A team of researchers has made significant progress in understanding the formation of massif-type anorthosites, enigmatic rocks that only formed during the middle part of Earth’s history. These plagioclase-rich igneous formations, which can cover areas as large as 42,000 square kilometers and host titanium ore deposits, have long puzzled scientists due to conflicting theories about their origins. A new study published in Science Advances on Aug. 14 delves into the intricate connections between Earth’s evolving mantle and crust and the tectonic forces that have shaped the planet over its history.
Led by Rice’s Duncan Keller and Cin-Ty Lee, the research team focused on studying massif-type anorthosites to test ideas about the magmas that formed them, specifically the Marcy and Morin anorthosites, classic examples from North America’s Grenville orogen that are approximately 1.1 billion years old. By analyzing the isotopes of boron, oxygen, neodymium, and strontium in the rocks, as well as conducting petrogenetic modeling, the researchers discovered that the magmas that formed these anorthosites were rich in melts derived from oceanic crust altered by seawater at low temperatures. They also found isotopic signatures corresponding to other subduction zone rocks such as abyssal serpentinite.
The research indicates that these giant anorthosites likely originated from the extensive melting of subducted oceanic crust beneath convergent continental margins. This finding suggests a direct connection between the formation of massif-type anorthosites and Earth’s thermal and tectonic evolution. The study combines classical methods with the novel application of boron isotopic analysis to massif-type anorthosites, suggesting that these rocks formed during very hot subduction that may have been prevalent billions of years ago. Since massif-type anorthosites do not form on Earth today, the new evidence linking these rocks to very hot subduction on the early Earth opens new interdisciplinary approaches for understanding how these rocks chronicle the physical evolution of our planet.
Cin-Ty Lee, the Harry Carothers Wiess Professor of Geology at Rice, highlights the study’s contributions by stating that this research advances our understanding of ancient rock formations and sheds light on the broader implications for Earth’s tectonic and thermal history. The study’s other co-authors include researchers from various institutions such as Colgate University, Woods Hole Oceanographic Institution, American Museum of Natural History, Washington State University, and Columbia University. The study was supported by NASA and the U.S. National Science Foundation, demonstrating the collaborative effort between different scientific entities to further explore Earth’s geological history.
Overall, the study offers new insights into the formation of massif-type anorthosites and their connection to Earth’s thermal and tectonic evolution. By analyzing the isotopic composition of these rocks and conducting petrogenetic modeling, the researchers were able to trace the origin of these enigmatic formations to very hot subduction processes that likely occurred billions of years ago. This research opens up new avenues for interdisciplinary approaches to understanding ancient rock formations and their implications for Earth’s geological history.
The discovery of the origins of massif-type anorthosites sheds light on the complex interplay between Earth’s mantle, crust, and tectonic forces throughout its history. By studying these rocks, scientists can better understand when plate tectonics began, how subduction dynamics operated in the distant past, and how the Earth’s crust has evolved over time. This research not only deepens our knowledge of ancient rock formations but also provides valuable insights into the broader implications for Earth’s geology and tectonic history. The findings of this study contribute to a more comprehensive understanding of Earth’s past and its evolution over billions of years.