Four-billion-year-old ancient rocks yield clues about Earth`s earliest crust

Washington: A researcher spent the better part of three years collecting and studying ancient rock samples from the Acasta Gneiss Complex in the Northwest Territories to understand the environment in which they formed.

Lead author Jesse Reimink said the timing and mode of continental crust formation throughout Earth`s history is a controversial topic in early Earth sciences. Reimink said continents today form when one tectonic plate shifts beneath another into the Earth`s mantle and cause magma to rise to the surface, a process called subduction. He said it`s unclear whether plate tectonics existed 2.5 billion to four billion years ago or if another process was at play. Working under the supervision of co-author Tom Chacko, Reimink spent his summers in the field collecting rock samples from the Acasta Gneiss Complex, which was discovered in the 1980s and found to contain some of the Earth`s oldest rocks, between 3.6 and four billion years old. Due to their extreme age, the rocks have undergone multiple metamorphic events, making it difficult to understand their geochemistry, Reimink says. Fortunately, a few rocks-which the research team dubbed "Idiwhaa" meaning "ancient" in the local Tlicho dialect-were better preserved. This provided a "window" to see the samples` geochemical characteristics, which Reimink says showed crust-forming processes that are very similar to those occurring in present-day Iceland. "This provides the first physical evidence that a setting similar to modern Iceland was present on the early Earth." The new study has been published in the journal Nature Geoscience.