Читать книгу Isotopic Constraints on Earth System Processes - Группа авторов - Страница 28

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 well‐resolved magnesium isotopic fractionations that were fit with a value of β _Mg = 0.045. The weight percent MgO and the isotopic fractionation across the natural felsic‐mafic contact was fit using the same effective diffusion coefficient that fit the experimental data and β _Mg = 0.045, which is effectively the same as that derived from the laboratory experiments.

Figure 1.5 The panel on the left shows a schematic of the piston cylinder assembly used to anneal juxtaposed melts of mafic and felsic powders made from rocks from the Vinalhaven igneous complex in Maine. The middle panel shows a backscattered electron image of glass recovered after quenching experiment GBM‐2. The panel on the right shows the temperature profile at run conditions extrapolated into the sample using temperatures (black diamonds) measured above and below the sample by the thickness of the spinel that developed in places where MgO and Al₂O₃ were in contact (see Watson et al., 2002, for a discussion of the spinel thickness temperature monitoring method).

The fact that the magnesium isotopic fractionation across the contact shown in Fig. 1.6 reaches about 3‰, whereas in experiment GBM‐2 the fractionation is somewhat less, does not contradict that the laboratory experiment is a very good analog of the natural system. The reason for the different degree of isotopic fractionation is that the magnitude of the isotopic fractionation depends on both the value of β and the concentration difference driving the chemical diffusion (see Fig. 9 in Richter et al., 2003). The smaller isotopic fractionation in experiment GBM‐2 compared to the natural case is due to the magnesium concentration difference in the laboratory experiment being less than that between the mafic and felsic rocks across the contact in the natural system. The important point is that the isotopic fractionation across a contact between felsic and mafic rocks from Vinalhaven and that found in the laboratory experiment can be explained by effectively the same value of β . This implies that the fractionation in the natural setting was dominantly due to the same simple diffusion process as in the laboratory experiment. Once it had been shown that that the magnesium zoning across the contact between the felsic‐mafic contact at Vinalhaven was due to diffusion, Chopra et al. (2012) went on to estimate a time scale of several hundred days during which diffusion took place with a cooling rate of about 1°C/day for the rocks now exposed at the surface.

Figure 1.6 The upper panel on the left shows the weight % MgO measured along the long axis of the glass recovered from experiment GMB‐2 together with a model curve calculated using an effective binary diffusion coefficient for magnesium that depends on the evolving SiO2 content of the melt. The lower panel on the left uses open circles with two sigma error bars to show the δ²⁶Mg‰ of thin slabs cut perpendicular to the long axis of the glass recovered from experiment GBM‐2. The two black diamonds in this panel show the δ²⁶Mg‰ of the mafic and felsic powders used to make the diffusion couple. The dashed curve is from a chemical diffusion calculation with δMg = 0.040 while the continuous black curve is from a calculation with δ = 0.040 but which also took into account the thermal isotope fractionation associated with the temperature profile shown in Fig. 1.5. The panels on right the show the weight percent MgO and the magnesium isotopic fractionation across an exposed contact between felsic and mafic rock in the Vinalhaven igneous complex in Maine, USA. The model curves in the panels on the right were calculated in the same way as those on the left except that the isotopic fractionation was fit using δMg = 0.045. The δ²⁶Mg‰ is reported relative to the magnesium isotope standard DSM3 of Galy et al. (2003).