ScienceWeek

GEOPHYSICS: ON CARBON ATOMS IN THE EARTH'S MANTLE

The following points are made by Mark Wilson (Physics Today 2003 October):

1) Earth scientists routinely monitor the levels of carbon they find in the planet's biomass, atmosphere, and oceans. But to judge by the composition of basaltic magma and the flux of carbon dioxide coughed out by volcanoes, the amount of carbon sequestered within Earth's voluminous mantle -- estimated to be a few hundred parts per million (ppm) by weight -- is roughly a thousand times larger than the amount on the surface. The presence of so huge a reservoir of carbon separated by mere kilometers from the comparatively tiny one on the surface has spurred researchers to figure out where and in what form the carbon is stored within Earth's interior.

2) As the dominant constituent of the upper 400 km of the mantle, olivine is a natural suspect. It is a solid mixture of 90% magnesium silicate and 10% iron silicate. Like any ionic mineral, it accepts small levels of impurities into the crystal structure. In the case of carbon, C4+ ions could substitute into tetrahedral Si4+ vacancies, for instance, although the radius mismatch between carbon and silicon cations would make the fit energetically unlikely. Substitution of carbon into magnesium vacancies is harder to imagine; no stable C2+ defect has ever been observed as an ionic substituent, probably because of its partially filled valence-electron shell in that charge state. Other possibilities come to mind, though: carbon diffusing into interstitial sites or line defects that could accommodate impurity ions.

3) Unfortunately, trace amounts of carbon in rock are hard to measure. The ubiquity of contaminant hydrocarbons in labs and the slow diffusion of carbon in silicates stymied experimenters in the mid-1980s. Typically, researchers took olivine from xenoliths -- inclusions that make their way to the surface inside volcanic rock. Hoping to ensure carbon-saturated samples, the researchers annealed the olivine at high pressure and temperature in the presence of a carbon-bearing phase. Reports of the solubility from competing groups varied widely. At the parts-per-million level, measurements must be ultrasensitive: The touch of a ChemWipe, fingerprint, or even stray atoms from the lubrication in a vacuum pump could ruin an experiment.

4) Keppler et al (1) have recently resolved the lingering uncertainty by pursuing a different tack. Instead of measuring carbon squeezed into olivine chunks spewed from volcanoes, they made their own proxy under conditions that would maximize the amount of carbon absorbed within the crystal. The recipe is simple enough: Take a stoichiometric mixture of magnesium oxide and silicon dioxide, 1% water by weight, and cook at gigapascal pressures and 1200°C mantle temperatures in a carbonate-rich melt. Such melts are thought to be common within Earth's mantle. Using that recipe, Keppler et al produced crystals of forsterite (90% of the solid mixture that makes up olivine). To distinguish dirt from carbon dissolved in the silicate, Keppler et al grew their crystals in carbonate made from carbon-13, which is 1% as abundant in nature as carbon-12, a procedure that.produced 100 times greater sensitivity. From their results, the authors conclude that the vast reservoir of carbon atoms in the mantle simply cannot be stuffed into the most common minerals within the mantle. So, carbon must reside either in reduced elemental form as graphite or diamond, or within a CO2-rich carbonate phase.(2,3)

References (abridged):

1. H. Keppler, M. Wiedenbeck, S. S. Shcheka, Nature 424, 414 (2003)

2. A. E. Saal, E. H. Hauri, C. H. Langmuir, M. R. Perfit, Nature 419, 451 (2002)

3. A. Marzoli et al., Science 284, 616 (1999)

Physics Today http://www.physicstoday.org

ScienceWeek http://www.scienceweek.com