An analysis of rock samples collected from the Superior Province, the region in Canada just north of the Great Lakes, suggests the samples contain components of ancient basaltic crust that existed more than 4.2 billion year ago (Hadean eon).
An artistic conception of the early Earth. Image credit: Simone Marchi / NASA.
Recreating the nature of Earth’s first crust is difficult because geologic activity has created turnover that drove most of it back into Earth’s interior.
While some slivers of 4-billion-year-old crust remain in the rock record, only isolated zircon mineral grains are dated to be older.
“Finding remnants of early Earth’s crust has proven difficult, but a new approach offers the ability to detect the presence of truly ancient crust that has been reworked into ‘merely’ really old rocks,” said study co-author Dr. Richard W. Carlson, Director of the Department of Terrestrial Magnetism at the Carnegie Institution for Science.
The novel approach examines variations in the abundance of an isotope of the element neodymium, which is created by the radioactive decay of a different element, samarium.
The isotope of samarium with a mass of 146 (samarium-146) has a half-life of only 103 million years. It decays to the isotope of neodymium with a mass of mass 142.
While samarium-146 was present when Earth formed, it became extinct very early in Earth’s history.
Researchers know of its existence from the study of very ancient rocks, especially meteorites and samples from Mars and the Moon.
Variations in the relative abundance of neodymium-142 compared to other isotopes of neodymium that didn’t originate from decaying samarium reflect chemical processes that changed the ratio of samarium to neodymium in the rock while samarium-146 was still present–basically before about 4 billion years ago.
Dr. Carlson and his colleague, University of Ottawa researcher Dr. Jonathan O’Neil, studied 2.7 billion-year-old granitic rocks that make up a portion of the eastern shore of Hudson Bay.
The abundances of neodymium-142 in these granites indicates that they were derived from the re-melting of much older rocks — rocks that were more than 4.2 billion years old – and that these ancient rocks were compositionally similar to the abundant magnesium-rich rock type known as basalt, which makes up all of the present day oceanic crust.
In more-recent times in Earth’s history, basaltic oceanic crust survives at Earth’s surface for less than 200 million years before it sinks back into Earth’s interior due to the action of plate tectonics.
The team’s findings, however, suggest that basaltic crust, which may have formed not long after Earth’s formation, survived at Earth’s surface for at least 1.5 billion years before later being re-melted into rocks that form a portion of the northernmost Superior craton, a geological formation that extends roughly from the Hudson Bay in Quebec to Lake Huron in Ontario.
“Whether this result implies that plate tectonics was not at work during the earliest part of Earth history can now be investigated using our tool of studying neodymium-142 variation to track the role of truly ancient crust in building up younger, but still old, sections of Earth’s continental crust,” Dr. Carlson said.
Details of the research were recently published in the journal Science.
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Jonathan O’Neil & Richard W. Carlson. 2017. Building Archean cratons from Hadean mafic crust. Science 355 (6330): 1199-1202; doi: 10.1126/science.aah3823
Source link: https://www.sci.news/geology/early-earths-crust-04709.html
