Oldest Crystal Tells a Tale of a Hospitable Early Earth

Reading the telltale chemical signature of a mineral sample determined to be the world's oldest known terrestrial material, OBES-sponsored scientists have reconstructed a portrait that suggests that the early Earth, instead of being a roiling ocean of magma, was cool enough to have water, continents, and conditions that could have supported life.

Moreover, the age of the 4.4-billion-year-old sample may also undermine accepted views on the formation of the moon. By probing a tiny grain of zircon, a mineral commonly used to determine the age of rocks, scientists from the University of Wisconsin-Madison, Curtin University in Australia, and the University of Edinburgh in Scotland have found evidence that 4.4 billion years ago, a mere 100 million years or so after the accretion of the Earth, temperatures had cooled to the 100^oC range, a discovery that suggests an early Earth far different from the one previously imagined. "This is an astounding thing to find for 4.4 billion years ago," says John W. Valley, a UW-Madison professor of geology and geophysics. He is co-author of two papers, one in Nature (Jan. 11) and another in press at Geochimica et Cosmochimica Acta. Both articles paint a picture of a precocious young Earth complete with a low-temperature environment, oceans, the beginnings of continents and conditions suitable for life.

"At that time, the Earth's surface should have been a magma ocean," says Valley. "Conventional wisdom would not have predicted a low-temperature environment. These results may indicate that the Earth cooled faster than anyone thought." Previously, the oldest evidence for liquid water on Earth, a precondition and catalyst for life, was from a rock estimated to be 3.8 billion years old.

The new picture of the earliest Earth is based on a single, tiny grain of zircon from western Australia found and dated by Simon A. Wilde, a professor in the School of Applied Geology at Curtin University of Technology in Perth, Western Australia. Valley worked with William H. Peck, a former UW-Madison graduate student and now an assistant professor of geology at Colgate University, to analyze oxygen isotope ratios, measure rare earth elements, and determine element composition in a grain of zircon that measured little more than the diameter of two human hairs. Peck, lead author of the Geochimica paper, conducted the work as part of his doctoral thesis at UW-Madison. Colin M. Graham's laboratory analyzed the zircon to obtain the oxygen isotope ratios. Graham is a contributor to the paper and professor of geochemistry at the University of Edinburgh.

According to Peck, the zircon has a high proportion of heavy oxygen isotopes (i.e., the sample is enriched in ¹⁸O and ¹⁷O, relative to the most abundant isotope ¹⁶O).  This feature is not common in other such minerals from the first half of the Earth's history.  Typically, such an isotopic "signature" arises when minerals interact with liquid water at low temperatures near the surface of the Earth. .

The conventional view of the infant Earth is that shortly after it first formed 4.5 to 4.6 billion years ago, the planet became little more than a swirling ball of molten metal and rock. Scientists believed it took a long time, perhaps 700 million years, for the Earth to cool to the point that oceans could condense from a thick, Venus-like atmosphere.

Complicating the picture is that for 500 million to 600 million years after the Earth was formed, the young planet was pummeled by intense meteorite bombardment. About 4.45 billion years ago, a Mars-size object is believed to have slammed into the Earth, creating the moon by blasting pieces of the infant planet into space. "This is the first evidence of crust as old as 4.4 billion years, and indicates the development of continental-type crust during intense meteorite bombardment of the early Earth," Valley says. "It is possible that the water-rock interaction (as represented in the ancient zircon sample) could have occurred during this bombardment, but between cataclysmic events."

Scientists have been searching diligently to find samples of the Earth's oldest rocks. Valley and Peck say such ancient samples are extremely rare because rock is constantly recycled or sinks to the hot mantle of the Earth. Over the great spans of geologic time, there is little surface material that has not been recycled and reprocessed in this way.

The tiny grain of zircon (which is a simple zirconium silicate, ZrSiO₄) found by Wilde in western Australia was embedded in a larger sample containing fragments of material from many different rocks, Valley says. Zircons dated at 4.3 billion years were reported from the same site a decade ago, but the new-found zircon grain is more than 100 million years older than any other known sample, giving scientists a rare window to the earliest period of the Earth. "This early age restricts theories for the formation of the moon," Valley says. "Perhaps the moon formed earlier than we thought, or by a different process."

Another intriguing question is whether or not life may have arisen at that early time. Low temperatures and water are preconditions for life. The earliest known biochemical evidence for life and for a hydrosphere is estimated at 3.85 billion years ago, and the oldest microfossils are 3.5 billion years old.  "It may have been that life evolved and was completely extinguished several times" in catastrophic, meteorite-triggered extinction events well before that, Valley says.

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