A team of scientists has made progress in unraveling the mysteries behind the creation of massif-type anorthosites, intriguing rocks that emerged during a crucial time in Earth’s history. These igneous formations, rich in plagioclase, can extend over vast regions of up to 42,000 square kilometers and contain valuable deposits of titanium ore. Their origins have long been a subject of debate and confusion among researchers.
A team of scientists has made progress in unraveling the mysteries behind the creation of massif-type anorthosites, intriguing rocks that emerged during a crucial time in Earth’s history. These igneous formations, rich in plagioclase, can extend over vast regions of up to 42,000 square kilometers and contain valuable deposits of titanium ore. Their origins have long been a subject of debate and confusion among researchers.
A recent study published in Science Advances on August 14 delves into the complex relationships between Earth’s changing mantle and crust, as well as the tectonic movements that have influenced the planet over time. The research also offers fresh insights into when plate tectonics began, how subduction processes functioned billions of years ago, and how Earth’s crust has evolved.
Led by Duncan Keller and Cin-Ty Lee from Rice University, the research team investigated massif-type anorthosites to examine theories regarding the magmas responsible for their formation. They specifically analyzed the Marcy and Morin anorthosites, which are well-known examples from North America’s Grenville orogen and date back approximately 1.1 billion years.
By studying isotopes of boron, oxygen, neodymium, and strontium in the rocks and performing petrogenetic modeling, the researchers found that the magmas that created these anorthosites were enriched with melts that originated from oceanic crust, which had been modified by seawater at low temperatures. They also identified isotopic signatures similar to those found in other rocks from subduction zones, such as abyssal serpentinite.
“Our findings suggest that these immense anorthosites are likely the result of significant melting of subducted oceanic crust beneath converging continental margins,” explained Keller, who is also the Clever Planets Postdoctoral Research Associate in Earth, Environmental and Planetary Sciences and the study’s lead author. “Because the mantle was hotter in the past, this discovery links the formation of massif-type anorthosites to the thermal and tectonic evolution of Earth.”
This research, which combines traditional methods with a pioneering use of boron isotopic analysis on massif-type anorthosites, indicates that these rocks formed during an era of extremely hot subduction, possibly prevalent billions of years ago.
Since massif-type anorthosites are not produced on Earth today, the new evidence connecting these rocks to the hot subduction processes of early Earth paves the way for innovative interdisciplinary approaches to understand how these formations reflect the physical development of our planet.
“This research enhances our knowledge of ancient rock formations and illuminates their broader implications for Earth’s tectonic and thermal history,” said Lee, the Harry Carothers Wiess Professor of Geology and professor of Earth, Environmental and Planetary Sciences, as well as a co-author of the study.
Other co-authors included William Peck from the Department of Earth and Environmental Geosciences at Colgate University; Brian Monteleone from the Department of Geology and Geophysics at Woods Hole Oceanographic Institution; Céline Martin from the Department of Earth and Planetary Sciences at the American Museum of Natural History; Jeffrey Vervoort from the School of the Environment at Washington State University; and Louise Bolge from the Lamont-Doherty Earth Observatory at Columbia University.
This research received support from NASA and the U.S. National Science Foundation.
