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