Recently completed work by DTM postdoctoral fellow Maud Boyet and staff scientist Rick Carlson offer new insights into Earth’s early history, suggesting a much more violent origin than previously imagined. The pair have been making new and more precise measurements of the neodymium isotope ratio (¹⁴²Nd/¹⁴⁴Nd) on chondritic meteorites—this ratio can vary with the decay of the now-extinct radioactive isotope ¹⁴⁶Sm of the element samarium. The pair’s research demonstrates a Nd isotope ratio that is slightly but measurably higher in terrestrial rocks than it is in meteorites, the presumed building blocks of the planet. This leads Boyet and Carlson to conclude that within the first 30 million years of the Earth’s formation the chemical composition of the mantle underwent significant change.

By combining these results with a growing set of observations from the Moon and Mars, scientists are building a stronger basis for the theory that the formation of the inner planets was a very violent process involving the collision of planetesimals, the release of early radioactive heat, and segregation of core material, all of which liberated enough energy to melt the Earth. As the planet cooled from this initially molten state, crystallization led to important differences in the composition and density of the solidified material and the remaining magma. Gravity then separated the heavier materials from the lighter to form compositionally distinct layers in the mantle. Boyet and Carlson infer that this process resulted in the sequestering of incompatible elements into the deep mantle. They suggest that this deep layer may coincide with the D” layer at the base of the mantle.

The discovery of Earth’s excess ¹⁴²Nd in the crust is made possible by advances in technique and instrumentation that are able to distinguish very small variations in isotopic ratios that were not previously detectable. A paper describing this work has been published in Science magazine.