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Diffusion in condensed phases is a ubiquitous but poorly understood phenomenon. For example, chemical diffusion, which is the transport of matter associated with chemical concentration gradients (Fick's law), is treated as a separate process from thermal transport (the Soret effect), which is mass transport induced by temperature gradients. In the past few years, large variations in the proportions of isotopes of Mg, Ca, Fe, Si and O found in silicate melts subject to thermal gradients have been found, but no physical mechanism has been proposed. Here we present a model of diffusion in natural condensed systems that explains both the chemical and isotopic fractionation of Mg, Ca and Fe in high-temperature geochemical melts. Despite the high temperatures associated with these melts (T > 1,000 °C), we find that consideration of the quantum-mechanical zero-point energy of diffusing species is essential for understanding diffusion at the isotopic level. Our model explains thermal and chemical mass transport as manifestations of the same underlying diffusion mechanism. This work promises to provide insights into mass-transport phenomena (diffusion and evaporation) and associated isotopic fractionations in a wide range of natural condensed systems, including the atmospheric water cycle, geological and geochemical systems and the early Solar System. This work might also be relevant to studies of mass transport in biological and nanotechnological condensed systems. [ABSTRACT FROM AUTHOR]

The isotope 146Sm undergoes α-decay to 142Nd, with a half-life of 103 million years. Measurable variations in the 142Nd/144Nd values of rocks resulting from Sm–Nd fractionation could therefore only have been produced within about 400 million years of the Solar System’s formation (that is, when 146Sm was extant). The 142Nd/144Nd compositions of terrestrial rocks are accordingly a sensitive monitor of the main silicate differentiation events that took place in the early Earth. High 142Nd/144Nd values measured in some Archaean rocks from Greenland hint at the existence of an early incompatible-element-depleted mantle. Here we present measurements of low 142Nd/144Nd values in 1.48-gigayear-(Gyr)-old lithospheric mantle-derived alkaline rocks from the Khariar nepheline syenite complex in southeastern India. These data suggest that a reservoir that was relatively enriched in incompatible elements formed at least 4.2 Gyr ago and traces of its isotopic signature persisted within the lithospheric root of the Bastar craton until at least 1.48 Gyr ago. These low 142Nd/144Nd compositions may represent a diluted signature of a Hadean (4 to 4.57 Gyr ago) enriched reservoir that is characterized by even lower values. That no evidence of the early depleted mantle has been observed in rocks younger than 3.6 Gyr (refs 3, 4, 7) implies that such domains had effectively mixed back into the convecting mantle by then. In contrast, some early enriched components apparently escaped this fate. Thus, the mantle sampled by magmatism since 3.6 Gyr ago may be biased towards a depleted composition that would be balanced by relatively more enriched reservoirs that are ‘hidden’ in Hadean crust, the D′′ layer of the lowermost mantle or, as we propose here, also within the roots of old cratons. [ABSTRACT FROM AUTHOR]

Small isotopic differences in the atomic abundance of neodymium-142 (142Nd) in silicate rocks represent the time-averaged effect of decay of formerly live samarium-146 (146Sm) and provide constraints on the timescales and mechanisms by which planetary mantles first differentiated. This chronology, however, assumes that the composition of the total planet is identical to that of primitive undifferentiated meteorites called chondrites. The difference in the 142Nd/144Nd ratio between chondrites and terrestrial samples may therefore indicate very early isolation (<30 Myr from the formation of the Solar System) of the upper mantle or a slightly non-chondritic bulk Earth composition. Here we present high-precision 142Nd data for 16 martian meteorites and show that Mars also has a non-chondritic composition. Meteorites belonging to the shergottite subgroup define a planetary isochron yielding an age of differentiation of 40 ± 18 Myr for the martian mantle. This isochron does not pass through the chondritic reference value (100 × ε142Nd = -21 ± 3; 147Sm/144Nd = 0.1966). The Earth, Moon and Mars all seem to have accreted in a portion of the inner Solar System with ∼5 per cent higher Sm/Nd ratios than material accreted in the asteroid belt. Such chemical heterogeneities may have arisen from sorting of nebular solids or from impact erosion of crustal reservoirs in planetary precursors. The 143Nd composition of the primitive mantle so defined by 142Nd is strikingly similar to the putative endmember component ‘FOZO’ characterized by high 3He/4He ratios. [ABSTRACT FROM AUTHOR]

Replying to D. J. Lacks, J. A. Van Orman & C. E. Lesher 482, 10.1038/nature10764 (2012)Lacks et al. argue that our model of isotopic fractionation in thermal gradients in silicate melts does not agree with measurements of the ratio of diffusivities seen in silicate melts. This statement is based on an over-interpretation of our model into non-steady-state applications, such as chemical fractionation, because the model we presented treats the quantized energy levels of the transition state as being equal to each other (the partition function Z(TS) = 1). This was warranted, as our main interest was in finding the steady-state solution to isotopic fractionation in a closed system (which is insensitive to the transition state). The potential importance of the transition state in determining the ratios of diffusivities of He isotopes in a geologic system has previously been noted, and future work will need to clarify the importance of the transition state for kinetic isotopic fractionation in silicate systems, particularly the ratio of diffusivities. [ABSTRACT FROM AUTHOR]

Replying to F. M. Richter 472, 10.1038/nature09954 (2011)In our Letter, we showed that the phenomenon of isotope fractionation in silicate melts by thermal diffusion, first reported in 1998 (ref. 2), can be characterised by a parameter ΔST that is independent of temperature and composition. (Here ΔST is the difference in the Soret coefficient, ST, between isotopes of a diffusing element.) Richter questioned this finding by plotting Mg isotope ratio versus temperature data (figure 1 in ref. 3) for a subset of experiments from figure 3f of ref. 1. [ABSTRACT FROM AUTHOR]

Arising from F. Huang, P. et al. 464, 396-400 (2010); Huang et al. replyIt was recently shown that relatively large (compared to analytical precision) steady state thermal isotope fractionations are produced in silicate melts whenever temperature differences are maintained for a sufficiently long time. Huang et al. reported new data on thermal isotopic fractionation of magnesium, calcium, and iron in silicate liquids, and claimed (1) that thermal isotopic fractionations in silicate liquids are independent of composition and temperature, and (2) that their 'results lead to a simple and robust framework for characterizing isotope fractionations by thermal diffusion in natural and synthetic systems'. Here I consider whether the data and arguments presented by Huang et al. support their claims. In summary, I caution against assuming (on the basis of the data presented by Huang et al.) that the thermal isotopic fractionations are independent of temperature and composition, or that a framework of the type claimed has been found. [ABSTRACT FROM AUTHOR]