From ScienceDaily:
From the early 20th century up through the 1980’s, geologists generally agreed that conditions during the Hadean period were utterly hostile to life. Inability to find rock formations from the period led them to conclude that early Earth was hellishly hot, either entirely molten or subject to such intense asteroid bombardment that any rocks that formed were rapidly remelted. As a result, they pictured the surface of the Earth as covered by a giant “magma ocean.”
This perception began to change about 30 years ago when geologists discovered zircon crystals (a mineral typically associated with granite) with ages exceeding 4 billion years old preserved in younger sandstones. These ancient zircons opened the door for exploration of the Earth’s earliest crust. In addition to the radiometric dating techniques that revealed the ages of these ancient zircons, geologists used other analytical techniques to extract information about the environment in which the crystals formed, including the temperature and whether water was present.
Since then zircon studies have revealed that the Hadean Earth was not the uniformly hellish place previously imagined, but during some periods possessed an established crust cool enough so that surface water could form — possibly on the scale of oceans.
The researchers decided to study zircon crystals, the only known survivors from the era. Apparently the zircons from the Hadean era grew in magmas different from the ones in which zircons formed in Iceland in the last 18 million years:
Most importantly, their analysis found that Icelandic zircons grew from much hotter magmas than Hadean zircons. Although surface water played an important role in the generation of both Icelandic and Hadean crystals, in the Icelandic case the water was extremely hot when it interacted with the source rocks while the Hadean water-rock interactions were at significantly lower temperatures.
What makes this find of more general interest is that life got started on Earth very early in its history:
Some recent research has identified life on land over two billion years ago, consisting of a fungus whose central cavity was filled with symbiotic bacteria. Some Australian fossils are said to date back to 3.5 billion years old, not long after the cooling of Earth’s crust from the bombardment by planetesimals (rocky objects) at 3.85 billion years ago. Of course, some such current findings may be revised. But the general trend has been to the discovery of ever-earlier instances of life on Earth. That means that very complex and precise sequences of events must have taken place in a short period of time. Also, we don’t know in detail what the conditions on early Earth were like so it is difficult to refine the search by ruling out whole classes of theories. More.
If this find—that the early climate was not hellish (Hadean) in all respects—is substantiated, we have better insight into how life was even possible in principle. We still don’t know how it got started.
See also:
Can all the numbers for life’s origin just happen to fall into place?
Maybe if we throw enough models at the origin of life… some of them will stick
and
The Science Fictions series at your fingertips (origin of life)
Follow UD News at Twitter!
Here’s the abstract:
Tangible evidence of Earth’s earliest (Hadean; >4.0 Ga) crust, and the processes and materials that contributed to its formation, exists almost entirely in a record of detrital zircon from Jack Hills, Western Australia, and a few other locations. Iceland, with its thick, juvenile, basaltic crust and relatively abundant silicic rocks, is considered a potential modern analog for the Hadean magmatic environment where >4 Ga zircon formed. We present the first extensive dataset for Icelandic zircon, with trace element and oxygen isotope compositions from samples that span the island’s history and full range of tectonic settings. This statistically robust zircon-based comparison between Iceland and the early Earth reveals distinctions in chemistry that suggest fundamental differences in magmatic environments. Whereas the d 18O signature of Hadean zircons generally exceed that of zircons equilibrated with mantle-derived magma (85%=5.3‰; median 6‰), almost all Icelandic zircons are characterized by a “light” oxygen signature (98%=5.3‰; median 3‰). Deviations from “juvenile” oxygen values indicate that many Hadean zircons and almost all Icelandic zircons grew from magmas with substantial contributions from materials that had interacted with surface waters. In the Hadean case, the interaction occurred at low temperatures, while in Iceland, it was a high-temperature interaction. Icelandic and Hadean zircons are also distinct in their Ti concentrations (Icelandic median concentration 12 ppm, Hadean median 5 ppm). Titanium in zircon correlates positively with temperature of crystallization, and this difference in median Ti concentration suggests a temperature difference of at least 50?̊C. Other differences in trace elements compositions are consistent with the interpretation that Icelandic and Hadean zircons grew in magmas with very different origins and histories (e.g., the heavy rare earth element Yb is almost an order of magnitude higher in Icelandic zircon). A comparison with elemental data for Phanerozoic zircon from different environments demonstrates that the Hadean population is unusually depleted in Ti, but otherwise similar to zircons from continental arc settings. Zircons from Iceland, and from modern evolving rift environments where oceanic lithosphere and upwelling asthenosphere are replacing continental lithosphere, are compositionally intermediate between mid-ocean ridge and continental arc zircon populations. The elemental distinctions are consistent with fractionation of zircon-bearing magmas under hotter and drier conditions in Icelandic, mid-ocean ridge, and evolving rift environments and cooler and wetter conditions in arc and, especially, Hadean environments. (paywall)