Geochemical study links ancient anorthosites to Earth’s hot subduction

A team of researchers has made progress in understanding the formation of massive anorthosites, enigmatic rocks that formed only midway through Earth’s history. These plagioclase-rich igneous rock formations, which can cover areas of up to 42,000 square kilometers and host deposits of titanium ore, have puzzled scientists for decades because of conflicting theories about their origins.

A new study published in Scientific progress On August 14, the exhibition highlights the complex connections between the evolution of the Earth’s mantle and crust and the tectonic forces that have shaped the planet throughout its history. It also offers new ways to explore how plate tectonics began, how subduction dynamics worked billions of years ago, and how the Earth’s crust evolved.

The research team, led by Duncan Keller and Cin-Ty Lee of Rice, studied massive-type anorthosites to test hypotheses about the magmas that formed them. The research focused on the Marcy and Morin anorthosites, classic examples of the Grenville orogen in North America that are about 1.1 billion years old.

By analyzing boron, oxygen, neodymium and strontium isotopes in the rocks and performing petrogenetic modeling, the researchers found that the magmas that formed these anorthosites were rich in melts from oceanic crusts altered by low-temperature seawater. They also found isotopic signatures that matched other subduction zone rocks such as abyssal serpentinite.

“Our research indicates that these giant anorthosites likely originated from massive melting of oceanic crust subducted beneath convergent continental margins,” said Keller, a postdoctoral research associate in Earth, environmental and planetary sciences and lead author of the study. “Because the mantle was warmer in the past, this process directly links the formation of massive-type anorthosites to the thermal and tectonic evolution of the Earth.”

The study, which combines classical methods with the novel application of boron isotope analysis to massive-type anorthosites, suggests that these rocks formed during very hot subduction that may have occurred billions of years ago.

Since massive-type anorthosites do not form on Earth today, the new evidence linking these rocks to very hot subduction on the early Earth opens up new interdisciplinary approaches to understanding how these rocks relate the physical evolution of our planet.

“This research advances our understanding of ancient rock formations and sheds light on the broader implications of Earth’s tectonic and thermal history,” said Lee, the Harry Carothers Wiess Professor of Geology, professor of Earth, environmental and planetary sciences and co-author of the study.

Other co-authors of the study are William Peck of Colgate University’s Department of Earth and Environmental Geosciences; Brian Monteleone of the Department of Geology and Geophysics at Woods Hole Oceanographic Institution; Celine Martin of the American Museum of Natural History’s Department of Earth and Planetary Sciences; Jeffrey Vervoort of Washington State University’s School of the Environment; and Louise Bolge of Columbia University’s Lamont-Doherty Earth Observatory.