Faculty Seminar Series

Dr. Doug Rumble, Geophysical Laboratory,
Carnegie Institution of Washington
Date: March 30^th, 12:00 - 1:00 pm

Abstract .pdf

Oxygen isotopes record processes of star and planet formation as well as planetary evolution. Here, the fractionations of oxygen isotopes characteristic of galactic and nebular processes are compared to those of planetary accretion and evolution.

Interstellar molecular clouds consist chiefly of H₂ and CO with stardust grains. The clouds are located along the mid-plane of the Milky Way galaxy. Stars are born in molecular clouds, nucleated by gravitational instabilities and supernova explosions. The intense radiation emanating from young stars disassociates CO molecules selectively. The rare molecules such as C¹⁷O and C¹⁸O break down faster than the more abundant C¹⁶O. The released ¹⁷O and ¹⁸O react with H, forming dust grains of water ice, preserving the isotopic heterogeneity initiated by photodissociation. Astronomers have mapped oxygen isotope variations by spectroscopic analysis of C¹⁷O and C¹⁸O in molecular clouds. Oxygen isotope heterogeneity created by photodissociation displays a distinctive pattern when plotted as δ¹⁸O (x-axis) vs δ¹⁷O (y-axis): data points lie along a line with slope = 1.0. Supernova explosions inject stardust into molecular clouds magnifying their heterogeneity in oxygen isotopes.

Once a molecular cloud begins to condense around a young star and forms a planet-forming nebula, the process of photodissociation continues to create a heterogeneous distribution of oxygen isotopes. With the condensation of planets and the incorporation of residual gas and dust, photodissociation ceases. The oxygen isotope heterogeneity inherited from the parental molecular cloud and accentuated in planetary nebula before planet condensation, however, is preserved in planets, asteroids and comets. Analysis of meteorites and solar wind samples captured by Genesis show that the Solar System lies along a line of slope 1, parallel to the line traced by astronomical observations of molecular clouds.

Planetary bodies begin heating immediately upon formation thanks to the presence of short-lived radioisotopes. The segregation of metal from silicate phases into the core generates heat as do the impacts of late-arriving planetesimals. Molten magma oceans effectively homogenize the inherited oxygen isotope heterogeneity. But, a new process of isotope fractionation begins. Oxygen isotopes are fractionated between the silicate melt of the magma ocean and the first crystals to precipitate. Zero-point energy differences between isotopically substituted phases drive equilibrium, mass-dependent distribution of oxygen isotopes between minerals, magmas, atmospheres, and hydrospheres. Oxygen isotope heterogeneity created by mass-dependent fractionation displays a distinctive pattern when plotted as δ¹⁸O vs δ¹⁷O: data points lie along a line with slope ~ 0.52.

 

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