This image shows a portion of the oldest-known rocks on Earth, dating from 4.28 billion years ago and found on the eastern shore of Canada's Hudson Bay. [Photo: Agencies]

Canadian bedrock more than four billion years old may be the oldest known section of the Earth's early crust, according to a study published Thursday by journal Science.

A team of scientists from the United States and Canada used geochemical methods to obtain an age of 4.28 billion years for samples of the rock, making it 250 million years more ancient than any previously discovered rocks. The findings offer scientists clues to the earliest stages of our planet's evolution.

The Nuvvuagittuq greenstone belt is an expanse of bedrock exposed on the eastern shore of Hudson Bay in northern Quebec in Canada and was first recognized in 2001 as a potential site of very old rocks.

Samples of the Nuvvuagittuq rocks were collected by geologists from Canada's McGill University and analyzed by researchers from McGill and the US Carnegie Institution.

By measuring minute variations in the isotopic composition of the rare earth elements neodymium and samarium in the rocks, they determined that the rock samples range from 3.8 to 4.28 billion years old.

"There have been older dates from Western Australia for isolated resistant mineral grains called zircons," says Richard Carlson at Carnegie Institution, "but these are the oldest whole rocks found so far." The oldest zircon dates are 4.36 billion years.

Before this study, the oldest dated rocks were from a body of rock known as the Acasta Gneiss, which are 4.03 billion years old. The Earth is 4.6 billion years old, and remnants of its early crust are extremely rare -- most of it has been mashed and recycled into Earth's interior several times over by plate tectonics since the Earth formed.

The rocks are significant not only for their great age but also for their chemical composition, which resembles that of volcanic rocks in geologic settings where tectonic plates are crashing together. "This gives us an unprecedented glimpse of the processes that formed the early crust," says Carlson.

(Xinhua News Agency September 26, 2008)