Isotopic Constraints on Earth System Processes. Группа авторов

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Soret Isotope Fractionation in Silicate Liquids

      The importance of laboratory experiments documenting kinetic isotope fractionation by temperature differences in silicate liquids is not so much because of potential applications to geologic problems (see reference to Bowen later in this section), but rather that certain features of the isotopic fractionation of samples recovered from piston cylinder experiments appear to have been significantly affected by thermal effects. A possible example of this was already noted in connection with the 44Ca/40Ca fractionation of about 3‰ in the basaltic part of the RB‐2 diffusion couple shown in Fig. 1.4. The suggestion that this “unexpected” isotopic fractionation might be the result of thermal isotope fractionation prompted the quantitative question of whether a few tens of degrees of temperature difference, which Watson et al. (2002) had shown was common for samples run in a piston cylinder apparatus, can explain isotopic fractionations of several per mil. This question was addressed in a series of experiments by Richter et al. (2008; 2009b; 2014a) that quantified the isotope fractionation of all the major elements, plus lithium and potassium, by a sustained temperature difference across a basalt melt.

Schematic illustration of the black squares that show the temperature derived from the spinel thickness at places above and below the molten basalt (shown schematically by the grey rectangle) where MgO and Al2O3 were in contact in the piston cylinder assembly used by Richter et al. Schematic illustration of the panel on the left shows the steady state distribution of the mole fraction of MgO and SiO2 as a function of temperature in glass recovered from two Soret experiments (SRT4 and SRT8) from Richter et al.

      Figure taken from Richter et al. (2008).

Schematic illustration of thermal fractionation of isotopes of the listed elements in molten basalt in terms of the parameter ohm defined as per mil per one atomic mass unit difference of the isotopes per 100°C.

      The question addressed in this section is whether the measurably large kinetic isotopic fractionations that were found in silicate melts also arise during diffusion in igneous minerals. Many of the laboratory experiments and natural examples focused on lithium because of its special attributes of large isotopic mass ratio and extremely

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