Wednesday, October 21, 2009

GSA update 4

My final day at GSA was fruitful. I started off in the "Earth, et al." session hosted by ODU's Nora Noffke. It was devoted to the Precambrian, and had some interesting talks about fluctuating oxygen levels, mineral evolution, microbially-induced sedimentary structures, and Neoproterozoic glaciation. This last one was most interesting to me: UMD's Jay Kaufman talked about field work he conducted in Siberia last summer, documenting a diamictite unit between Ediacaran strata and Cambrian strata. There's even a carbon isotope excursion to match up with it! Cool... literally.

I had lunch with my friend David Dantzler, who I hadn't even realized was at the conference, until I saw him come in to one of the Darwin-focused sessions. In the afternoon, I attended another eight talks, including some on greenstone belts in South Africa, some on geological education, and a couple about the evolution of orogens, with an emphasis on South America. (One of these was an excellent talk by Brian Romans about his field area in Patagonia.) I finished up with Kim Hannula's talk about the geoblogosphere's role in supporting women geoscientists. Then it was time to bug out: back to the hotel, then to the airport, then to Los Angeles, then to Dulles, where I arrived this morning at 6:30am. On the flight, I took an Advil PM, put in earplugs and wore one of those little eye-masks so I could get some decent amount of sleep... Mixed success on that front. Once I got to Dulles, I got some coffee, and headed straight to work! It's good to be back in the familiar environs of my office and lab again. Thanks for a great conference, everyone!

Also: GSA is maintaining a webpage summarizing the various posts from registered geobloggers. It's incomplete, but a useful idea: a repository for all the stuff being said about the conference from the various attending geobloggers.

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posted by Callan Bentley at 12:53 PM 0 Comments

Tuesday, January 27, 2009

Andesitic meteorites and what they mean

Blogging on Peer-Reviewed ResearchJames Day (of the University of Maryland, College Park) presented last Wednesday at the Geological Society of Washington. He gave a talk entitled "Evidence for evolved crust formation in the early solar system." I would describe this presentation as a "game-changer," and I'll tell you why.

James described the Antarctic discovery* of two pieces of a new kind of meteorite with an andesitic composition. A clear fusion crust indicated it was a meteorite, and not just a rock from the Antarctic crust. (Isotopic evidence corroborates this, as you'll see.) The meteorite was in two pieces, which are respectively referred to as Graves Nunatuk (GRA) 06128 and 06129. Here's a plot from James' (et al.'s) Nature paper a few weeks ago showing the meteorite's composition:

meteor_comp

Black dots are actual measurements, and the gray blob is the calculated composition based on variations in mineralogy and mineral major element compositions. The meteorite has an 207Pb-206Pb age of 4.5 billion years, and oxygen isotopes plot far off the terrestrial fractionation trend:

not_from_earth
Everything from our planet plots on that upper horizontal line (including the Moon). This sample of evolved crust is therefore not from the Earth or the Moon. James also ruled out Mercury and Venus as potential sources, and suggested that it may be a fragment of a parent body in the asteroid belt. As the diagram above shows, the oxygen isotopes suggest an affinity with a group of meteorites called brachinites. (As near as I can tell, brachinites are usually ultramafic. At any rate, there have never been andesitic meteorites of any flavor known prior to GRA 06128/9.)

Highly siderophile element patterns suggest that there was no core formation in the parent body (these elements were still present in the sample; indicating they had not sequestered themselves in a metallic core). James also reported that pyroxene exsolution lamellae work by another group indicates a shallow depth of formation, on the order of 15-20 meters depth. (This, however, is extrapolated from pyroxene exsolution lamellae work on the Skaergaard Intrusion in Greenland; how well the method translates to an asteroid forming at the dawn of our solar system is another question. This generated a lot of questions at the GSW talk.) Large amounts of Na-rich plagioclase in GRA 06128/9 suggest partial melting of 10-30% in regions of the parent body. Assuming a chondritic, oxidized, volatile-rich starting composition, this could generate the large amount of Na-rich plagioclase seen in the samples.

So they're andesitic in composition, but otherwise like brachinites. In an e-mail to me, James noted that, "they have uncannily similar HSE patterns (and key ratios like Pd/Ir etc. are similar), O isotopes are in the right ballpark, they required about 30% partial melting (whether they are residues or cumulates; we haven't quite figured that out yet) and the accessory phases in these meteorites also imply a volatile rich parent body."

So why should you care? Why would I call this a "game changer?" It's because it really stretches our thinking. The nebular hypothesis of the solar system's formation has meteorites' composition as the starting material for the rocky planets. On earth, this meteoritic ("chondritic") composition compacted under the influence of gravity, then differentiated into layers based on density (a process facilitated by higher temperatures due to more radioactive decay early in the planet's history). Dense iron and nickel flowed down to make the core (joined by those HSEs), the medium-weight stuff became the 'silicate Earth' (mantle + crust), and the lightweight stuff formed an early atmosphere, most of which was likely stripped away by the erosive effects of the solar wind. (This is inferred to have taken place before the development of a magnetic field.)

Then, over time, the ultramafic-composition mantle partially melted to form basaltic-composition oceanic crust, which probably at first appeared like the surface of a lava lake (e.g. Kilauea Iki). This basaltic scum participated in a rudimentary form of plate tectonics, which encouraged partial melting via subduction (and the generation of a new atmosphere, but that's another story). The resulting magma would likely have been andesitic. In other words, on Earth, our andesite comes from plate tectonics, and that likely took a while to get going.

The assumption, in other words, was that crustal evolution ("distillation," in my parlance) took some serious time on a serious planet. But if crust evolved to andesitic compositions this early on non-Earth, non-plate-tectonic, non-planetary bodies, it really changes our understanding of early-formed materials in the solar system. I am reminded of the example of the Jack Hills zircons in Australia. Preserved as part of sedimentary rocks, these zircons crystallized about 4.4 billion years ago. Isotopic examination of the Jack Hills zircons suggest that they formed in a granitic rock. And granites are the most evolved of igneous rocks (the highest "proof"). Granites make up continental crust.

So the Jack Hills zircons similarly stretched our conception of when the earliest evolved crust formed on the planet Earth. I mean; Earth had granites 4.4 billion years ago? Prior to their discovery, most geologists would not have predicted so early a date for evolved crust. But the evidence suggests that's indeed how it was. And now, thanks to James Day's study, our imaginations are being similarly stretched regarding the origins of evolved crust on extraterrestrial bodies, too.

What else is there we don't know about our planet, our solar system? Probably a lot.
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Original paper in Nature: James M. D. Day, Richard D. Ash, Yang Liu, Jeremy J. Bellucci, Douglas Rumble III, William F. McDonough, Richard J. Walker & Lawrence A. Taylor. "Early formation of evolved asteroidal crust." Nature 457, 179-182 (8 January 2009). doi:10.1038/nature07651

Nature Podcast discussing (among other things) the meteorites.

Press release from the University of Maryland.
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* By the Antarctic Search for Meteorites program, which has blogged their expeditions in the past, and apparently just concluded the 2008-09 search.

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posted by Callan Bentley at 11:40 AM 1 Comments