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Richard W. Carlson

Richard Carlson

Geochemist Richard Carlson's studies are directed at understanding the chemical and physical processes that formed the terrestrial planets and determined their current structures. To address this broad question, Carlson's research topics range from chronological investigations of early igneous processes on small planetary objects to broad chemical and physical studies of the characteristics of both old and young crust-forming processes on Earth.


Drilling samples from a 3.4-billion-year-old-lava in South Africa

Fieldwork is integral to Richard Carlson’s research. Here he uses a specially adapted chain saw to drill samples from a 3.4-billion-year-old lava flow in South Africa.

Carlson's current research on early solar system history includes investigations of the Rb-Sr, Sm-Nd, Lu-Hf and U-Th-Pb radiometric isotope systems in order to define the time scales of melting on the eucrite parent body and also on Mars from investigation of the nakhlite meteorites believed to originate from our neighbor planet. With colleague Erik Hauri and post-doctoral associate Maria Schönbachler, he has examined the silver isotopic composition of a variety of meteorites and terrestrial samples in order to better understand the processes and timing of core formation. Palladium has a comparatively short-lived isotope (107Pd) that decays to a silver isotope (107Ag), with a half-life of 6.5 million years. Carlson and fellow researchers developed a new method for high-precision silver isotope ratio analysis that provides ages precise to within a million years or less on meteorites that are 4.56 billion years old. This advance means that the Pd-Ag dating technique can now be used on a broader range of meteorites, as well as on terrestrial rocks to determine when Earth’s iron-metal core segregated to the center of the planet.

Carlson's work has also focused on understanding how continents are formed on Earth and how their formation has affected the composition of the mantle. Over the past decade, he has pursued Re-Os isotope analyses of a variety of mantle samples. One result of this work is the demonstration that old sections of continental crust generally are underlain by similarly old sections of mantle that are depleted in those elements and removed by melting to form the crust. One example of this work is the Kaapvaal Project, a multidisciplinary project focusing on the old crust of southern Africa. Through radiometric analyses of mantle xenoliths—fragments of mantle brought to the surface by volcanic eruptions from as deep as 220 km—and from seismic imaging, Carlson and colleagues found that the geological evolution of the crust of the craton is strongly tied to that of the underlying mantle. These analyses indicate that thick mantle roots date to the Archean era (3.9 to 2.5 billion years ago), suggesting that the Kaapvaal Craton has not changed significantly over the last 3 billion years.

More recently, Carlson has returned to one of his early interests: understanding the causes of continental intraplate volcanism. While the basic processes causing volcanism along ocean ridges and the volcanic arcs associated with subduction zones are reasonably well understood, the factors leading to volcanism well removed from plate boundaries are not as well defined. Carlson is a participant in a new project combining geology, geochemistry, geochronology, geodynamics and seismology to study the crust and upper mantle structure beneath the High Lava Plains of eastern Oregon. This area has been one of the most volcanically active areas in North America over the last 20 million years, despite the fact that it sits well behind the active Cascade arc and at the northern boundary of the Basin and Range Province.

SELECTED PUBLICATIONS

  • Carlson, R. W., ed. 2003. Treatise on Geochemistry, Vol. 2: The Mantle and Core, Elsevier Science.

  • Carlson, R. W., and E. H. Hauri. 2001. Extending the 107Pd-107Ag chronometer to low Pd/Ag meteorites with the MC-ICPMS, Geochim. Cosmochim. Acta 65, 1839-1848.

  • Carlson, R. W., and G. W. Lugmair. 2000. Timescales of planetesimal formation and differentiation based on extinct and extant radioisotopes, in Origin of the Earth and Moon, K. Righter and R. M. Canup, eds., Tucson, University of Arizona Press, pp. 25-44.

  • Carlson, R.W., D.G. Pearson and D.E. James. 2005. Physical, chemical and chronological characteristics of continental mantle, Rev. Geophys. 43, doi:10.1029/2004RG000156,2005.

  • Boyet, M. and R.W. Carlson. 2005. 142Nd evidence for early (>4.53 billion year) global differentiation of the silicate earth, Science 309, 576-581, 2005.

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