4.3 Analytical Method
4.4.7 Fast diffusion mechanism
Both Mg and the REE show unusually fast diffusion mechanism in addition to their expected diffusion mechanism. These fast diffusion profiles are measured at almost an identical rate (with the exception of AGV58) (Figure 82) and is the fastest rate of diffusion in plagioclase.
This rate is not effected by ionic radius, melt composition or plagioclase composition (in the range An58-An68)
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Figure 82: A comparison between the very fast diffusivities of both Mg and the rare earth elements (REE).
4.5 Discussion
Combining the data obtained in this study and published values for diffusion in labradorite, Arrhenius relationships were improved. Comparing the Arrhenius relationships for An~70 plagioclase, K and Sr (Cherniak and Watson, 1994; Giletti and Shanahan, 1997) have roughly parallel slopes as do; Ba, REE and Mg (Cherniak, 2002; Cherniak, 2003; Faak et al., 2013).
Figure 83: A comparison of the Arrhenius relationships of published diffusion data in An~70 plagioclase. (Cherniak, 2002; Cherniak, 2003; Cherniak and Watson, 1994; Faak et al., 2013; Giletti and Shanahan, 1997)
129 The relationship between the diffusivities of Ba and Sr change slightly with anorthite content of the plagioclase, suggesting there may be an effect of ionic radius on the diffusivities of the divalent cations in the M-site that changes with anorthite content. In simpler minerals such as forsterite, the ionic radius of the cations has no great effect on the diffusivity of the element, nor does the charge (Spandler and O’Neill, 2009).
Comparing literature data, the diffusivities of Ba and Sr change with anorthite content, with albite rich plagioclase diffusing significantly faster than anorthite rich plagioclase (Cherniak, 1996, 2002; Cherniak and Watson, 1994; Giletti and Casserly, 1994). Ba is affected by temperature much more than Sr (Figure 83). This suggests at high temperatures, Ba will diffuse faster than Sr, as discovered in An70 + corundum, mullite buffer at 1290 °C.
The data presented here suggests that the diffusion of the divalent cations in the M-site is affected by ionic radius (Figure 76). The smaller divalent cations are found to diffuse faster than the larger divalent cations.
Sr diffusivities in literature and this study are highly variable; attributed to crystal orientation effects (Cherniak, 2010). These experiments were not orientated but no other element shows a significant variation in the diffusion rate other than strontium. Cherniak (2010) notes that the only other element to show anisotropy is lead (Pb). The reason for this is still unknown. In these experiments, only sample AGV58 shows a significantly slower diffusion rate than any of the other experiments, and was only resolvable in three profiles.
The strontium partition coefficients are much higher than equilibrium concentrations in all experiments. All other extremely short diffusion profiles have partition coefficients close to equilibrium values which suggests it is not a problem with defining the placement of the interface.
When examining the melt Sr concentrations by nanoSIMS (Figure 56), the concentration changes gradually over 1-2μm. This may be an artefact of the analytical method or could represent local equilibrium of the melt immediately adjacent to the plagioclase crystal. This would cause the partition coefficients for these elements to be incorrect as the melt in the centre of the well is not in equilibrium with the crystal. Diffusion of major elements in melts is very fast (Guo and Zhang, 2016). Watson (1979) investigated the diffusion of calcium in melt and gives a result of approximately logDCamelt = -6.2 at 1 kbar and 1200 °C and assuming that strontium behaves the same way in a silicate melt, due to similar
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charge and size, it should diffuse equally as fast. The diffusion of major elements is known to be very fast, however the diffusion of trace elements in melts is less well understood. To resolve this, the diffusion of trace elements in silicates melts could be investigates in a similar experimental set up as the Watson (1979) experiments. This would be an interesting study with applications for all mineral diffusion and partitioning. If trace elements diffuse slowly through melt, the “equilibrium melt” that is in contact with the growing face of the crystal might have a different composition than the “bulk melt” and therefore significantly impact partition coefficients.
Magnesium diffusion in plagioclase has previously been investigated by a number of authors (Costa et al., 2003; Faak et al., 2013; Faak et al., 2014; LaTourrette and Wasserburg, 1998; Van Orman et al., 2014).
Costa et al. (2003) show that the diffusion of trace elements, specifically Mg, can be tied to concentration gradients in the major elements. This is particularly relevant in the simple system experiments as some diffusive exchange of CaO was observed with the EPMA. The simple system experiments vary by an anorthite number of <2 between the core and the rim. Even though the results of trace element concentrations are not significantly changed when the internal standard for laser data is selected at Ca43 of Si29, this change in major element concentration may have affected the diffusion profile itself. This assumption would need to be corrected by running additional experiments that are ensured to be in equilibrium and comparing the equilibrium diffusion with these experiments that cause changed in the plagioclase chemistry.
Here it was discovered that Mg partitions preferentially onto the tetrahedral site (CaMgSi3O8) however there may also be some contribution of the M-site (MgAl2Si2O8). For Mg diffusion, there are two competing diffusion mechanisms.
In a few experiments, these two diffusion mechanisms work in direct competition with each other, with the fast mechanism diffusing out and the slow mechanism diffusing in. If the effective partition coefficient of the fast diffusion “out” is calculated, it is found to contribute approximately 80% of the equilibrium partition coefficient. As tetrahedral Mg is known to be the main contributor to Mg and plagioclase, it is assumed this fast diffusion represents tetrahedral Mg (CaMgSi3O8). These fast diffusivities also have a linear relationship with Be diffusivities, a known tetrahedral coordinated divalent cation. The fast diffusion mechanism was found to occur during high silica activity buffers in Faak et
131 al. (2013), however in the silica buffered experiments presented in this study there was no noticeable difference between the two.
Therefore, CaMgSi3O8 diffuses more quickly than MgAl2Si2O8. If this assertion is true, it is the opposite of what would be assumed for these mechanisms. As CaMgSi3O8 requires a change balance exchange, it would assumedly diffuse much slower than MgAl2Si2O8, which does not require a charge balance.
Mg does not have significant diffusional anisotropy (Faak et al., 2013; Van Orman et al., 2014), therefore the variation in diffusivities in the simple system experiments are likely due to the experiment being cut at an angle to the interface. The shorter diffusion profiles are effected less by this change in angle.
The fast diffusion mechanism is only observed within the natural type diffusion experiments. This is because of the extremely low magnitude of the compatibility of this type of diffusion. As the concentration gradient in the natural melts were much higher than the simple systems, this diffusion mechanisms was able to be distinguished from the background values.
The fast diffusion for both Mg and REE have very similar rates, suggesting that this is a common diffusion mechanism. Faak et al. (2013) found that the fast diffusion rates for Mg coincided with SiO2 buffered experiments. We do not see the fast diffusion rates in our simple system buffered experiments, though the diffusion rates are highly variable. Eu2+ is orders of magnitude faster than the trivalent REE. If Eu2+ diffusion occurs in plagioclase crystals, the europium anomalies would be exaggerated. With its very high compatibility, it could be used as an indicator of oxygen fugacity changes in the system. Mn and Fe are assumedly diffusing as divalent cations. These cations diffuse at a rate similar to each other and do not change significantly within the tested temperature range.