Isotopic Constraints on Earth System Processes. Группа авторов

Figure 1.4 Figure taken from Richter et al. (2003) showing the δ⁴⁴Ca of slabs cut perpendicular to the long axis of glass recovered from rhyolite‐basalt diffusion couples RB‐2 and RB‐3. A normalized distance scale with units x/t ^1/2 , where x is in microns and t is the run duration in hours is used in order that the data from the two experiments of different size and duration can be plotted along the same normalized distance axis. The thin horizontal line shows the initial ⁴⁴C/⁴⁰Ca of the rhyolite and basalt relative that of a CaCO₃ salt standard. The thicker black line is the result of a model calculation with the mass‐dependence of the diffusion coefficient of calcium calculated as with β = 0.075.

      1.3.2. Isotope Fractionation between Melts from a Natural Setting

The relevance to geology of the kinetic isotope fractionations by diffusion in melts as documented by the laboratory experiments depends on whether similar fractionations can be found in natural settings. Chopra et al. (2012) addressed this in a detailed study, in which the magnesium isotopic fractionations in diffusion couples juxtaposing felsic and mafic powdered rock samples from the Vinalhaven intrusive complex in Maine were compared with the isotopic fractionations they measured across an exposed contact between these two rock types. Fig. 1.5 shows the piston cylinder assembly used by Chopra et al. (2012) for the experiments along with a backscattered electron image of glass recovered from their experiment GBM‐2. The figure also shows the temperatures measured in the piston cylinder assembly above and below the sample and a temperature profile extrapolated the measured temperatures into the sample while it was at run conditions. The temperatures in the piston cylinder assembly were determined by the thickness of spinel that grew where Al₂O₃ and MgO were in contact at various places in the assembly based on a calibration of the growth rate of spinel as a function of temperature and pressure by Watson et al. (2002). The calibration is used to convert the spinel thickness into the time‐averaged temperature over the duration of the experiment.

Fig. 1.6 shows the results of experiment GBM‐2 from Chopra et al. (2012) in which melt derived from rock powers made from the mafic and felsic rocks of the Vinalhaven igneous complex were juxtaposed in a piston cylinder assembly, shown schematically in Fig. 1.5. The temperature differences across the experimental sample turned out to be important for distinguishing between that part of the isotopic fractionation due to chemical diffusion versus that due to thermal diffusion. Fig. 1.6 also shows the weight % MgO and magnesium isotopic fractionation measure in rock powders that were drilled at spots across an exposed contact between the felsic and mafic rocks at Vinalhaven. The MgO data on the felsic side of the contact shows quite a bit of scatter due to the powders being mixtures of different proportions of relatively coarse‐grained minerals with different MgO content. The isotopic fractionation data are less scattered reflecting negligible isotopic fractionation between the minerals at magmatic temperatures. The weight percent MgO and the isotopic fractionation across the glass recovered from the piston cylinder experiment GBM‐2 was fit with model profiles from a diffusion calculation done in much the same way as for calcium in Section 1.3.1, with ( is the weight percent SiO₂) and with β _Mg = 0.040. The isotopic fractionation of the sample from this experiment had values at each end that were more positive than that of the powders used to make the sample. Such positive isotope fractionations close to the end of the sample are similar to the increasingly positive calcium isotopic fractionation in the basalt side of the glass recovered from experiment RB‐2 (see Fig. 1.4). In the case of GBM‐2, the temperature difference across the sample had been measured and this was used to take into account the thermal isotope fractionation of magnesium due to temperature differences of 10°C and 25°C between the center of the sample and the felsic and mafic ends, respectively. The thermal isotopic fractionation of magnesium was calculated using parameters derived from Soret experiments by Richter et al. (2008), and discussed in the next section. Fig. 1.6 shows model calculated isotope fractionation profiles with and without taking into account the thermal isotope fractionation, and in both cases the data are still fit with β _Mg = 0.040 for that part of the fractionation associated with chemical diffusion. A second diffusion couple, GBM‐1, also had

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