The School of Earth and Atmospheric Sciences Presents Dr. Elizabeth "Beth Ann" Bell, University of California Los Angeles
Hellish or Habitable? Constraints on the Environment of Early Earth
Several lines of evidence suggest that life on Earth may be as or more ancient than 3.8 Ga. As such, there is a real possibility that Earth became habitable within its first few hundred million years. The chief difficulty in assessing various hypothesized scenarios for the early Earth’s environment is the lack of a known rock record prior to 4.02 Ga.
This earliest eon of Earth history (the Hadean) can be studied directly only by detrital minerals in later sediments. The most well-studied suite of Hadean minerals is the Jack Hills detrital zircons (Western Australia), ranging to nearly 4.4 Ga in age and containing a variety of geochemical and petrological information about the Hadean magmas in which they crystallized.
While pointing to the composition of at least part of the Hadean crust, these zircons and their cargo of mineral inclusions also provide indirect evidence for conditions in the surface and near-surface environment of Hadean Earth through a variety of isotopic systems and trace elements.
A group of zircons with anomalous chemistry ca. 3.9 Ga may have formed through recrystallization – potentially representing some of the first terrestrial evidence for the Late Heavy Bombardment. Carbonaceous mineral inclusions may provide evidence for Hadean carbon cycling: an isotopically light graphite inclusion in a 4.1 Ga zircon may provide evidence for life on Earth by 4.1 Ga. By more fully exploiting mineral inclusions and trace element chemistry, especially in Hadean-Archean zircons from sites other than Jack Hills, we can develop a better grasp on the diversity of materials making up the Hadean crust and the diversity of their thermal histories during hypothesized bombardment episodes.
These nontraditional mineral records may help to more clearly constrain not only the igneous crustal composition but potentially also the surficial environment and geodynamic settings at the dawn of life.