Researchers have proposed a radical solution to a cosmochemical mystery that has baffled scientists for over a century: how numerous small, glassy chondrules had become embedded within chondrites, the largest class of meteorites.
Chondrules are visible as round objects in this electron image of a polished thin section made from the Bishunpur meteorite from India; the dark grains are iron-poor olivine crystals (Steven Simon)
First described in 1877 by the British mineralogist Henry Sorby, chondrules are small, round granules of olivine or pyroxene. Sorby suggested that they might be ‘droplets of fiery rain’ which somehow condensed out of the cloud of gas and dust that formed the Solar System.
“Researchers have continued to regard chondrules as liquid droplets that had been floating in space before becoming quickly cooled, but how did the liquid form? “There’s a lot of data that have been puzzling to people,” explained Prof Lawrence Grossman from the University of Chicago, senior author of the study published in the July issue of Geochimica et Cosmochimica Acta.
The study reconstructs the sequence of minerals that condensed from the solar nebula, the primordial gas cloud that eventually formed the Sun and planets. The authors have concluded that a condensation process cannot account for chondrules. Their favorite theory involves collisions between planetesimals, bodies that gravitationally coalesced early in the history of the Solar System.
“That’s what my colleagues found so shocking, because they had considered the idea so kooky,” Prof Grossman said.
Prof Grossman re-evaluated the theory after Dr Conel Alexander at the Carnegie Institution of Washington and his colleagues supplied a missing piece of the puzzle. They discovered a tiny pinch of sodium in the cores of the olivine crystals embedded within the chondrules.
When olivine crystallizes from a liquid of chondrule composition at temperatures of approximately 2,000 degrees Kelvin, most sodium remains in the liquid if it doesn’t evaporate entirely. But despite the extreme volatility of sodium, enough of it stayed in the liquid to be recorded in the olivine, a consequence of the evaporation suppression exerted by either high pressure or high dust concentration. No more than 10 percent of the sodium ever evaporated from the solidifying chondrules.
Prof Grossman’s team has calculated the conditions required to prevent any greater degree of evaporation. They plotted their calculation in terms of total pressure and dust enrichment in the solar nebula of gas and dust from which some components of the chondrites formed.
“You can’t do it in the solar nebula. That’s what led him to planetesimal impacts. That’s where you get high dust enrichments. That’s where you can generate high pressures,” Prof Grossman said.
When the temperature of the solar nebula reached 1,800 degrees Kelvin, it was too hot for any solid material to condense. By the time the cloud had cooled to 400 degrees Kelvin, however, most of it had condensed into solid particles.
Prof Grossman and his colleague Dr Alexei Fedkin from the University of Chicago propose a scenario in which planetesimals, consisting of metallic nickel-iron, magnesium silicates and water ice, condensed from the solar nebula, well ahead of chondrule formation. Decaying radioactive elements inside the planetesimals provided enough heat to melt the ice. The water percolated through the planetesimals, interacted with the metal and oxidized the iron. With further heating, either before or during planetesimal collisions, the magnesium silicates re-formed, incorporating iron oxide in the process. When the planetesimals then collided with each other, generating the abnormally high pressures, liquid droplets containing iron oxide sprayed out.
“That’s where your first iron oxide comes from, not from what I’ve been studying my whole career,” Prof Grossman said. He and his colleague have now reconstructed the recipe for producing chondrules.
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Bibliographic information: Alexei Fedkin, Lawrence Grossman. Vapor saturation of sodium: Key to unlocking the origin of chondrules. Geochimica et Cosmochimica Acta, vol. 112, July 2013, pp. 226-250; doi: 10.1016/j.gca.2013.02.020
