New Research Opportunities in the Earth Sciences - New Research Opportunities in the Earth Sciences - page 18

This raises several fundamental questions, such as:

• What is the critical oxygen concentration for early life forms?

• What was the role of the late heavy bombardment near 3.9 Ga on the

terrestrial environment?

• At what time did the earliest continental crust stabilize?

• When did plate tectonics initiate, and what environmental effects did this

transition have?

Figure 2.4 Images of Earth's oldest crustal rocks. Left: Acasta Gneiss of north-
central Canada (Bowring et al., 1990). Right: The 4.28-Ga “faux-amphibolite” from
the Nuvvuagittuq supracrustal belt in northern Quebec. SOURCE: O'Neil et al.
(2008). Reprinted with permission from AAAS.

BOX 2.2
Earth's Oldest Solids: Hadean Zircons

The oldest known terrestrial solids are zircon crystals. Zircons are extremely resistant
to both chemical and physical destruction and hence have the potential to survive billions of
years of reprocessing after their formation. Fortunately, they also carry a range of
mineralogical, geochemical, and isotopic tracers that document their age and environment of
formation.
The oldest known zircons come from the Jack Hills region of Western Australia,
where they are found in metamorphosed rocks originally deposited in a fan-delta setting (see
Figure B2.2; Spaggiari et al., 2007). Although they span a range of ages, many of the Jack
Hills zircons formed in the Hadean Eon (>3.8 Ga), and the oldest among them crystallized
<250 million years after the birth of the solar system (e.g., Compston and Pidgeon, 1986).
Because they provide a unique window into the early Earth, the Jack Hills crystals have been
intensively studied in the past decade. A generally consistent story emerges from analyses of
the trace element and isotopic composition of the zircons as well as the assemblage of
mineral inclusions trapped within them (e.g., Wilde et al., 2001; Cavosie et al., 2005; Watson
and Harrison, 2005; Trail et al., 2007; Hopkins et al., 2008; Harrison, 2009). The zircons
appear to be igneous and formed at relatively low temperatures, suggesting crystallization
from magma at or near water saturation. Inclusion mineralogy and oxygen isotope data
indicate the magma may have formed from melting of a felsic protolith that had interacted
extensively with an early hydrosphere, possibly an ocean. Geobarometry and thermometry of