Isotopic Constraints on Earth System Processes. Группа авторов

lithium source in an evacuated silica ampule together with an oxygen fugacity buffer (Ni‐NiO, Fe‐FeO, or FeO‐Fe₂O₃) and then placed in a furnace at 900°C for a prescribed amount of time. Richter et al., 2014b

The data shown in Fig. 1.11 were so unexpected that Richter et al. (2014b) ran additional experiments with different durations, different orientations relative to the cleavage of the augite, and different fO₂. Most, but not all, of these experiments had results like those in Fig. 1.11. However, in some cases, as shown in Fig. 1.12, the lithium concentration profile and associated isotopic fractionation was much more like what one might have expected based on many previous the diffusion experiments in liquids and other solids. The main difference between results of experiment LiPx8 shown in Fig. 1.11 and those from LiPx22 shown in Fig. 1.12 was the oxygen fugacity, which was buffered at 10^–12 bars in experiment LiPx8 versus a value 10^–15 bars in experiment LiPx22. The model calculations that produced step‐like profiles required a larger concentration of vacancies compared to the lithium concentration at the boundary, while the calculations the resulted in smooth profiles like in Fig. 1.12 resulted when the initial concentration of vacancies was the same as or less than the lithium concentration at the grain edge. An important result is that regardless of the shape of the lithium concentration profile, the isotopic data can be fit using the same value of β _Li = 0.27.

Figure 1.11 The panel on the left uses black circles to show the lithium concentration (normalized to a value of one in the interior) measured along a traverse across a 700 μm augite grain from experiment LiPx8 from Richter et al. (2014b). Distance is normalized by the size of the grain. This experiment was annealed for 842 hours at 900°C and buffered by Ni–NiO (log fO₂ ~–12). The lithium concentration was measured along one line while the isotopic fractionation shown in the panel on the right was measure along two parallel lines. Comparing the isotopic data from the two lines provides a good measure of the uncertainty of the isotopic measurements. The lithium isotopic fractionation is reported as using the average of the ratio measured in the interior of the grain as the standard. The solid lines are the results of a model calculation based on lithium occupying two types of sites in the augite and β_Li = 0.27.

      Figure from Richter et al. (2014b).

Figure 1.12 Lithium concentration and isotopic fractionation normalized by the average values in the interior portion of the grain that had not been affected by diffusion. These data are from experiment LiPx20 from Richter et al. (2014b) in which lithium diffused into the grain for 672 hours at 900°C with the oxygen fugacity buffered by wüstite–magnetite (log fO₂= –15). The data shown here were measured across only one boundary layer rather than across the entire grain. The isotopic data are shown with 2σ error bars derived from 20 or more repeated analyses at each spot. The model profile that fits the lithium concentration data used a two‐lithium species model with the initial vacancy concentration in the interior equal to the lithium concentration imposed as a boundary condition at the grain edge. The modeled profile of the isotopic fractionation associated with the diffusion of lithium into the grain shown by the black line was calculated assuming β_Li = 0.27.

      1.5.2. Natural Examples of Lithium Zoning and Isotopic Fractionation by Diffusion in Pyroxenes

      Several well‐documented instances of large lithium isotopic fractionations in natural pyroxenes from terrestrial rocks (Jeffcoate et al., 2007; Parkinson et al., 2007) and from Martian meteorites (Beck et al., 2006) had been reported prior to the lithium diffusion experiments described. This section gives two examples that use the parameters derived from the experiments to interpret the significance of the isotopic fractionation, or the lack thereof, of zoned pyroxene grains from terrestrial rocks and from a Martian rock.

Figure 1.13 Lithium concentration data normalized by the average value in the interior (left‐hand panels) and isotopic fractionation data relative to the average in the interior that defines δ⁷Li = 0.0‰ (right hand panels with ±2σ of clinopyroxene grain CPX 95.11.14.10 from a Solomon Island lava flow measured by Parkinson et al. (2007). The black lines are from model calculations that assumed that the lithium concentration and isotopic composition was initially uniform. The isotopic profile was calculated using δ⁷Li = 0.0‰ as the boundary condition and with the relative mobility of ⁷Li compared to ⁶Li specified using β_Li = 0.25. Richter et al., 2014b.

Fig. 1.13 shows the lithium concentration and isotopic fractionation measured by Parkinson et al. (2007) across a clinopyroxene grain from a lava flow in the Solomon Islands. The data are very much like those from the experiment shown in Fig. 1.12 and the model profiles that fit the isotopic fractionation data were calculated using a value of β _Li = 0.25, which is effectively the same as that determined by the Richter et al. (2014b) experiments ( β _Li = 0.27). A number of other examples of lithium‐zoned pyroxene grains with associated isotopic fractionations that quantitatively conform to the expectation derived from the laboratory experiments can be found in recent papers by Richter et al. (2014b; 2016). It’s pretty clear that the lithium zoning of these grains is the result of the diffusion of lithium.

Isotopic measurements across zoned mineral grains are important regardless of whether significant isotopic fractionations are found. A good illustration of this is a lithium‐zoned pyroxene grain from the Martian meteorite Shergotty, where the important point is precisely that there is little if any significant isotopic fractionation despite it being strongly zoned in lithium abundance. Fig. 1.14 shows the lithium concentration and isotopic fractionation data that was measured on a transect across the pyroxene grain from Shergotty with the CAMECA 1270 ion microprobe at the Centre de Recherches Pétrographiques et Géochimiques in Nancy, France. The lithium concentration data can be reasonably fit