Magnesium isotope fractionation in silicate melts by chemical and thermal diffusion

Variations of Li and Mg isotope ratios in bulk chondrites and mantle xenoliths

“…The model integrates the crystal data to acquire a whole-rock xenolith 629 isotope ratio, which initially becomes isotopically lighter as Li and Mg diffuse in, and 630 eventually returns to an equilibrium value with time (Fig. 9, shown best-fit our data, and are similar to those established experimentally (Richter et al, 636 2003;Richter et al, 2008;Richter et al, 2009) (Fig. 10).…”

“…Mg offers the potential to clarify this ambiguity. As the 78 major mantle cation, addition of recycled material will have minimal effect on its 79 isotopic ratio, whereas diffusive processes will likely fractionate Mg together with Li 80 (Richter et al, 2003;Richter et al, 2008). Hence we have analysed whole rock Li and 81 Mg isotopic ratios on a range of xenoliths, including samples in which variable 82 amounts of diffusive fractionation have already been identified (Jeffcoate et al, 2007;83 Rudnick and Ionov, 2007), a suite of xenoliths from a mantle wedge setting that might 84 display primary heavy Li isotope signatures and additional samples that have 85 experienced a range of metasomatic enrichment processes.…”

Grain boundary mobility of carbon in Earth's mantle: A possible carbon flux from the core

“…Isotopes are now known to be fractionated in geological systems by diffusion processes: lighter isotopes diffuse slightly faster than their heavy counterparts. In molten silicates, for example, the ratio of the diffusivities of two isotopes is inversely proportionate to their mass ratio raised to an exponent of Ϸ0.05-0.2 (21)(22)(23). Because the relative mass difference between 12 C and 13 C is significant (as it is for light isotopes in general), grainboundary diffusion could be an effective mechanism in producing isotopically light sources of carbon.…”

The influence of melt infiltration on the Li and Mg isotopic composition of the Horoman Peridotite Massif

“…Chemical diffusion experiments showed as much as 7 ‰ fractionation 69 of 26 Mg/ 24 Mg, which is the second largest effect for a metal cation, after Li (Richter et al, 70 2003;Richter et al, 2008;Chopra et al, 2012). It has also been shown that Mg isotopic 71 fractionation can result from thermal diffusion Richter et al, 2008), 72 although it has long been known that this process more significant in laboratory experiments 73 than in nature (Bowen, 1921;Lesher and Walker, 1988;Walker et al, 1988). Recent work has 74 empirically suggested that such Soret diffusion of Mg is not an important geological 75 phenomenon .…”