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Possible oldest-known rocks found

Sept. 25, 2008
Courtesy Carnegie Institution
and World Science staff

Sci­en­tists have iden­ti­fied what they say may be the old­est known rocks, a sec­tion of Ca­na­di­an bed­rock more than four bil­lion years old.

Sci­en­tists at the Car­ne­gie In­sti­tu­tion in Wash­ing­ton, D.C. used ge­o­chem­i­cal meth­ods to es­ti­mate an age of 4.28 bil­lion years for sam­ples of the rock. That would make it 250 mil­lion years more an­cient than any pre­vi­ously dis­cov­ered rocks, the re­search­ers said.

Rocks at the Nuv­vua­git­tuq Belt. (Cour­tesy Jon­a­than O'Neil)

The find­ings, which of­fer sci­en­tists clues to the ear­li­est stages of our plan­et’s ev­o­lu­tion, are pub­lished in the Sep­tem­ber 26 is­sue of the re­search jour­nal Sci­ence.

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

Sam­ples of the stone were col­lect­ed by ge­ol­o­gists from McGill Uni­ver­s­ity in Mont­real and an­a­lyzed by Jon­a­than O’Neil, a PhD stu­dent at McGill, and Rich­ard Carl­son at the Car­ne­gie In­sti­tu­tion’s De­part­ment of Ter­res­tri­al Mag­net­ism. 

By meas­ur­ing min­ute varia­t­ions in the com­po­si­tion of ra­dio­ac­t­ive vari­ants of the el­e­ments ne­o­dym­i­um and sa­mar­i­um in the rocks, O’Neil and Carl­son de­ter­mined that the rock sam­ples range from 3.8 to 4.28 bil­lion years old. 

The old­est dates came from rocks termed “faux am­phi­bo­lite,” which the re­search­ers in­ter­pret to be an­cient vol­can­ic de­posits. 

“There have been old­er dates from West­ern Aus­tral­ia for iso­lat­ed re­sist­ant min­er­al grains called zir­cons,” said Carl­son, “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 in the North­west Ter­ri­to­ries, 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, Carl­son said. “This gives us an un­prec­e­dent­ed glimpse of the pro­cesses that formed the early crust,” he added.

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Scientists have identified what they say may be the oldest known rocks, a section of Canadian bedrock more than four billion years old. Scientists at the Carnegie Institution in Washington, D.C. used geochemical methods to estimate 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, which offer scientists clues to the earliest stages of our planet’s evolution, are published in the September 26 issue of the research journal Science. The Nuvvuagittuq greenstone belt is an expanse of bedrock exposed on the eastern shore of Hudson Bay in northern Quebec and was first recognized in 2001 as a potential site of very old rocks. Samples of the Nuvvuagittuq rocks were collected by geologists from McGill University in Montreal and analyzed by Jonathan O’Neil, a PhD student at McGill, and Richard Carlson at the Carnegie Institution’s Department of Terrestrial Magnetism. By measuring minute variations in the composition of radioactive variants of the elements neodymium and samarium in the rocks, O’Neil and Carlson determined that the rock samples range from 3.8 to 4.28 billion years old. The oldest dates came from rocks termed “faux amphibolite,” which the researchers interpret to be ancient volcanic deposits. “There have been older dates from Western Australia for isolated resistant mineral grains called zircons,” said Carlson, “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 in the Northwest Territories, 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,” said Carlson.