Volume diffusion rates for five rare-earth elements were measured in single crystals of natural diopside at pressures of 0.1MPa to 2.5GPa and temperatures of 1050 to 1450C. Polished pre-annealed crystals were coated with a thin film of rare-earth element oxides, then held at constant temperature and pressure for times ranging from 20 to 882h. Diffusion profiles in quenched samples were measured by secondary ion mass spectrometry depth profiling. At 1atm, with the oxygen fugacity controlled near to the quartz-fayalite-magnetite buffer, the results for diffusion normal to (001) were described by:

Ce:      log10[D(m2/s)] = -4.10-(463kJ/mol/2.303RT)

Dy:     log10[D(m2/s)] = -3.31-(461kJ/mol/2.303RT)

La:      log10[D(m2/s)] = -4.22-(466kJ/mol/2.303RT)

Nd:     log10[D(m2/s)] = -2.95-(496kJ/mol/2.303RT)

Yb:     log10[D(m2/s)] = -4.64-(411kJ/mol/2.303RT)

The diffusion rates decreased significantly with increasing ionic radius, with La being a factor of ∼35 slower than Yb. The relationship between diffusivity and ionic radius was consistent with a model in which elastic strain played a critical role in governing the motion of an ion through the crystal lattice. The activation volumes for Yb and Ce diffusion, at constant temperature and oxygen fugacity, were 9.0cm3/mol and 8.9cm3/mol, respectively; corresponding to an order of magnitude decrease in diffusivity as the pressure was increased from 0 to 3GPa at 1200C. The diffusion of Nd was such that grain-scale isotopic equilibrium in the mantle could be achieved in ∼1My under conditions near to the periodotite solidus (∼1450C at 2.5GPa). The equilibration time was much longer under P,T conditions of the lithospheric mantle or at the eclogite solidus (∼1Gy at 1.5GPa and 1150C). Because of the relatively strong decrease in diffusivity with pressure (two orders of magnitude between 2.5 and 15GPa along an adiabatic temperature gradient) Nd transport in clinopyroxene would be effectively frozen at pressures approaching the transition zone, on time-scales of less than 100My. Rare earth element diffusion rates were slow enough that significant disequilibrium up-take of REE by growing clinopyroxene phenocrysts could be preserved under natural conditions of basalt crystallization.

Rare Earth Element Diffusion in Diopside: Influence of Temperature, Pressure, and Ionic Radius, and an Elastic Model Fordiffusion in Silicates. J.A.Van Orman, T.L.Grove: Contributions to Mineralogy and Petrology, 2001, 141[6], 687-703