The Trouble with Zircons

Posted on March 25, 2013 in Dating Methods, Geology, Physics

Geologists’ favorite tool for dating rocks at millions and billions of years old has revealed problems with interpretation.

Zircons are hard minerals that can contain uranium.  To secular geologists, they act like ideal time capsules, sealed from outside influences. They are used frequently to date deposits with a claimed high precision, but is it possible to trust them too much?

The abstract of a paper in Geology by American scientists claims that detrital zircons show “no drainage link between southern California rivers and the Colorado Plateau from mid-Cretaceous through Pliocene.”  The zircons told a tale at odds with views that the Grand Canyon began cutting hundreds of millions of years ago.

Although Paleogene headwaters of southern California rivers extended into the eastern Mojave Desert, Sonoran Desert, and Mogollon Highlands, our results indicate that these headwaters did not extend as far as the Colorado Plateau. This conclusion conflicts with the hypothesis of a Paleogene southwest-flowing Arizona River, but supports late Miocene–Pliocene drainage reorganization and integration of the Colorado River coincident with development of the Salton Trough and Gulf of California.

The canyon is young, in other words.  The interpretation, however, required reversing the assumed amount of slippage along the San Andreas Fault and deciding which zircons had been locally recycled.

Another paper in Geology said that zircons can give meaningless dates if not properly interpreted.  In “Mobilization of radiogenic Pb in zircon revealed by ion imaging: Implications for early Earth geochronology,” a team from Sweden, Poland and Australia began with a warning: “Zircon is arguably the most commonly used geochronometer, but the reliability of ages obtained requires a full understanding of processes that might compromise the integrity of its U-Pb systematics.”

In an effort to quantify the effects of metamorphism, the team used an ion microprobe on Antarctic zircons and found concordances and disconcordances.  “Ion images of uranogenic Pb reveal a surprising micrometer-scale patchy distribution that is unrelated to crystal morphology or damage,” they said.  The abstract cautioned,

The 207Pb/206Pb ratios within these subdomains correspond to apparent zircon ages as old as 4.2 Ga. These are interpreted as artifacts of ancient redistribution of radiogenic Pb, a process that can generate meaningless ages, and are not relicts of ancient (including Hadean) zircon. Scanning ion imaging thus facilitates identification of unsupported radiogenic Pb and enables testing of the validity of old ages from zircon known to have a long and complicated history.

Careful reading of that statement might raise more questions than the geologists are asking.  Who does the interpreting?  What rules of interpretation are employed?  Do those rules change over time?  How do they know when and where an “ancient redistribution” of radiogenic lead occurred?  If that process can produce meaningless ages, what other undiscovered processes might also generate meaningless ages?  Who decides what is a relict of ancient zircon or not?  How is “unsupported” radiogenic lead identified?  Can rules for the validity of zircon ages be established independent of the consensus geologic time scale?  Who knows how long and complicated the history of these zircons was?  How is it determined it was a long history, independent of the belief they were ancient?  If the history is complicated, what unknowns, unknown unknowns and unknowable unknowns might have contributed to the complications?

Despite these concerns, Geology routinely updates its official time scale.  The latest was published in December 2012, claimed to be astonishingly accurate for time periods no human has ever observed: “Over the past 100 years, the confluence of process-based geological thought with observed and approximated geologic rates has led to coherent and quantitatively robust estimates of geologic time scales, reducing many uncertainties to the 0.1% level.”

We’ll leave it to geophysicists to evaluate the import of these papers, but philosophers of science and competent readers skeptical of ruling consensi should be alert to hand-waving exercises used by practitioners within the guild who might be blind to questions outside the paradigm.

 

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