Traces of the oldest meteorite impact?

In trying to piece together the origin of the continents and the early history of th earth, scientists turn to the oldest rocks on the planet, dating from the Archeaneon 2.5 billion to at least 3.7 billion years ago. But there are only three sites in the world where rocks that were exposed at the surface during the Archean have remained relatively unaltered. Now, at two fo these sites, researchers from Louisiana State University in Baton Rouge have identified layers of tiny spherical silicate grains that they suspect are the debris from meteorite impacts--the oldest known impacts on the earth.

If this interpretation is correct, then "we can start to quantitatively assess ... the rate of meteorite impacts ... during the Archean and whether [meteorites] played a more significant role is shaping the outer part of the earth's surface then than they do today," says igneous petrologist Gary R. Byerly, who co-authored a paper describing the finds with sedimentologist Donald R. Lowe in the January GEOLOGY.

But before scientists can start thinking about the cosmic consequences of these silicate spherules, lowe and Byerly will have to convince their colleagues, some of whom have very different ideas about the origin of the grains. In particular, Maarten J. de Wit from the University of Witwatersrand in Johannesburg, South Africa, has concluded that similar spherules were originally formed during volcanic eruptions. De Wit will present his theory this March at the Lunar and Planetary Science Conference in Houston.

Lowe and Byerly discovered the silicate spherules in the course of mapping and sampling rocks from the 3.2-billion-to 3.5-billion-year-old geologic formation in South Africa called the Barberton Greenstone Belt. According to Byerly, other researchers had found similar grains at a few isolated spots in the Barberton greenstone in the past, but none had appreciated just how widespread the spherule layer is. Lowe and Byerly think the layer, which is as thick as 0.5 meter in some places, can be traced over an area of hundreds and possibly thousands of square kilometers.

While the structure of these particles suggests that they were formed by the quenching of liquid silicate -- a process involved in both volcanic eruptions and meteorite impacts -- the researchers think that the "immense" expanse of the layer is one factor favoring a meteorite impact. "It's almost inconceivable," says Lowe, "that the grains could be carried this far during volcanic processes."

Another of Lowe and Byerly's arguments against a volcanic origin is that the two kinds of spherules mixed in the layer differ so radically in composition that they could not have come from the same magma source. What's more, they say, the compositions match those of the underlying rocks had they been thrown up and melted by the force of a meteorite impact. Lowe notes that there are no glass shards or other volcanic materials in the spherule layer, and he says there are cracks in the rocks that might have been produced by an impact.

The researchers also discovered Archean spherules in western Australia and most recently found two more spherule layers in South Africa. Lowe and Byerly are less certain whether these were formed by meteorite impacts.

De Wit says he holds to the volcanic idea because spherules with the same internal textures as those found by Lowe and Byerly are commonly found embedded in "pillows" of lava in many Archean greenstone belts. Some scientists have suggested that the spherules were created when one chemical compound separated from the rest of the liquid lava. De Wit suggest that the spherules studied by Lowe and others were released from the hardened lava through erosion because the lava matrix was more susceptible to weathering than were the spherules. Because there were so few of these embedded spherules in later times, there must have been some volcanic process operating in the Archean that is no longer common, he says.

If the meteorite interpretation is right, it will have a bearing not only on earth processes but on the history of the solar system as well. The spherules found in South Africa and Australia resemble chondrules, or spherical particles found in chondritic meteorites and in lunar soils. According to Elbert A. King at the University of Houston, scientists have been arguing over the origin of chondrules for two centuries. If Lowe and Byerly are correct, then more chondrules might be formed by impacts -- occurring on the parent bodies, which eventually are broken into meteors -- than previously imagined, he says. "And if that's ture," says King, "then large impacts probably play an even more major role in early solar system history than we now believe."