NOVA Geoblog

Tuesday, April 14, 2009

Another Namibia shot: The Hoba Meteorite

Following on yesterday's Namibiferific post, I'd also like to share this image:

More on this, the largest known intact meteorite on the Earth's surface.

Labels: africa, meteors, namibia, travel

Tuesday, January 27, 2009

Andesitic meteorites and what they mean

James 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:

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:

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.

Labels: antarctica, astronomy, australia, basalt, gsw, hadean, igneous, isotopes, meteors, minerals

Monday, October 20, 2008

Meteorites in Ordovician limestones!

My friend Mark Tyra (a College Park geology master, like myself) just alerted me to a cool site which explores the presence of meteorites preserved in Ordovician limestones in Sweden. Check it out!

Labels: limestone, meteors, sweden, websites

Friday, September 26, 2008

Virginia's extraterrestrial impact crater

The largest meteorite (or maybe comet?... we don't really know which) impact crater in the United States is in Virginia, underneath the lower Chesapeake Bay. In the Eocene, a large bolide (unidentified space chunk) slammed into the Earth. Dating of microfossils found in the same sedimentary layers as impact ejecta have provided a date of ~35.5 Ma for the event. The impactor hit on the continental shelf offshore of Eocene Virginia, carving through the Atlantic-deposited sediments there and gouging into the crystalline bedrock beneath (igneous and metamorphic rocks like the modern Piedmont province, but buried beneath Coastal Plain layers).

The crater was discovered over a ten-year process that began with offshore sampling near Atlantic City, New Jersey in the mid-1980s. Those drill cores came up with a layer of ejecta (including shocked quartz and little beads of glass called tektites) among the late Eocene layers of sediments. Searching around, eventually the crater was seismically imaged by oil exploration in the Chesapeake Bay in the mid-1990s.

Centered on Cape Charles, Virginia, the crater is about 50 miles across, but appears wider as sedimentary layers adjacent to the hold have slumped inward along listric faults. The James, York, and Rappahannock Rivers all trend into this depression, and ultimately the crater is probably responsible for the Susquehanna River taking on its southerly course. When sea level rose and flooded the valley of the Susquehanna, the Chesapeake Bay was formed.

A similar impact structure offshore of New Jersey, the Toms Canyon Impact Crater, may have formed at the same time as the impactor broke into pieces before impacting.

The lead-off image to this post is by the team at the U-Haul trucking company, which performs a terrific public service by finding out interesting things about the different states (and Canadian provinces) and posting them on the sides of their trucks with eye-catching graphics. A great many of the topics they choose are about geology, from minerals to fossils to impact craters to cartography and canyons. A while ago, I wrote an article for Geotimes looking at their program.

More information on the crater:

Wikipedia's entry on the crater.
W&M Geology Department's page about the crater.
USGS team examining the crater.
National Geographic article (2001).

Labels: art, cenozoic, coastal plain, geology, meteors, new jersey, piedmont, rivers, virginia

Thursday, August 28, 2008

"Target Earth" article in National Geographic

Just now getting around to reading last month's issue of National Geographic, but it's a good thing I got to it yesterday rather than tomorrow -- because today's the day I talk about comets, asteroids, and meteors in Physical Geology class, and one of the topics that people always love to talk about is what happens when those big dumb space chunks smack into Earth.

The article's a good read, and illustrated with magnificent images, like this classic 1972 image of a fireball over Jackson Lake, Wyoming. That's the Tetons in the background. Check it out.

Labels: meteors, news, wyoming

Friday, February 22, 2008

Meteor in Oregon

National Geographic has a news piece about a large fireball seen over Oregon early on Tuesday. A bunch of people witnessed it falling and then explode in the sky. The article suggests that in the days to come, we may find chunks of the exploded space rock in impact sites around the Pacific Northwest (assuming it's not that spy satellite the Navy shot down around the same time...). Check it out.

Labels: meteors

Saturday, December 22, 2007

Mars may get an asteroid impact next month

January 30 may be a bad day on Mars. A space rock discovered in November of this year has a 1-in-75 chance of smacking into the red planet on that day. The rock is about the same size as the one inferred to have leveled the forest in a big swath of Siberia in 1908 (in what is called the Tunguska Event.) The amount of energy released then is estimated to be approximately the equivalent of a 15 megaton nuclear bomb. More details here.

Labels: astronomy, mars, meteors