FOLLOWING IN FANG-ZHEN TENG’S FOOTSTEPS: ISOTOPE FRACTIONATION BY DIFFUSION IN MINERALS AND IN GRAIN BOUNDARIES

The field of kinetic stable isotope fractionation by diffusion in geological material has rapidly expanded over the last ten years to include experiments and natural examples involving silicate melts, major minerals such as pyroxene and olivine, and transport in grain boundaries. Sometimes the demonstration of large isotope fractionations in laboratory experiments preceded their being rigorously documented in a natural setting. This was the case for kinetic isotope fractionation between silicate melts. The opposite was true in the case of grain boundary diffusion, where a remarkable paper by Fang-Zhen Teng and collaborators at the University of Maryland showed very large isotopic fractionation of lithium that had diffused from the Tin Mountain pegmatite into the surrounding amphibolite along grain boundaries. This work was the key motivation for our recent laboratory experiments demonstrating very large isotopic fractionation of lithium that diffused along grain boundaries in polycrystalline olivine. Another very important work by Fang-Zhen showed how isotopes should be used distinguishing between mineral zoning due to growth from an evolving melt versus that due later diffusive exchange between a mineral and its surrounding. This work has stimulated both applications of this idea to olivine grains from a Hawaiian lava lake, and our laboratory studies documenting isotopic fractionation by diffusion in olivine and pyroxene.