Grains of garnet sand tell the story of traveling from the surface to about 75 miles in the Earth, then returning to the surface before ending up on a beach

Garnet sand beach on Goodenough Island, Milne Bay Province, Papua New Guinea. These garnets originate from rocks such as gneiss (see photo below), and are then concentrated as a placer deposit on the beach. Photos: Professor Paul Fitzgerald

Scientists read the rock cycle that happened in less than about 10 million years

On a beach on a remote island in eastern Papua New Guinea, the grains of garnet sand reveal their story of traveling from the surface into the Earth (~75 miles), and then returning to the surface before ending up on a beach as today’s sand grains. Over the course of this geologic journey, the rock type changed as some minerals were changed, and other materials were included (trapped) within the newly formed garnets.

The story is preserved in garnet compositions, as well as in their trapped inclusions: solids (e.g., very rare minerals such as coesite – a high pressure form of quartz), liquids (e.g., water) and gases (e.g., CO2).
Suzanne Baldwin, Thonis Family Professor in the Department of Earth and Environmental Sciences at Syracuse University’s College of Arts and Sciences, has led many field expeditions to Papua New Guinea, a country located in the southwestern Pacific to the north of Australia.

By reading the rock record, researchers revealed the recycling pathway from the surface to deep within the upper mantle and then back to the surface as a result of tectonic and sedimentary processes. The compositions of that sand also hold various key components that reveal how quickly this recycling happened. In this case, transit through the rock cycle happened in less than ~10 million years. This may seem like a long time, but for these geologic processes, it is actually remarkably short.

The garnet sands are just the latest piece of the puzzle to understand the geologic evolution of this region. It is the only location on Earth where active exhumation of high- and ultrahigh-pressure metamorphic rocks is occurring during the same rock cycle that produced these metamorphic rocks.

Suzanne Baldwin, Thonis Family Professor, examining a gneiss, a type of metamorphic rock on a field expedition to Goodenough Island, Milne Bay Province, Papua New Guinea.

The international group of researchers, including Joseph Gonzalez ’19 Ph.D. from Syracuse University (now a European Research Council postdoctoral researcher at the University of Pavia, Italy), Ph.D. student Jan Schönig and Professor Hilmar von Eynatten from the University Göttingen in Germany, and Professor Hugh Davies (formerly from the University of Papua New Guinea, now at The Australian National University), revealed how the trapped inclusions in garnet sand can be used to determine rock recycling processes within active plate boundary zones.

At active plate boundaries, like the one the team studied in eastern Papua New Guinea, converging tectonic plates slide toward each other with one plate moving underneath the other to form a subduction zone. During this process, rocks are subducted deep into the Earth.

Over time, forces on the plate boundaries may change and rocks can be exhumed to the surface through a process known as lithospheric deformation.

The trapped inclusions preserve a record of crustal subduction and rapid exhumation linking upper mantle and surface processes on these short geologic timescales. By applying their approach to both modern sediments and sedimentary rocks, researchers can now reveal the tempo of rock recycling processes throughout Earth’s history.

Source: Syracuse University

Proceedings of the National Academy of Sciences (PNAS)

(19.01.2021, USA: 01.19.2021)