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A group of scientists has made significant progress in unraveling the origins of massif-type anorthosites, a type of igneous rock composed largely of plagioclase that formed exclusively during Earth's middle history. These vast rock formations, some of which cover up to 42,000 square kilometers and are associated with titanium ore deposits, have long puzzled researchers due to varying theories a by Clarence Oxford
Los Angeles CA (SPX) Aug 15, 2024
A group of scientists has made significant progress in unraveling the origins of massif-type anorthosites, a type of igneous rock composed largely of plagioclase that formed exclusively during Earth's middle history. These vast rock formations, some of which cover up to 42,000 square kilometers and are associated with titanium ore deposits, have long puzzled researchers due to varying theories about their formation.
In a study published on August 14 in 'Science Advances', the team provides new insights into the complex interactions between Earth's evolving mantle and crust and the tectonic forces that have shaped the planet over time. This work offers new perspectives on the origins of plate tectonics, the functioning of subduction processes billions of years ago, and the overall development of Earth's crust.
The research, led by Rice University's Duncan Keller and Cin-Ty Lee, focused on understanding the magma origins of massif-type anorthosites by examining well-known examples from the Grenville orogen in North America, specifically the Marcy and Morin anorthosites, which are about 1.1 billion years old.
Through the analysis of boron, oxygen, neodymium, and strontium isotopes in the rocks and detailed petrogenetic modeling, the researchers concluded that the magmas responsible for these anorthosites were enriched with melts originating from oceanic crust that had been altered by low-temperature seawater. Additionally, they identified isotopic signatures similar to those found in subduction zone rocks such as abyssal serpentinite.
"Our research indicates that these giant anorthosites likely originated from the extensive melting of subducted oceanic crust beneath convergent continental margins," said Keller, the Clever Planets Postdoctoral Research Associate, Earth, Environmental and Planetary Sciences and the study's lead author. "Because the mantle was hotter in the past, this process directly connects the formation of massif-type anorthosites to Earth's thermal and tectonic evolution."
The study also employs a novel approach by applying boron isotopic analysis to massif-type anorthosites, suggesting these rocks were formed during periods of extremely hot subduction, which may have been a common occurrence billions of years ago.
Since massif-type anorthosites are not formed on Earth today, the new evidence linking these rocks to early Earth's hotter subduction zones offers a fresh perspective for understanding the physical evolution of our planet.
"This research advances our understanding of ancient rock formations and sheds light on the broader implications for Earth's tectonic and thermal history," said Lee, the Harry Carothers Wiess Professor of Geology, professor of Earth, environmental and planetary sciences and study co-author.
Other contributors to the study include William Peck from Colgate University's Department of Earth and Environmental Geosciences, Brian Monteleone from Woods Hole Oceanographic Institution's Department of Geology and Geophysics, Celine Martin from the Department of Earth and Planetary Sciences at the American Museum of Natural History, Jeffrey Vervoort from Washington State University's School of the Environment, and Louise Bolge from Columbia University's Lamont-Doherty Earth Observatory.
Research Report:Mafic slab melt contributions to Proterozoic massif-type anorthosites
Related Links
Rice University
Explore The Early Earth at TerraDaily.com