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Sacramento's conference on geochemistry: Echoes of ancient Earth identified

The Goldschmidt conference is the world's leading annual conference on geochemistry, and it's come to Sacramento this week, all the way from Europe. The theme of the conference is geochemistry. For example, the current topic at today's talks emphasized how scientists may have identified echoes of ancient Earth.

Sacramento's conference on geochemistry: Echoes of ancient Earth identified.
Photo by Kevork Djansezian/Getty Images

A group of scientists believe that a previously unexplained isotopic ratio from deep within the Earth may be a signal from material from the time before the Earth collided with another planet-sized body, leading to the creation of the Moon. This may represent the echoes of the ancient Earth, which existed prior to the proposed collision 4.5 billion years ago. This work is being presented at the Goldschmidt conference in Sacramento, California, which is being held this week downtown at the Sacramento Convention Center on 13th and J streets.

The currently favored theory says that the Moon was formed 4.5 billion years ago, when the Earth collided with a Mars-sized mass, which has been given the name “Theia”. According to this theory, the heat generated by the collision would have caused the whole planet to melt, before some of the debris cooled and spun off to create the Moon.

Now however, a group of scientists from Harvard University believe that they have identified a sign that only part of the Earth melted, and that an ancient part still exists within the Earth’s mantle, says a news release on how scientists may have identified echoes of ancient Earth, from this week's Goldschmidt conference in Sacramento.

According to lead researcher Associate Professor Sujoy Mukhopadhyay (Harvard), “The energy released by the impact between the Earth and Theia would have been huge, certainly enough to melt the whole planet. But we believe that the impact energy was not evenly distributed throughout the ancient Earth. This means that a major part of the impacted hemisphere would probably have been completely vaporized, but the opposite hemisphere would have been partly shielded, and would not have undergone complete melting.”

The team has analysed the ratios of noble gas isotopes from deep within the Earth’s mantle, and has compared these results to isotope ratios closer to the surface. The found that 3He to 22Ne ratio from the shallow mantle is significantly higher than the equivalent ratio in the deep mantle.

Professor Mukhopadhyay commented, according to the Goldschmidt conference news release that “This implies that the last giant impact did not completely mix the mantle and there was not a whole mantle magma ocean.”

Additional evidence comes from analysis of the 129-Xenon to 130-Xenon ratio. It is known that material brought to the surface from the deep mantle (via mantle plumes) has a lower ratio than that normally found nearer the surface, for example in the basalts from mid-ocean ridges. Since 129-Xenon is produced by radioactive decay of 129-Iodine, these xenon isotopes put a time stamp on the formation age of the ancient parcel of mantle to within the first 100 million years of Earth’s history.

Professor Mukhopadhyay continued, according to the news release, “The geochemistry indicates that there are differences between the noble gas isotope ratios in different parts of the Earth, and these need to be explained. The idea that a very disruptive collision of the Earth with another planet-sized body, the biggest event in Earth’s geological history, did not completely melt and homogenize the Earth challenges some of our notions on planet formation and the energetics of giant impacts. If the theory is proven correct, then we may be seeing echoes of the ancient Earth, from a time before the collision.”

Commenting, Professor Richard Carlson (Carnegie Institute of Washington), Past President of the Geochemical Society said, according to the news release, "This exciting result is adding to the observational evidence that important aspects of Earth's composition were established during the violent birth of the planet and is providing a new look at the physical processes by which this can occur.”

Dr. Sujoy Mukhopadhyay presented the talk, “Chemical heterogeneities survive giant impacts and mantle convection” to the Goldschmidt conference, Sacramento, California, on this morning on June 9, 2014. The Goldschmidt conference is the world's leading annual conference on geochemistry.

You may wish to check out the abstract of the new study or see the PDF format article online, "Chemical heterogeneities survive giant impacts and mantle convection." Authors are S. Mukhopadhyay, S.T. Stewart, J. M. Tucker, R. Parai, and S. Lock, from the Department of Earth and Planetary Sciences, Harvard University.

Heterogenetities in the Earth?

The giant impact phase of Earth’s accretion likely produced multiple magma oceans. In particular, the Moon-forming giant impact is often thought to have produced a whole mantle magma ocean, which should have erased pre-existing chemical heterogeneities within the Earth.

The researchers argue that the ratio of 3He to 22Ne in the present day mantle does record multiple magma ocean episodes during Earth’s accretion. A simple explanation for why the noble gas signature still persists in the present-day mantle may be the lower recycling efficiency of the noble gases compared to elements like Nd and W, according to the study's abstract. To find out what elements He, Ne, Nd, and W are, there's always the table of elements to refer to, which is online, at the site, Periodic Table of the Elements by WebElements. For example Ne stands for neon, He is helium, and W is tungsten. Check out that WebElements website for other elements mentioned. You can learn a lot about geochemistry, since when you click on the element, a whole new page opens up with a lot of information in an article format all about that element.