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Diffusive mass transport usually tends to erase heterogeneity in chemical composition, but it may lead to large isotope fractionation because heavier isotopes diffuse slower than lighter isotopes. The diffusivity ratio of two isotopes can be related to the inverse atomic mass ratio as [1]: D2/D1 = (m1/m2)^b, with b being an empirical parameter ranging between 0 and 0.5. The b parameter for diffusive isotope fractionation in silicate melts has been determined for Li, Cu, K, Ca, Mg, Fe, and Ti [1, 2, and references therein]. There is a general but scattered trend of b value increasing with diffusivity [2, 3]. There are theoretical relations between b and diffusing species, but it has been difficult to apply such relations to the elements investigated so far because the diffusing species of these elements in silicate melts are not known. 

The diffusion of H, with diffusing species of H2 or H2O, provides an opportunity to apply and test the theoretical relations. The H diffusion potentially provides an excellent example for possible large variation of the b value for an element depending on the diffusing species. In pure silica glass, molecular H2 and D2 diffusion has been investigated [4,5, and references therein], with b value ranging from 0.29 to 0.47. The theoretical b value is 0.5. In silicate melts, water diffusion has been investigated extensively [e.g., 6]. Existing experimental charges of water diffusion will be prepared for SIMS measurement of D/H ratio profiles. The data will be used to determine b value. Then the significance of speciation and isotope fractionation will be discussed. 

References: [1] Richter et al., 1999, Geochim. Cosmochim. Acta, 63, 2853. [2] Zhou et al., 2025, Earth Planet. Sci. Lett., 651, 119176. [3] Watkins et al., 2011, Geochim. Cosmochim. Acta, 75, 3103. [4] Lee et al., 1962, J. Chem. Phys., 36, 1062. [5] Shang et al., 2009, Geochim. Cosmochim. Acta, 73, 5435. [6] Zhang and Behrens, 2000, Chem. Geol., 169, 243.