Practice Problems Problem W4.1. Plutonic Equivalents of Surface Igneous Rocks. Igneous rocks that form when lava cools on the surface correspond to equivalent rocks that form when magma cools underground. WQ4.1. What is the plutonic equivalent of rhyolite? WQ4.2. What is the plutonic equivalent of basalt? Problem W4.2. Identifying Plutonic Rocks. Imagine examining a plutonic rock that contains visible quartz, potassium feldspar, and plagioclase. WQ4.3. What is this rock, most likely? Problem W4.3. How Composition Varies with Temperature. This problem requires some explanation, probably recapping what you already have read or heard. Magmas crystallize (solidify), not at a single temperature, but over a range of temperatures. This is different from your daily experience with the crystallization (freezing) of water, which occurs at a single temperature, not over a range. At normal pressure, water crystallizes to ice at its single crystallization temperature of 0ºC (32ºF). Why should a liquid like magma behave differently, with a range of crystallization temperatures? The short answer is that water is a simple substance, with the simple formula H2O, whereas magmas are very complex substances with complex chemical formulas. Let us amplify this. An ice crystal grows from a tiny nucleus of frozen water, adding more water particles as it grows. Thus, an ice crystal is essentially pure frozen water, all the way through. From the earliest-formed center of the crystal to its last-formed outer rim, the ice crystal has a uniform composition, entirely of H2O molecules (Figure WS4.1A). Water-freezing-into-ice is a single-component system because only one type of molecule, H2O, is involved in the crystallization. We can show this by drawing a simple phase diagram (Figure WS4.1B). It shows water changing from its liquid phase to its solid phase at 0ºC. Figure WS4.1 (A) Ice crystal. (B) Water phase diagram at 1 atmosphere of pressure. The phase diagram shows water changing from its liquid phase to its solid phase at 0°C (32°F). Magmas are far more complex. In the melt are numerous elements, such as sodium, calcium, potassium, aluminum, silicon, oxygen, carbon, and so on. As a result, several different minerals crystallize from the melt as it cools. One of these, plagioclase feldspar, actually varies in composition within a single crystal. We often find plagioclase crystals that contain zones of different compositions (Figure WS4.2A). Consequently, the phase diagram for plagioclase is more complex than the phase diagram for ice (Figure WS4.2B). Plagioclase is a two-component system, which grades from a sodium-rich formula to a calcium-rich formula (see bottom of Figure WS4.2B). It is this complexity of composition that allows plagioclase to crystallize over a range of temperatures, as shown in the figure. (This is true even at any single pressure.) In the figure, the curve represents the composition of plagioclase crystals as they form at different temperatures. Looking deeper, consider a magma having a composition indicated by the sample at temperature point T1. As this magma cools to temperature T1, a calcium-rich plagioclase crystal will grow in the melt. This first little bit of crystal will be 80% calcium and 20% sodium. But as the magma cools further, a more sodium-rich plagioclase will solidify from the melt. At temperature point T2, plagioclase crystals then forming will be half calcium and half sodium. Even later-forming (cooler) sodium-rich plagioclase will grow around and encase the earlier-formed calcium-rich plagioclase crystals. Eventually, when the magma has cooled to temperature T3, pure sodium plagioclase will crystallize, forming the last layer of zoned plagioclase crystals, like that shown in Figure WS4.2A. This phenomenon is so dependable that the composition of plagioclase can be read directly from temperature in the phase diagram. For example, a crystal forming at 1200°C (2,150°F) will be 75% sodium and 25% calcium. And this is just one example. Plutonic rocks are made up of many different minerals, not just plagioclase. Each mineral and its range of composition contributes to the overall temperature range over which a magma crystallizes. The net result is a large variation in mineral content among plutonic igneous rocks. Figure WS4.2 (A) Plagioclase crystal in cross-section. (B) Phase diagram of plagioclase at fixed pressure. The full phase diagram for plagioclase feldspar at a given pressure is shown in Figure WS4.3. The solid curve indicates temperatures at which different compositions of plagioclase crystallize in the melt, and is called the solidus. The dashed line indicates temperatures of different liquid compositions, and is called the liquidus. The liquidus and solidus divide the phase diagram into three areas: all-liquid melt liquid plus some solid crystals of plagioclase all solid plagioclase crystals Let us interpret this diagram, using the sample magma indicated. As our magma cools, it doesn’t initially change composition; it simply lowers in temperature (indicated by the straight line downward). The composition of the liquid is shown by the x-axis: in this case, 30% calcium feldspar. As the magma cools to the temperature of the liquidus (reaches T1), plagioclase crystals begin to form. You can read their composition from the horizontal line drawn to C1. Thus, at T1, there is a liquid having the composition of about 30% calcium feldspar, plus a few individual crystals of plagioclase having the composition 70% calcium feldspar. To keep this simple, assume that once a crystal of plagioclase forms, it will not chemically react with the magma around it. This isolation from the melt means that different layers of crystalline material will surround the initial “seed” of high-calcium plagioclase. This whole process is called fractional crystallization, and it results in highly zoned plagioclase crystals. Figure WS4.3 Full phase diagram for plagioclase feldspar. WQ4.4. In Figure WS4.3, use a ruler to determine the various compositions at T1, T2, T3, and T4. At temperature T1: composition of the melt is ______ % calcium feldspar. composition of the plagioclase just formed is ______ % calcium feldspar. At temperature T2: composition of the melt is ______ % calcium feldspar. composition of the plagioclase just formed is ______ % calcium feldspar. At temperature T3: composition of the liquid material is ______ % calcium feldspar. composition of the plagioclase just formed is ____ % calcium feldspar. At temperature T4: composition of the last drop of liquid and the final rim of the plagioclase crystal is ____ % calcium feldspar.