Noble Gases and Siderophile Elements in the Mantle: Unconventional Experimental Results and Their Implications for Chemical Geodynamics

Recent and ongoing experimental studies reveal unexpected behavior of noble gases and siderophile elements that may affect future geodynamic interpretations. Ar-uptake experiments on mantle minerals provide insight into Ar compatibility (solubility) and diffusivity in these phases. Contrary to expectation, solubility results suggest that Ar behaves as a compatible element during mantle melting, favoring residence in point defects in minerals over 'escape' to the melt. In terms of diffusion, Ar is sufficiently mobile in olivine and orthopyroxene to ensure effective dispersal from relatively K-rich minerals on a geodynamically short time scale. Solid-liquid equilibration during melting of the MORB source is also likely; however, Ar diffusion is slow enough for disequilibrium to arise during phenocryst growth in a pre-eruption setting. The main implications of these results are that: 1) Ar degassing of the mantle through MORB volcanism may be an inefficient process; and 2) the existence of deeply-sequestered Ar (and other noble gases?) is plausible. New data on the behavior of siderophile elements (Os, Ir, Ru, Rh, Pt, Re, Au, W, Mo, Co, Cu and C) in polycrystalline MgO and synthetic peridotite reveal that these elements are highly mobile in the grain boundaries of mantle assemblages and analogs. W, Au and C have the highest diffusivities (10-8 to 10-7 m2/s); Ir, Ru, Re and Os are at the low end of the range (~10-12 to 10-11 m2/s). There is no discernible correlation between grain-boundary diffusivity and any particular property of the atoms or elements. These new data imply that Earth's outer core may 'leak' siderophile elements into the lower mantle over distances exceeding 100 km in 4 GYr for W, Au and C. Although not significant in itself as a whole-mantle transport process, grain boundary diffusion appears capable of 'contaminating' the lower mantle over a sufficient distance to enable entrainment of a core signature into plume- or convective mantle flow. If the outer core contains C, our data suggest that there may be a significant core-to-mantle flux of this element.