Richard W. Carlson (Carnegie Institution for Science)
"The Nature of Earth's First Crust"
Earth's oldest rocks date back to about 4.3 billion years old, leaving the first 260 million years of Earth history unsampled. Ancient crustal rocks from the Moon and Mars provide a window into this missing period of Earth history, but various lines of evidence suggest that Earth's first crust likely does not well match the examples seen on our neighbor planets and moons. This talk will discuss studies of the oldest crustal rocks from Arctic Canada, Greenland, and South Africa that use both traditional geochemical and geochronological approaches, but also measurements of the decay products of short-lived radionuclides. The short-lived radionuclides record the processes that were occurring on Earth prior to the formation of Earth's oldest surviving crust and hence provide a glimpse of the nature Earth's first crust.
About the Speaker
Rick Carlson’s current research focus includes the study of nucleosynthetic heterogeneity in the early Solar nebula, attempts to more precisely determine the age of Earth’s moon, and various projects aimed at understanding the origin and evolution of continental crust on Earth and the consequences of its formation for the chemical constitution and structure of Earth’s interior.
The nucleosynthesis studies focus on precise isotopic measurements of Cr, Sr, Ba, Nd, Sm and Hf (e.g. Carlson et al., 2007; Qin et al., 2011) to detect the isotopic anomalies, correlate them with the responsible nucleosynthetic processes, and then map their distribution in different types of meteorites and the terrestrial planets. The work contributes to our understanding of the stellar nucleosynthetic processes that created the elements in our Solar system, to the question of how well mixed the Solar nebula was before the start of planet formation, and how compositional heterogeneities in the nebula may have influenced the composition of the terrestrial planets.
A second focus involves age dating of lunar crustal rocks using various radioactive chronometers to determine the time of Moon formation, the growth of its first crust, and whether Moon formation is recorded in Earth history as would be expected if the Moon formed by ejection of materials into Earth orbit by a giant impact into Earth. The work has provided the most precise date (4360 ± 3 Ma) for the age of the anorthositic crust on the Moon (Borg et al., 2011). Ongoing studies are focusing on whether the rocks of the lunar crust conform to the stratigraphy predicted from the crystallization of a lunar magma ocean that would be expected to form in the aftermath of such a giant impact and whether that event can be correlated with similar early differentiation events on Earth (Carlson and Boyet, 2008; Carlson et al., 2014).
A related research effort is examining sections of the oldest crust on Earth. One of these is the Nuvvuagittuq terrane of northern Quebec where work done by postdoctoral fellow Jonathan O’Neil provided a strong argument that this crustal section formed via convergent margin related volcanic processes at 4.3 Ga (O’Neil et al., 2012). Work is ongoing with postdoctoral fellow Hanika Rizo and University of Maryland collaborator Richard Walker on 182W isotope variation in ancient rocks from both Quebec and Greenland. The W results, due to the decay of 182Hf, may record either the initial magma ocean differentiation of Earth, or, perhaps more likely, the delivery of additional quantities of W by meteorite bombardment after core formation was complete on Earth.
A project nearing completion examines the cause of modern volcanism in the Pacific Northwest. Combination of geochronologic and geochemical studies of the volcanic rocks along with seismic imaging of the underlying mantle have revealed an interesting interplay between mantle motions induced by the subducted oceanic plate with those that may be related to uprise of a column of hot material from the deep mantle responsible for volcanism along the Snake River Plain – Yellowstone trace (e.g. James et al., 2011; Long et al., 2012’ Till et al., 2013). A similar effort is underway in Mongolia in order to understand why this mid-plate locality displays high elevations, active faulting and young volcanism. Carlson’s part of the study, working with collaborator Dmitri Ionov, is focused on the chemical and isotopic examination of the abundant mantle xenoliths from recent volcanism in central Mongolia.
All of this work involves Carlson’s interest in the development of mass spectrometric procedures (Carlson, 2013) that allow the discovery of isotopic variations at increasing levels of precision.
About this Series
Weekly talks given by leading thinkers in the areas of geology, geophysics, geobiology, geochemistry, atmospheric science, oceanography, climatology, and planetary science. Lectures take place on Wednesdays from 3:45pm in MIT Building 54 room 915, unless otherwise noted.