Friday, November 5, 2010

Zircon Dating: Hadean not so Hadean?

This will include no advice about pursuing a relationship with a mineral.

But it will include some hypotheses on early environmental makeup with the help of half-lives and Australia. In the form of zircons.

So a little bit of background on atmospheric origins: two of the leading theories on how our atmosphere formed that I've found: planetary degassing and comet impact. One of the flaws with the latter are that the D/H (deuterium/hydrogen) ratios of comets that we've studied do not match the D/H of our own planet--Hale-Bopp for one--which reinforces the idea that, while it is possible that some of the responsibility may fall to comets, it can't only come from comets.

What is less known about the atmosphere's formation is the question of when? This is where zircon comes to the rescue.

Deep in the heart of Jack Hills in Western Australia lies some of the oldest pieces of zircon that can give us a hint about the age of our atmosphere. Through measurements utilizing uranium decay (into lead) and the measurements of temperature related to titanium composition, zircons suggest a different Hadean eon.

"For example, oxygen isotope compositions of these ancient zircons suggest the presence of a terrestrial hydrosphere and stable continents only 200 Myr after accretion (Mojzsis et al. 2001)."

The composition of the zircons gives us evidence of the type of environment they were in when they formed, hinting that--since these zircons are in the range of 4.2-4.4 billion years old--the very early Earth had a burgeoning atmosphere.

"Excesses of 129Xe in mantle-derived samples relative to the atmosphere have been interpreted to indicate the presence of live parent 129I in the deep Earth following early degassing of the atmosphere (e.g., Staudacher and Allegre 1982; Allegre et al. 1983) since 129I decays to 129Xe with a half life (t1/2) of only 16 million years. If true, then it is argued that the present atmosphere and hydrosphere must have formed by ~4.4 Gyr (Podosek 1970)."

This posits a different way of dating zircons--not by uranium and lead, but with xenon and iodine, giving further evidence to an earlier-than-though atmosphere.

Lunine textbook


  1. Interesting post NIk! Its incredible that the zircon they found in those rocks is basically almost as old as the Earth itself. I'm just wondering how this compares to carbon dating... I don't think carbon can date much past 75,000 years or so, so the fact that xenon and iodine can date back so much further is really cool!

  2. it is amazing that minerals from 4.4 billion years ago could still exist undisturbed. how could they not have metamorphosed during that time?! i looked up more about this mineral and it realy seems like the perfect dating mineral. they are so durable and resistant to chemical attack that they rarely go away and all kinds of metamorphosis and deformation the core of the zircon itself remains unchanged, and preserves the chemical characteristics of the rock in which it originally crystallized. also it just happens to have uranium in its core allowing to be the perfect 'clock.' crazy!