In today’s Academic Minute, Dr. Richard Walker of the University of Maryland reveals what we know about how the Earth weathered the period of heavy bombardment.
Richard Walker is a professor of geology at the University of Maryland where he serves as Director of the Isotope Geochemistry Laboratory. His research interests include the geochemical evolution of the Earth's crust and mantle and the origin of early solar system materials. He holds a Ph.D. from Stony Brook University.
Dr. Richard Walker – Earth and the Formation of the Moon
As with most of the chemical elements, tungsten and hafnium consist of different isotopes. each are characterized by having the same number of electrons and protons but different numbers of neutrons. An isotope of Tungsten, Tungsten 182, is of special interest to geologists and meteoriticists because it was partly produced by the radioactive decay of an unstable isotope of hafnium, Hafnium 182. Radioactive Hafnium 182 was present when the Solar System formed, but all of it converted to Tungsten 182 within the first 50 million years This means that small variations in the abundance of this isotope of Tungsten could only have been created within the first 1% of Solar System history.
We used a mass spectrometer to measure a very small, but significant enrichment in the abundance of Tungsten 182, in some 2.8 billion year old volcanic rocks from Russia. The lava that crystallized to form these unusual rocks, was created by melting of the Earth’s mantle at great depths. The slightly greater amount of Tungsten 182 in these rocks means that the lavas sampled a portion of the mantle that formed very early in Earth history, Earth most likely grew to its current size by a series of collisions of bodies of increasing size, over what may have been as much as tens of millions of years. Collisions involving bodies the size of the Moon, or even Mars are so energetic that they can lead to complete melting of the resulting composite bodies, which would tend to mix away isotopic differences like we identified.
Our new findings, therefore, indicate that some of the building blocks of the Earth somehow survived the violent construction period of Earth’s formation. Preservation of such early materials deep within the mantle lends hope that some of the other diverse building blocks of the Earth may eventually be identified.