Valley worked with William H. Peck, a former UW-Madison graduate student and now an assistant professor of geology at Colgate University, to analyze oxygen isotope ratios, measure rare earth elements, and determine element composition in a grain of zircon that measured little more than the diameter of two human hairs.
Peck, lead author of the Geochimica paper, conducted the work as part of his doctoral thesis at UW-Madison. Colin M. Graham's laboratory analyzed the zircon to obtain the oxygen isotope ratios. Graham is a contributor to the paper and professor of geochemistry at the University of Edinburgh.
"What the oxygen isotopes and rare earth analysis show us is a high oxygen isotope ratio that is not common in other such minerals from the first half of the Earth's history," Peck says. In other words, the chemistry of the mineral and the rock in which it developed could only have formed from material in a low-temperature environment at Earth's surface.
The accepted view on an infant Earth is that shortly after it first formed 4.5 to 4.6 billion years ago, the planet became little more than a swirling ball of molten metal and rock. Scientists believed it took a long time, perhaps 700 million years, for the Earth to cool to the point that oceans could condense from a thick, Venus-like atmosphere.
Complicating the picture is that for 500 million to 600 million years after the Earth was formed, the young planet was pummeled by intense meteorite bombardment. About 4.45 billion years ago, a Mars-size object is believed to have slammed into the Earth, creating the moon by blasting pieces of the infant planet into space.
"This is the first evidence of crust as old as 4.4 billion years, and indicates the development of continental-type crust during intense meteorite bombardment of the early Earth," Valley says. "It is possible that the water-rock interaction (as represented in the ancient
'"/>
Contact: John Valley
valley@geology.wisc.edu
608-263-5659
University of Wisconsin-Madison
10-Jan-2001