Plagioclase melting is similar to olivine melting. Figure 5.05 shows the plagioclase melting diagram determined by Bowen (1913). The diagram is separated into three regions by two saturation curves. The lower (solidus) curve shows the temperatures at which plagioclase begins to melt on heating as a function of plagioclase composition. The upper (liquidus) curve shows the temperatures at which plagioclase begins to crystallze on cooling as a function of the magma composition. The region between the two curves indicates temperatures and compositions for which there are two phases in equilibrium, plagioclase and liquid. The proportions of the two phases can be determined with the lever rule. Click on the diagram to see a larger version.
Solid plagioclase has the same challenges as olivine does in needing to change its chemical composition during melting or crystallization. However, because changing the plagioclase composition requires the chemical exchange NaSi = CaAl, diffusion is much slower. Therefore, zoned plagioclase crystals with CaAl-richer cores and NaSi-richer rims are very common. As with olivine, the retardation and extinction angle of plagioclase vary with chemical composition, so the evidence for zoning of plagioclase crystals can be visible in thin section under crossed polarized light. Figure 5.00 at the front of this chapter shows an example of this concentric zoning.
Press "Enter" after you type in the number.
Yes. The solidus temperature for a NaAlSi3O8-CaAl2Si2O8 plagioclase crystal that is 60 wt% CaAl2Si2O8, is about 1327°C.
No. Use the "Coordinates" button on Figure 5.05 to find the temperature where the 60 wt% CaAl2Si2O8 vertical line crosses the solidus and try again.
Yes. The solidus temperature for a NaAlSi3O8-CaAl2Si2O8 plagioclase crystal that is 60 wt% CaAl2Si2O8, is about 1327°C.
No. The solidus temperature for a NaAlSi3O8-CaAl2Si2O8 plagioclase crystal that is 60 wt% CaAl2Si2O8, is about 1327°C. You should be able to answer this correctly. Reload the page and use the "Coordinates" and "Show Phase %" buttons on Figure 5.05 to make sure you can find the solidus temperature.
Yes. At equilibrium, there will be 87 wt% liquid and 13 wt% plagioclase. No. At equilibrium, at 1400°C, there will be 87 wt% liquid and 13 wt% plagioclase. You can determine this with the lever rule. You can see this result on Figure 5.05 if you use the "Coordinates" and the "Show Phase %" buttons.