5.4 Olivine Melting Diagram

Olivine is a common mineral with a chemical composition that ranges from pure Mg-olivine (forsterite) to pure Fe-olivine (fayalite). Other elements, notably Mn, can be found in olivine, but olivine compositions are largely a two-component mixture of forsterite and fayalite. A phase diagram based on the melting of olivine looks a lot like the water-ethanol boiling diagram. Figure 5.03 shows the olivine melting diagram determined by Bowen and Schairer (1935). The diagram is separated into three regions by two saturation curves. The lower (melting) curve shows the temperatures at which olivine will begin to melt on heating as a function of olivine composition. This lower curve is called a solidus because no liquids occur at equlibrium below this curve. The upper (liquidus) curve shows the temperatures at which olivine begins to crystallize 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, olivine and liquid. The proportions of the two phases can be determined with the lever rule. Click on the diagram to see a larger version.

If you follow the melting of olivine using the "Show Phase %" button on the olivine melting diagram, you will see that the liquid that forms is richer in iron than the crystal that is melting. This means that the olivine crystal that is melting must change its chemical composition to become more magnesium rich. This is more difficult to do for solid olivine than it is for a water-ethanol solution to change its composition during boiling. The olivine might change by regrowing as a new, more forsteritic olivine. Or it might change through diffusional exchange of Mg and Fe. Time and high temperatures both facilitate the changes needed to maintain crystal-liquid equilibrium.

The same problem exists when a magma of an olivine composition is cooled. The first olivine to grow is richer in magnesium than the magma, making the magma richer in iron. However, as the magma crystallizes olivine, the olivine that grows should become more fayalitic. Diffusion is relatively fast in olivine at the high temperatures of olivine growth, so many olivines in igneous rocks have nearly homogeneous compositions. But in some cases, olivine phenocrysts have iron-richer rims as can be seen in Figure 5.04, a photomicrograph of an olivine crystal in a basalt from a Mauna Loa volcano lava flow, Hawaii. The second-order blue retardation of the rim on the first-order red olivine crystal in crossed polarized light is due to the increased iron content of the rim.

If an Mg₂SiO₄-Fe₂SiO₄ liquid that is 40 wt% Fe₂SiO₄ is cooled, what will be temperature when the liquidus is reached and crystallization of olivine can begin? Enter the temperature (number only) in °C.

You may use Figure 5.03 and its "Coordinates" and "Show Phase %" buttons to help you find the answer this question. Press "Enter" after you type in the number.


Plagioclase Melting