Geologic history written in garnet sand
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.
Credit: Professor Paul Fitzgerald
On a beach on a remote island in eastern Papua New Guinea, a country located in the southwestern Pacific to the north of Australia, garnet sand reveals an important geologic discovery. Similar to messages in bottles that have traveled across the oceans, sediments derived from the erosion of rocks carry information from another time and place. In this case the grains of garnet sand reveal a story of traveling from the surface to deep into the Earth (~75 miles), and then returning to the surface before ending up on a beach as 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. Her team’s latest results on this tectonically active region have just been published in the prestigious journal Proceedings of the National Academy of Sciences (PNAS).
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.