be distorted through Earth’s history.” Moreover, since the nanospheres are not uniformly distributed, the date could depend on the sample selected, like a biopsy missing the cancer. “The inhomogeneous distribution of lead in zircon might adulterate the ages measured with high-spatial resolution ion probe technique.”

Update 4/13/15: Smithsonian scientists used zircons to date the formation of the Panama land bridge, according to Science Daily and Science Magazine (see full paper in Science). This is supposedly the time when North American animals could reach South America. The new date of 13-15 million years “could rewrite the geological history of the Americas,” Lizzie Wade writes in Science Magazine, because it is “more than 10 million years earlier than previously thought.” A lot of evolutionary weight is being placed on these zircons.

In the ICR publication Acts & Facts, Dr. Vernon R. Cupps has been publishing a detailed analysis of how results can be corrupted in radiometric dating.  He has shown numerous ways that deceptively old dates can be produced, depending on the assumptions used. The mathematical techniques are sound, but like with computer programs, wrong assumptions can make for garbage-in, garbage-out conclusions.  Those interested may wish to study this new PNAS paper to see how often this problem occurs in practice.