Oldest crystal reveals e...( zircon sample) could have occurred dur...)

Scientists have been searching diligently to find samples of the Earth's oldest rocks. Valley and Peck say such ancient samples are extremely rare because rock is constantly recycled or sinks to the hot mantle of the Earth. Over the great spans of geologic time, there is little surface material that has not been recycled and reprocessed in this way.

The tiny grain of zirconium silicate or zircon found by Wilde in western Australia was embedded in a larger sample containing fragments of material from many different rocks, Valley says. Zircons dated at 4.3 billion years were reported from the same site a decade ago, but the new-found zircon grain is more than 100 million years older than any other known sample, giving scientists a rare window to the earliest period of the Earth. "This early age restricts theories for the formation of the moon," Valley says. "Perhaps the moon formed earlier than we thought, or by a different process."

Another intriguing question is whether or not life may have arisen at that early time. Low temperatures and water are preconditions for life. The earliest known biochemical evidence for life and for a hydrosphere is estimated at 3.85 billion years ago, and the oldest microfossils are 3.5 billion years old.

"It may have been that life evolved and was completely extinguished several times" in catastrophic, meteorite-triggered extinction events well before that, Valley says. The research conducted by Valley, Peck, Graham and Wilde was supported by the National Science Foundation, the U.S. Department of Energy, the U.K. Natural Environment Research Council and a Dean Morgridge Wisconsin Distinguished Graduate Fellowship.

'"/>

Contact: John Valley
valley@geology.wisc.edu
608-263-5659
University of Wisconsin-Madison
10-Jan-2001