By James Morgan
Science reporter, BBC News
The rocks contain structures which might indicate life was present
Earth's most ancient rocks, with an age of 4.28 billion years, have been found on the shore of Hudson Bay, Canada.
Writing in Science journal, a team reports finding that a sample of Nuvvuagittuq greenstone is 250 million years older than any rocks known.
It may even hold evidence of activity by ancient life forms.
If so, it would be the earliest evidence of life on Earth - but co-author Don Francis cautioned that this had not been established.
"The rocks contain a very special chemical signature - one that can only be found in rocks which are very, very old," he said.
The professor of geology, who is based at McGill University in Montreal, added: "Nobody has found that signal any place else on the Earth."
"Originally, we thought the rocks were maybe 3.8 billion years old.
"Now we have pushed the Earth's crust back by hundreds of millions of years. That's why everyone is so excited."
Ancient rocks act as a time capsule - offering chemical clues to help geologists solve longstanding riddles of how the Earth formed and how life arose on it.
But the majority of our planet's early crust has already been mashed and recycled into Earth's interior several times over by plate tectonics.
Before this study, the oldest whole rocks were from a 4.03 billion-year-old body known as the Acasta Gneiss, in Canada's Northwest Territories.
The only things known to be older are mineral grains called zircons from Western Australia, which date back 4.36 billion years.
Professor Francis was looking for clues to the nature of the Earth's mantle 3.8 billion years ago.
He and colleague Jonathan O'Neil, from McGill University, travelled to remote tundra on the eastern shore of Hudson Bay, in northern Quebec, to examine an outcrop of the Nuvvuagittuq greenstone belt.
They sent samples for chemical analysis to scientists at the Carnegie Institution of Washington, who dated the rocks by measuring isotopes of the rare earth elements neodymium and samarium, which decay over time at a known rate.
The rocks turned out to be far older than first thought
The oldest rocks, termed "faux amphibolite", were dated within the range from 3.8 to 4.28 billion years old.
"4.28 billion is the figure I favour," says Francis.
"It could be that the rock was formed 4.3 billion years ago, but then it was re-worked into another rock form 3.8bn years ago. That's a hard distinction to draw."
The same unit of rock contains geological structures which might only have been formed if early life forms were present on the planet, Professor Francis suggested.
The material displays a banded iron formation - fine ribbon-like bands of alternating magnetite and quartz.
This feature is typical of rock precipitated in deep sea hydrothermal vents - which have been touted as potential habitats for early life on Earth.
"These ribbons could imply that 4.3 billion years ago, Earth had an ocean, with hydrothermal circulation," said Francis.
"Now, some people believe that to make precipitation work, you also need bacteria.
"If that were true, then this would be the oldest evidence of life.
"But if I were to say that, people would yell and scream and say that there is no hard evidence."
Fortunately, geologists have already begun looking for such evidence, in similar rocks found in Greenland, dated 3.8 billion years.
"The great thing about our find, is it will bring in people here to Lake Hudson to carry out specialised studies and see whether there was life here or not," says Francis.
"Regardless of that, or the exact date of the rocks, the exciting thing is that we've seen a chemical signature that's never been seen before. That alone makes this an exciting discovery."