Web Extra Friday, July 20, 2007
Controversy brews over Earth's early years
Until recently, no one really had much understanding about what the first 500 million years on Earth were like, largely because no rocks seemed to have been preserved from this period. Now, however, thanks to yet another new analysis of 4.2-billion-year-old rocks, the picture is becoming clearer — Earth may have been wet as far back as 4.3 billion years ago.
Lacking a clear geologic record, in the past, scientists drew sweeping conclusions on the condition of the infant Earth based on tenuous scraps of evidence, says Mark Harrison of the University of California at Los Angeles and the Australian National University. Scientists envisioned enormous meteorites pulverizing early Earth, and oceans of churning magma extruding from supervolcanoes. From 4.56 to about 3.8 billion years ago, Earth’s surface is thought to have been devoid of crust and water — hot, dry and lifeless. These erstwhile times of fire and brimstone were aptly dubbed the Hadean Eon, and thus stood the paradigm.
This all began to change in 2001, when groups led by Harrison and John Valley of the University of Wisconsin at Madison found detrital zircon crystals in metasedimentary rocks at Jack Hills, Western Australia, that were more than 4.2 billion years old.
The zircons, and the mineral inclusions within them, suggested that as early as 4.3 billion years ago, a solid crust had formed and even that surface water had driven cycles of erosion and deposition of earth materials, Harrison says. Subsequent studies reached similar conclusions based on other methods.
Principal among them was a 2005 paper in Science by Bruce Watson of Rensselaer Polytechnic Institute and Harrison that used the titanium content in the Jack Hills zircons as a zircon crystallization thermometer. They found that the Hadean zircons had crystallized at a relatively cool 700 degrees Celsius. Because water has the near unique ability to lower the liquid-solid transition of earth materials, Watson and Harrison took the surprisingly low crystallization temperature of the Jack Hills zircons as evidence that water was available, and perhaps even abundant, during the formation of zircons as old as 4.35 billion years. Unsurprisingly, this controversial conclusion has been questioned and criticized in numerous other papers.
Now, in a paper published in this month's issue of Geology, Harrison and Watson, together with Amos Aikman of the Australian National University, further support their original conclusion of a water-rich Hadean Earth.
Criticism of the 2005 paper fell primarily into two camps. One major criticism questioned the validity of the zircon crystallization thermometer. In a 2006 paper in Science, Andrew Glikson and Allen Nutman (both formerly of the Australian National University) suggested that that the method for determining how and when zircon grains crystallized — called zircon saturation thermometry, and developed by Watson and Harrison — indicates that zircons may have been among the last minerals to crystallize, thus explaining their low crystallization temperatures at Jack Hills.
In the July Geology paper, Harrison, Watson and Aikman refute this critique. Glikson and Nutman had used a different thermometer to measure the temperatures of the zircon crystals, which underestimated the onset of zircon crystallization, Harrison says. Comparing the results of the two thermometers in this way “is like comparing apples and oranges,”’ he says.
Harrison, Watson and Aikman also refute the second primary critique, which questioned whether iron-magnesium-rich volcanic rocks — called mafic fractionates — that can contain zircon, might have been a significant source for the zircon grains found at Jack Hill. Mafic zircons tend to crystallize at higher temperatures than Hadean zircons from Jack Hills (averaging 800 compared to 700 degrees Celsius). However, some scientists, including Valley, found that zircons in mafic melts actually crystallize over a range of temperatures, the lowest end of which overlaps with the temperature values documented by Watson and Harrison in Jack Hill zircons.
"The majority of Jack Hill zircons yield temperatures around 680 degrees [Celsius]," Harrison says, "while the mafic zircons average temperatures nearly 100 degrees [Celsius] higher with relatively very few around 700 [degrees Celsius].” Thus, he says, “to have derived all the cooler zircons from a mafic source would require a differential selection and preservation process." In fact, there is such a process at work, according to Harrison and co-authors. It works, however, in the opposite direction, as low-temperature zircons tend to be higher in uranium and thorium whose radioactivity helps destroy the grains. "If you look at the statistics, there is a roughly 1 in a hundred-thousand, billion, billion chance that the Hadean Jack Hill zircon population came from a mafic source," Harrison says.
According to Harrison, the view of the early Earth devoid of water and continents "is largely a myth we created in the absence of observational evidence." The indirect observations informing the Hadean model were derived from observations of the moon, where many of the largest impact craters peppering its surface dated to the end of the Hadean. "If the moon got whacked, we got whacked even harder. For every major impact on the moon, then maybe there were a dozen impacts on Earth,” he says. “But impacts only last a moment and there’s over 500 million years of Earth history missing," he adds, suggesting the Earth's crust and hydrosphere had ample time to recover between impacts.
Thus far the Jack Hills zircon grains are the oldest Earth materials ever found, though Harrison doubts they will stay that way indefinitely. "I have no doubt that there are 4.4-billion-year-old terrestrial rocks, or older, intact, that we can discover. The rub is that I don't think they're on this planet," he says. He points to the hypothesis that Earth materials were knocked off this planet and onto the moon during large meteorite impacts throughout the Hadean. These are the same impacts that led scientists to infer hellish conditions on Earth during the Hadean. If recovered, the impact-transported materials may provide a clearer record of the conditions on Earth after its formation.
In the meantime, Harrison and his colleagues continue to search for evidence of our primordial planet. "We’re building the forensic case slowly,” Harrison adds. “This is a true detective story."
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