Friday, September 26, 2008

The world's oldest rock is punny

I finally found time to read the paper describing the new World's Oldest Rock. I wanted to blog about why news reports seemed cautious about trusting its age (for instance, see Richard Kerr's write-up in Science). I need to spend more time thinking through the isotope geochemistry of the heavy rare-earth elements before that's possible, unfortunately. (Maybe a fan of extinct isotopes could help, since the argument is based on decay of 146Sm?)

But the problem, really, is that I got distracted by the mineralogy of the gneiss in question. The rock is described as a "faux-amphibolite," and is made of plagioclase, biotite, quartz, some garnet, and...

...wait...

...cummingtonite.

That's right. The world's oldest rock contains one of the favorite minerals of dirty-minded geology majors.

Other discussions in the geoblogosphere:

The Volcanism Blog
NOVA Geoblog
goodSchist (Chris mentions familiarity with the 146Sm-142Nd system; maybe he could help evaluate the paper)
olelog
About.com geology

(And they did a good job of linking to discussions in the media, so I won't try.)

7 comments:

1&2 said...

Har har! Cummingtonite jokes will never get old; in fact, I think I need to make a (Mg,Fe)7 Si8 O22 (OH)2 based t-shirt...

Chris said...
This comment has been removed by the author.
Chris said...

I started writing a comment, but it ended up being so long I turned it into a blog post:
Short- and long-lived radio nuclide dating and the world’s oldest rocks.

I'm sure Chuck at Lab Lemming will correct me somewhere, but this is how I understand it to work.

Kim said...

Chris - I tried to post this comment to your blog, but it didn't work. (I got an error about a precondition evaluated to false.) So I'm posting it here, so I won't lose the thought.

Thanks. I was wondering whether the technique as used on Earth is related to the more familiar (to me) epsilon-Nd technique. As I understand it, Sm and Nd fractionate during partial melting of the mantle, but not during partial melting or metamorphism in the crust. (Though I know garnet can be dated by Sm/Nd, so there's got to be some partitioning during garnet growth, at least.)

Also, I got the impression from the paper that they searched pretty hard for zircon in the rocks, and could only find it in felsic pods that looked texturally late. (And which gave younger ages.)

Chris said...

I've fixed the problem on my site, turns out having the word "dating" in the URL set of the spam filter. (new url is here)

I'm a little rusty on partial melting REE fractionation insofar as the mantle/crust goes, so I've put it on my "read again list".

The Sm-Nd dating used in garnets will use the Sm isotopes with MUCH longer half lives than 146Sm.

146Sm = half life of ~100 Myr
147-149Sm = half lives of hundreds of billions of years.

147, 148 and 149Sm all decay to different Nd isotopes, which makes for a handy suite of nuclides.

I haven't read the paper yet (will grab a copy at school on Monday), but not finding zircons in textually appropriate phases is a real shame.

Chuck said...

The paper is on my to-do list, along with Bennett et al. 2007 (142Nd in Greenland and WA).

Short answer is that everything got reset at 3.7 during metamorphism, but an 4.2Ga protolith is the most sensible interpretation of the 142Nd.

andrew said...

I think the whole thing is just a bunch of blueschist.