A Tale of Early Earth Told in Zircons

摘要

No rock or mineral record has been preserved from Earth's "dark age"— the mysterious time after accretion of the planet about 4560 million years ago. Thanks to a continuous effort to find the oldest pieces of our planet, however, the duration of this unknown era is becoming shorter and shorter. Following the development of modern isotopic dating, the extent of the dark age was established at about 800 to 1000 million years by the discovery of exceptionally old rocks in western Greenland. The discovery of still older grains of zircon (zirconium silicate, a common if not very abundant com- ponent of crustal rocks, and an extraordinarily resilient mineral) in archean sedimentary rocks in Western Australia reduced the dark age to 400 to 300 million years and recently to less than 200 million years, a mere 5% of Earth's life span. That this was an eventful time is clear from studies of the Moon and Mars, where internally driven magmatism and differentiation ceased quickly or slowed down. We know that the processes that shaped further evolution of these bodies—large-scale mantle differentiation and formation of the primary crust— were occurring within the first few hundred million years.