5.5 Plagioclase Melting Diagram

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.

If a plagioclase crystal that is 60 wt% CaAl₂Si₂O₈ is heated, what will be temperature when the solidus is reached and and the first drop of liquid forms? Enter the temperature (number only) in °C.

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Azetropic Point