An important topic that has not yet been explored by experiments involves kinetic isotope fractionation by diffusion in grain boundaries. Diffusion along grain boundaries in unsaturated rocks is commonly many orders of magnitude larger than in the minerals themselves and thus it is the dominant mass transport process in such rocks. There is evidence from natural settings that significant isotopic fractionation of lithium is associated with diffusion along grain boundaries in coarse‐grained metamorphic rocks (see the classic study by Teng et al., 2006) but as yet no complementary laboratory experiments have reported. The experimental design developed by Thomas and Watson (2014) to monitor the transport of magnesium along grand boundaries in quartzite would a good way of producing samples with which to measure the isotope fractionation of elements that diffused along grain boundaries.
Some recent diffusion experiments showed results that still are quite surprising and raise issues that could eventually be understood by further laboratory experiment and theoretical methods including molecular dynamics calculations. For example, recent experiments raise the question of why diffusion in igneous minerals such as clinopyroxene and olivine is more effective at fractionating isotopes of a major element (e.g., Mg) and of a trace element (e.g., Li) than diffusion in a silicate melt? Another question whose answer might illuminate fundamental thermodynamic properties of silicate melts is why Soret diffusion in a basalt liquid is so effective at fractionating isotopes, or put another way, why is the isotopic fractionation associated with the mass flux driven by a thermal gradient so much larger than that of the flux driven differences in concentration?
The situation with regard to experiments that determine high‐temperature kinetic isotope fractionation of silicate liquids by evaporation is similar to that of diffusion in that only a limited parameter range has so far been thoroughly explored. For the most part these experiments involve evaporation into vacuum whereas evaporation in the early solar system involved evaporation of molten precursors to the CAIs now found in chondritic meteorites in a finite gas pressure of hydrogen and more oxidizing conditions later on when chondrules were melted. Richter et al. (2002) did report the results of the chemical and isotopic fractionation of a set of experiments in which a CAI‐like liquid was evaporated at 1500°C and PH2 = 2×10‐4 bars. They found that despite the evaporation rates in these low‐PH2 experiments being about two orders of magnitude faster than in vacuum, the magnesium isotopic fractionation of the evaporation residues as a function of the amount of magnesium lost was not measurably different than in vacuum. The isotopic composition of the evaporation residues from these experiments were measured with much less precision compared to what can be done with a modern ion probe, so there now could well be a resolvable difference between evaporation in vacuum and in low pressure hydrogen. The reason why one should consider repeating these earlier evaporation experiments in hydrogen is that the more precisely one can determine the amount of Mg and Si volatilized based on the isotopic fractionation of individual CAIs, the more precisely one can determine the composition of their precursor. More accurate estimates of the composition of CAI precursors will allow for more compelling validation or refutation of a fundamental proposition in cosmochemistry: that the precursors of the CAIs were condensates from a common well‐mixed solar compositions gas.
Expanding the laboratory evaporation experiments to higher pressures and more oxidizing gas mixtures will be challenging. The design of the furnaces that are now used to determine the isotopic fractionation of evaporation residues are limited to low gas pressures of less than about 10–3 bars and limited in the range of gas compositions in order that the furnace parts not also evaporate under oxidizing conditions. One will need to develop new experimental approaches in order to carry out evaporation experiments under a range of conditions that may have prevailed in the early solar nebula. A promising new approach for this involves the evaporation of molten beads in a laser heated aerodynamic levitation furnace. In such an apparatus a molten sample is floated on top of a stream of a prescribed gas composition and heated to high temperature by a laser. Winpenny et al. (2019) have shown that materials evaporated in such a furnace do become isotopically fractionated. The levitation experiments have their own limitation in that the gas flow cannot be varied at will because of the flow needed to levitate a sample of a given size. However they do provide quantitative information on evaporation into a flowing gas, which is the sort of evaporation that could take place in impact plumes. Fedkin et al. (2015) have argued that the chemical compositions of at least some types of chondrule are best explained as having once been molten droplets in impact plumes
The present report limited itself to kinetic isotope effects in high‐temperature silicate systems. The interested reader will find a discussion of a large number of works reporting kinetic isotope fractionations in aqueous solutions in Watkins et al. (2017).
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