Mysterious origins of Earth's largest igneous rocks revealed

Ancient rocks known as massif-type anorthosites have long puzzled scientists, presenting an enigma that has endured for decades and sparked intense debate within the geological community.

These vast, plagioclase-rich igneous formations, which trace back to the middle part of Earth’s history, have consistently eluded precise classification due to the myriad of conflicting theories about their origins and the processes that led to their formation.

However, recent breakthroughs in research and technology may be rewriting what we thought we knew about these mysterious rocks and providing new insights into Earth’s geologic past.

Geological connections in ancient rocks

A new study highlights the intricate connections between Earth’s evolving mantle and crust and the tectonic forces that have shaped the planet throughout its history.

This research simultaneously ushers in new methods to probe deeper into the beginnings of plate tectonics, the dynamics of subduction billions of years ago, and the unfolding story of the Earth’s crust.

The research team, led by Duncan Keller and Cin-Ty Lee from Rice University, studied massif-type anorthosites to test ideas about the magmas that formed them.

The research focused on the Marcy and Morin anorthosites, classic examples from North America’s Grenville orogen that are about 1.1 billion years old.

By analyzing isotopes of boron, oxygen, neodymium, and strontium in the rocks, as well as conducting petrogenetic modeling, the researchers uncovered that these anorthosites originated from magmas rich in melts derived from oceanic crust that had been altered by seawater at low temperatures.

The mystery of the ancient rocks

Study lead author Duncan Keller is the Clever Planets Postdoctoral Research Associate in Earth, Environmental, and Planetary Sciences at Rice University.

“Our research indicates that these giant anorthosites likely originated from the extensive melting of subducted oceanic crust beneath convergent continental margins,” said Keller.

“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 team’s exploration ventured into the depths of the Marcy and Morin anorthosites, where they identified isotopic signatures analogous to other rocks found in subduction zones, such as abyssal serpentinite.

The findings suggest that these rocks were formed during very hot subduction, a phenomenon that may have been prevalent billions of years ago.

The study combined classic methods with the novel application of boron isotopic analysis to these massif-type anorthosites.

A revolutionary conclusion

As massif-type anorthosites are not presently forming on Earth, this new evidence linking these rocks to the intense subduction heat of the early Earth opens unique interdisciplinary approaches for understanding how these rocks chronicle 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 Cin-Ty Lee, the Harry Carothers Wiess Professor of Geology and co-author of the study.

Technological advances in geological research

To achieve these discoveries, the researchers utilized a combination of cutting-edge technology and traditional geological techniques.

Advances in isotope geochemistry and high-precision instrumentation have played a crucial role in unveiling the mysteries embedded within ancient rock formations.

For example, the deployment of sophisticated mass spectrometers allowed the team to measure isotopic ratios with unprecedented accuracy, ultimately refining our understanding of how massif-type anorthosites were formed.

Moreover, the integration of computational modeling with field and laboratory data has enabled scientists to simulate the geodynamic processes that shaped these enigmatic rocks. These models provide insights into the conditions of the early Earth, offering a virtual glimpse into the complex interactions between tectonic plates, mantle convection, and crustal formation.

The collaborative nature of this study, drawing expertise from multiple institutions, highlights the importance of interdisciplinary research in advancing our knowledge of Earth’s geological history.

By combining resources and expertise, the team was able to push the boundaries of what is known about massif-type anorthosites, setting the stage for future research endeavors that could further unravel the secrets of our planet’s ancient past.

The full study was published in the journal Science Advances.

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