New theory on how Earth鈥檚 crust was created
More than 90% of Earth鈥檚 continental crust is made up of silica-rich minerals, such as feldspar and quartz. But where did this silica-enriched material come from? And could it provide a clue in the search for life on other planets?
Conventional theory holds that all of the early Earth鈥檚 crustal ingredients were formed by volcanic activity. Now, however, 平特五不中 earth scientists Don Baker and Kassandra Sofonio have published a theory with a novel twist: some of the chemical components of this material settled onto Earth鈥檚 early surface from the steamy atmosphere that prevailed at the time. 听
First, a bit of ancient geochemical history: Scientists believe that a Mars-sized planetoid plowed into the proto-Earth around 4.5 billion years ago, melting the Earth and turning it into an ocean of magma. In the wake of that impact 鈥� which also created enough debris to form the moon -- the Earth鈥檚 surface gradually cooled until it was more or less solid. Baker鈥檚 new theory, like the conventional one, is based on that premise.
The atmosphere following that collision, however, consisted of high-temperature steam that dissolved rocks on the Earth鈥檚 immediate surface -- 鈥渕uch like how sugar is dissolved in coffee,鈥� Baker explains. This is where the new wrinkle comes in. 鈥淭hese dissolved minerals rose to the upper atmosphere and cooled off, and then these silicate materials that were dissolved at the surface would start to separate out and fall back to Earth in what we call a silicate rain.鈥�
To test this theory, Baker and co-author Kassandra Sofonio, a 平特五不中 undergraduate research assistant, spent months developing a series of laboratory experiments designed to mimic the steamy conditions on early Earth. A mixture of bulk silicate earth materials and water was melted in air at 1,550 degrees Celsius, then ground to a powder. Small amounts of the powder, along with water, were then enclosed in gold palladium capsules, placed in a pressure vessel and heated to about 727 degrees Celsius and 100 times Earth鈥檚 surface pressure to simulate conditions in the Earth's atmosphere about 1 million years after the moon-forming impact. After each experiment, samples were rapidly quenched and the material that had been dissolved in the high temperature steam analyzed.
The experiments were guided by other scientists鈥� previous experiments on rock-water interactions at high pressures, and by the 平特五不中 team鈥檚 own preliminary calculations, Baker notes. Even so, 鈥渨e were surprised by the similarity of the dissolved silicate material produced by the experiments鈥� to that found in the Earth鈥檚 crust.
Their resulting paper, published in the journal Earth and Planetary Science Letters, posits a new theory of 鈥渁erial metasomatism鈥� -鈥� a term coined by Sofonio to describe the process by which silica minerals condensed and fell back to earth over about a million years, producing some of the earliest rock specimens known today.
鈥淥ur experiment shows the chemistry of this process,鈥� and could provide scientists with important clues as to which exoplanets might have the capacity to harbor life Baker says.
鈥淭his time in early Earth's history is still really exciting,鈥� he adds. 鈥淎 lot of people think that life started very soon after these events that we're talking about. This is setting up the stages for the Earth being ready to support life.鈥�
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Funding for the research was provided by an NSERC Discovery grant to Baker and an NSERC Summer Undergraduate Research Assistant grant to Sofonio.
鈥淎 metasomatic mechanism for the formation of Earth鈥檚 earliest evolved crust,鈥� Don R. Baker, Kassandra Sofonio, Earth and Planetary Science Letters, 1 April 2017.听
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