Possible Echoes of Ancient Earth Identified


Giant Impact Forms The Moon



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.





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.


Commenting, Professor Richard Carlson (Carnegie Institute of Washington), Past President of the Geochemical Society, said, "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."

Science Contacts:
Prof. Sujoy Mukhopadhyay

Prof. Richard Carlson

Tom Parkhill
Goldschmidt Press Officer
+44 131 208 3008 (CEST)

ABSTRACT "Chemical Heterogeneities Survive Giant Impacts and Mantle Convection" S. Mukhopadhyay, S. T. Stewart, J. M. Tucker, R. Parai, and S. Lock Dept. of Earth & Planetary Sciences, Harvard University, Cambridge, MA 02138, USA

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. We argue that the ratio of 3He to 22Ne in the present day mantle does record multiple magma ocean episodes during Earth's accretion. However, the 3He/22Ne ratio of the Earth's shallow depleted mantle is significantly higher than the deep mantle indicating that the last giant impact did not generate a whole mantle magma ocean. Although the energy associated with the Moon-forming giant impact was sufficient to melt the whole planet, the impact energy is heterogeneously deposited; the impacted hemisphere is shocked to the point of partial vaporization, but the opposite hemisphere experiences modest heating that does not result in completely melting. As a result, chemical heterogeneities persist through the giant impact phase of accretion.

Additional evidence for the preservation of early-formed heterogeneities in the deep mantle is provided by 129Xe/130Xe ratios. Deep mantle plumes have a lower ratio of 129Xe/130Xe compared to the source of mid-ocean ridge basalts (MORBs). The Xe signature requires a region of the deep mantle to be less degassed and to have separated from the shallower MORB source by 4.45 Ga (since 129I, which produces 129Xe, is extinct after ~100 Ma); i.e., neither the giant impact phase nor mantle convection has efficiently homogenized the mantle. The persistence of noble gas signatures produced very early in Earth history, such as those associated with the 129I-129Xe system, may appear to be in conflict with other extinct nuclide systems such as 146Sm-142Nd or 182Hf-182W. While isotopic anomalies in 142Nd and 182W are present in the Hadean and Archean mantle, the present-day mantle appears to be homogeneous. The observation requires Sm-Nd and Hf-W fractionation within the first few hundred Ma but also the subsequent destruction of the chemical fractionation through recycling and mantle mixing. 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.

Please follow SpaceRef on Twitter and Like us on Facebook.