Earth Materials. John O'Brien. Читать онлайн. Newlib. NEWLIB.NET

Автор: John O'Brien
Издательство: John Wiley & Sons Limited
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Жанр произведения: География
Год издания: 0
isbn: 9781119512219
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°C) at point A. Once the system moves incrementally below A, it moves into the melt plus solid field. This means that crystallization of the melt begins at point A. To determine the composition of the first crystals, a horizontal line (A–B), called a tie line, is constructed between the liquidus and the solidus. The tie line represents the composition of the two phases (liquid and solid solution) in equilibrium with each other at that temperature. The intersection of the tie line with the liquidus (point A) represents the composition of the liquid (~An50), because the melt has just begun to crystallize. The tie line intersection with the solidus (point B) represents the composition of the first solid solution mineral (~An90) to crystallize from the melt.

      As the system continues to cool, the composition of the melt continues to change incrementally down the liquidus line (e.g., to point C) while the composition of the crystalline solid solution simultaneously changes composition as it moves incrementally down the solidus line (e.g., to point D). This process continues as liquid compositions evolve down the liquidus and solid compositions evolve down the solidus until the latter reaches the vertical system composition line where it intersects the solidus at point F. Any further cooling brings the system into the 100% solid field. The tie line E–F at this temperature indicates that the last drops of liquid in the system have the composition ~An10, whereas the final solid crystals will be the same as the system composition (→An50).

      The precise proportion of melt and solid at any temperature can be determined by the lever rule. The lever rule states that the proportion of the tie line on the solidus side of the system composition represents the proportion of liquid in the system, whereas the proportion of the tie line on the liquidus side of the system composition represents the proportion of crystals in the system. In Figure 3.7, the proportion of tie line A–B on the solidus side of the system composition line is ~100% and the proportion on the liquidus side of the system composition line is ~0%. This makes sense because crystallization has just begun. So tie line A–B indicates that, just as crystallization begins, ~0% solids of composition An90 coexist with ~100% liquid of composition An50,. As the system cools (1) the percentage of crystals increases at the expense of the melt; (2) crystal composition evolves down the solidus; and (3) liquid composition evolves down the liquidus during continuous melt–crystal reaction and additional crystallization.

      We can check this by drawing tie lines between the liquidus and the solidus for any temperature in which melt coexists with solids. Tie line C–D provides an example. In horizontal (An) units, this tie line is ~45 units long (An86 – An41 = 45). The proportion of the tie line on the liquidus side of the system composition (x) that represents the percentage of crystals is 20% (9/45). The proportion of the tie line on the solidus side (y) that represents the percentage of liquid is 80% (36/45). The system is 20% crystals of composition An86 and 80% liquid of composition An41. As the system cooled from temperature A–B to temperature C–D, existing crystals reacted continuously with the melt and new crystals continued to separate from the melt. Therefore, the percentage of crystals progressively increased as crystal composition evolved incrementally down the solidus line and melt composition evolved incrementally down the liquidus line. When the system has cooled to the solidus temperature (1225 °C), the proportion of the tie line (E–F) on the liquidus side approaches 100% indicating that the system is approaching 100% solid and the proportion on the solidus side approaches 0%, implying that the last drop of liquid of composition An10 is reacting with the remaining solids to convert them into An50. We can use the albite–anorthite phase diagram to trace the progressive crystallization of any composition in this system. The lever rule can be used for compositions and temperatures other than those specifically discussed in this example.

      The crystallization behavior of plagioclase in which An‐rich varieties crystallize at high temperatures and react continuously with the remaining melt to form progressively lower temperature Ab‐rich varieties forms the basis for understanding the meaning of the continuous reaction series of Bowen's reaction series, as discussed in Chapter 8. Phase stability diagrams summarize what happens when equilibrium conditions are obtained. In the real world, disequilibrium conditions are common so that incomplete reactions between crystals and magmas occur. These are discussed in the section of Chapter 8 that deals with fractional crystallization.

      Why are phase diagrams important in understanding igneous processes? Several important concepts concerning melting in igneous systems are illustrated in the plagioclase phase diagram.

      1 All partial melts are enriched in low temperature components, in this case albite, relative to the composition of the original rock.

      2 The smaller the amount of partial melting that occurs in a system, the more enriched are the melts in low temperature constituents such as albite.

      3 Progressively larger percentages of partial melting progressively dilute the proportion of low temperature constituents.

      4 If melts separate from the remaining solids, the solids are enriched in high temperature, refractory constituents.

      During crystallization, the liquidus indicates the temperature at which a system of a given composition (An content) begins to crystallize; and the stable composition of any liquid in contact with crystals in the melt plus solid field. During crystallization, the solidus represents the stable composition of any solid crystals that are in contact with liquid in the melt plus solid field as crystallization continues and the temperature of final crystallization for a system of given composition.