Isotopes are elements that contain the same number of protons Z but different numbers of neutrons N. Because of the slight difference in their mass (Z+N), their zero-point energies differ. When two different substances are in equilibrium, the isotopes partionate, with the heavier isotope going preferentially in the substance that forms the strongest bonds.
Fractionation of isotopes is inferred to be usually small at magmatic temperature, as zero-point energy differences and other quantum mechanic effects become negligible compared to thermal excitation of the nuclei.
However, light isotopes still experiment fractionation at magmatic temperature. For instance, we showed that hydrogen isotopes fractionate between silicate melts and aqueous fluids at 1500 MPa and 700°C, with fluid-melt fractionation factors as high as ~100 ‰ (Dalou et al., 2015). They even reach 600 ‰ at around T = 300-400 °C and P = 200-300 MPa, conditions relevant to phreatomagmatic activity for instance.
Such high fractionation factors may have been promoted by intramolecular effects in silicate melts. Indeed, we show in a new study available in the Geochemical Perspective Letters journal, an open-source journal from the European Association of Geochemistry, that actually protons and deuterons, the main isotopes of hydrogen, fractionate between the tetrahedral units that are present in the molecular disordered network of silicate melts. Such intramolecular fractionation might enhance any effect at high temperature, and possibly explains the results of Dalou et al. (2015).
Hydrogen isotopes present a very strong mass difference and are quite unique, but such intramolecular fractionation effect seems not to be limited only to the hydrogen isotopes. Indeed, the fractionation factors of the isotopes of nitrogen and carbon between silicate melts and aqueous fluids (Mysen and Fogel, 2009; Mysen et al. 2010) also show a dependence to the melt tetrahedral network disordered structure. Furthermore, the fractionation of sulfur isotopes between silicate melts and metals also depends on the amount of trivalent network former cations (aluminium, boron...) in the melt, and, hence, on the melt structure (Labidi et al., 2016). This relates to a change in the sulfur environment, in term of coordination number and bond strength, with changing the melt bulk chemical composition and molecular structure. Interestingly, during planetary formation, the sulfur coordination number should increase with increasing the depth of the core-mantle differentiation. This will change the fractionation of sulfur isotopes between the planetary mantle and core. This may explain differences between the sulfur isotopic budget of the mantles of Mars and Earth, two planets for which the core-mantle segregation depth is expected to have been different.
Therefore, intramolecular effects happening in the disordered structure of silicate melts may affect light isotopes. While the extent of such control remains to be assessed, they are of potential interest to improve our understanding of the behaviour of isotopes and their use for solving various problems linked to planetary formation and differentiation, volcanology, the budget of volatile elements, to cite only a few examples.
References:
Dalou C., Le Losq C., Mysen B. O. (2015) In situ study of the fractionation of hydrogen isotopes between aluminosilicate melts and coexisting aqueous fluids at high pressure and high temperature - Implications for the dD in magmatic processes. Earth and Planetary Science Letters 426, 158-166.
Labidi J., Shahar A., Le Losq C., Hillgren V. J., Mysen B. O., Farquhar J. (2016) Experimentally determined sulfur isotope fractionation between metal and silicate and implications for planetary differentiation. Geochimica et Cosmochimica Acta 175, 181-194.
Mysen, B.O., Fogel, M.L. (2010) Nitrogen and hydrogen isotope compositions and solubility in silicate melts in equilibrium with reduced (N+H)-bearing fluids at high pressure and temperature: effects of melt structure. American Mineralogist 95, 987-999.
Mysen, B.O., Fogel, M.L., Morrill, P.L., Cody, G.D. (2009) Solution behavior of reduced C O H volatiles in silicate melts at high pressure and temperature. Geochimica et Cosmochimica Acta 73, 1696-1710.