However, Jonathan O'Neil and Don Francis of McGill University in Montreal, Canada, later collected samples of the Hudson Bay rock that appeared to be even older than 3.8 billion years.
O'Neil worked with study co-author Carlson on the analysis of these rock samples, which had been intruded by the zircon-containing rock.
"So the unit we worked on had to be older than 3.8 [billion years old], but you didn't know if it wasn't 3.8 plus a day," Carlson said, noting that the samples lacked easily datable zircon crystals.
Instead, the researchers looked for variations in isotopes, or the atomic structures, of the rare earth metals samarium and neodymium.
One isotope of neodymium is formed from the decay of a samarium isotope that existed only at the beginning of the solar system.
The presence of that neodymium isotope in variable quantities and other clues revealed an age of 4.28 billion years, Carlson explained.
The study will appear in tomorrow's issue of the journal Science.
University of Colorado's Mojzsis said the data used to reach the 4.28-billion-year-old date "meets the highest standards"—but he is unconvinced the rocks themselves are that old.
The rocks could be derived from 4.28-billion-year-old sediments, he noted, similar to rocks that form from pre-existing, older rocks.
"We need to be cautious about this and, frankly speaking, skeptical," he said.
Mojzsis added that he is currently looking for a rock in the Hudson Bay region that contains 4.28-billion-year-old zircon, which he said would help resolve the debate.
Mojzsis's research is funded, in part, by a grant from the National Geographic Society's Committee for Research in Exploration. (National Geographic News is owned by the National Geographic Society.)
If the rocks themselves are shown to be 4.28 billion years old, he added, they would represent the only known piece of crust that has survived from the Hadean, the earliest era of Earth's history.
Most pieces of Earth's crust have been mashed up, melted, and reformed time and again through the process of plate tectonics.
Pinpointing when Earth's crust-mantle and plate tectonics systems got underway remains an active area of geological research, Mojzsis added.
The Hudson Bay rocks, he said, reinforce the idea that the early Earth "was one that had dynamic rock cycling and volcanism and formation of crust from the earliest days."
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