These Rocks May Be the Final Remnants of Earth’s Early Crust

About four billion years ago, magma from Earth’s mantle intruded the primitive crust of a nascent planet. Over the next period, nearly all of the planet’s early crust melted back into the mantle, leaving behind a small remnant near the site of this intrusion that still exists today.

This remnant is part of the Nuvvuagittuq Greenstone Belt along Hudson Bay’s coast in Canada. Recent analyses of the rock’s radioisotope signatures have sparked debates among geologists about whether it is indeed the oldest rock on Earth or simply very ancient.

In a study published in 2008, Jonathan O’Neill from the University of Ottawa and his team posited that the surrounding rocks could be as old as 4.3 billion years, dating back to the Hadean eon—just a few hundred million years after Earth’s formation.

While there have been discoveries of older mineral grains, these ancient Hadean rocks provide critical insights into Earth’s formative years, possibly shedding light on geological enigmas like the onset of plate tectonics and early ocean compositions.

The method used for dating the rocks has drawn controversy, particularly regarding the claimed age of 4.3 billion years. Traditionally, old rocks are dated utilizing a robust mineral known as zircon, but these volcanic rocks lack zircon. “No one can date these rocks using the popular techniques,” O’Neill remarks.

Instead, researchers analyzed the isotopes of neodymium and samarium within the rock. As samarium decays, it generates different isotopes of neodymium at predictable rates, allowing the ratio of isotopes to serve as a “clock” marking the time since the rock crystallized from magma. Interestingly, two isotopes of samarium can decay at differing rates, acting as two parallel chronometers. Disagreement arose among researchers about whether the rock was genuinely Hadean, as the two clocks provided inconsistent age estimates.

“I’m not convinced that most of the early Earth research community agrees,” states Richard Walker at the University of Maryland.

Currently, O’Neill’s team is assessing the neodymium and samarium isotopes in the rock formations dating back 4.3 billion years. By definition, such intrusions are younger than the surrounding rock layers, implying that dating an intrusion yields the minimum age for the enclosing rocks.

In the findings, the two chronological indicators tell the same tale, indicating the rocks’ age to be approximately 4.16 billion years. “Both clocks yield identical results,” O’Neill states. This consistency bolsters the theory that the surrounding rocks were indeed solidified during the Hadean eon, making them potentially the only known remnants of Earth’s ancient crust.

“I believe they present the strongest argument possible,” asserts Graham Pierson from the University of Alberta, Canada.

“The simplest interpretation of this data is that these represent the oldest rocks on Earth,” says Jesse Reimink at Pennsylvania State University. Nevertheless, he cautions that this may not be the final word on the subject, stating, “When it comes to the oldest rocks and minerals, absolute certainty is hard to come by.”