Earth Materials. John O'Brien

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СКАЧАТЬ any temperature in the anorthite plus liquid field. When the system cools to approach the eutectic temperature (tie line E–I), it contains a proportion of anorthite crystals in equilibrium with a liquid of composition ~An₄₂. At the eutectic temperature, both diopside and anorthite simultaneously crystallize isothermally in eutectic proportions (58% diopside, 42% anorthite) until no melt remains.

      Several important concepts emerge from studies of the equilibrium crystallization of two‐component eutectic systems such as diopside–anorthite:

      1 Which minerals crystallize first from magma depends on the specifics of melt composition

      2 Separation of crystals from the melt generally causes melt composition to change

      3 Multiple minerals can crystallize simultaneously from a magma.

      This means that no standard reaction series, such as Bowen's reaction series (Chapter 8), can be applicable to all magma compositions because the sequence in which minerals crystallize or whether they crystallize at all is strongly dependent on magma composition, as well as on other variables. It also means that the separation of crystals from liquid during magma crystallization generally causes magma compositions to change or evolve through time. These topics are discussed in more detail in Chapter 8, which deals with the origin, crystallization and evolution of magmas.

      Phase diagrams can also provide simple models for rock melting and magma generation. To do this, we choose a composition to investigate starting at subsolidus temperatures low enough to ensure that the system is 100% solid, and then gradually raise the temperature until the system reaches the solidus line where partial melting begins. As temperature continues to rise, we can trace the changes in the composition and proportions of melts and solids, using the lever rule, until the system composition reaches the liquidus, which implies that it is 100% liquid. Let us examine such melting behavior, using the two compositions previously used in the discussion of crystallization. A solid system of composition 20% anorthite (An₂₀) and 80% diopside (Di₈₀) will remain 100% solid until it has been heated to a temperature of 1274 °C where it intersects the solidus. Further increase in temperature causes the system to enter the melt plus diopside field as indicated by tie line E–F. The composition of the initial melt is given by the intersection of the tie line with the liquidus (point E), so that first melts have the eutectic composition (An₄₂), and the composition of the remaining, unmelted solids is indicated by the intersection of the tie line with the solidus (point F = An₀ = Di₁₀₀). As the system is heated incrementally above the eutectic, the tie line (E–F) is 42 An units long and the proportion of the tie line on the liquidus side is ~52% (22/42) indicating that the system contains 52% diopside crystals, and the proportion on the solidus side is ~48% (20/42), indicating that all the anorthite and some of the diopside have melted at the eutectic to produce a liquid of composition An₄₂. At the eutectic temperature, both diopside and anorthite simultaneously melt isothermally until the remaining anorthite is completely melted. The proportion of crystals that melt during eutectic melting (48% of the system) is given by the lever rule and is 42% anorthite crystals and 58% diopside crystals as reflected in the melt composition. Further increases in temperature cause more diopside to melt. This increases the amount of melt and changes the melt composition toward less An‐rich compositions as melt composition. As temperature continues to increase, melt composition evolves up the liquidus toward progressively diopside‐enriched, anorthite‐depleted compositions. When the temperature approaches the liquidus temperature for the bulk composition (An₂₀) of the system, the lever line (A–B) clearly indicates that the system consists of nearly 100% melt (An₂₀) and nearly 0% diopside (An₀) as the last diopside is incorporated into the melt. For the composition An70, the initial also have the eutectic composition (An42).

      Several important concepts emerge from an examination of melting behavior in two‐component systems such as diopside–anorthite:

      1 The composition of first melts in such systems is the same – is invariant – for a wide range of system compositions

      2 Melt compositions depend on the proportion of melting so that increasing degrees of partial melting cause liquid compositions to change

      3 Changes in liquid composition depend on the composition of the crystals being incorporated into the melt.

Invariant melting helps to explain why some magma compositions (e.g., basaltic magmas) are more common than others, because some magma compositions can be generated by partial melting of a wide variety of available source rock compositions. The dependence of melt composition on the degree of partial melting suggests that it might be an important influence on ultimate melt composition. The ways in which magma composition depends on the incorporation of constituents from crystals in contact with the melt is also discussed in Chapter 8 in conjunction with a discussion of magma origin and evolution.

      3.2.5 Two‐component phase diagram: albite–orthoclase

Mineral compositions may offer vital clues to the conditions under which they were produced. This is well illustrated by the temperature‐dependent substitution of potassium (K⁺¹) and sodium (Na⁺¹) in the alkali feldspars (Na,K)AlSi₃O₈, as illustrated by the albite–orthoclase phase diagram (Figure 3.9). Because albite‐rich plagioclases and potassium feldspars are abundant in felsic/acidic igneous rocks, a significant component of continental crust, this diagram is useful in understanding their formation.

      At high temperatures (>~620 °C at 1 atm pressures) a complete substitution solid solution series exists between the two end members. These are the potassium feldspar orthoclase (KAlSi₃O₈) and the sodium plagioclase feldspar, albite (NaAlSi₃O₈). Feldspar crystals that form at high temperatures can have any proportions of orthoclase (Or) or albite (Ab) end member. Actual proportions depend largely on the composition of the system; that is, the availability of potassium and sodium ions. Because a complete solid solution exists between the two end members, crystallization and melting in this system share many similarities with the albite–anorthite system (see Figure 3.7) discussed earlier. For systems with <40% Or, initial crystals are rich in the albite plagioclase component. As plagioclase crystals continue to separate on cooling, they react continuously with the melt so that crystal composition changes down the solidus as the remaining liquid changes composition down the liquidus, both toward increasing Or content until no melt remains.

Figure 3.9 Albite–orthoclase phase diagram at atmospheric pressure.

      The result is a solid composed of sodic plagioclase, with a potassic orthoclase component in solid solution. For systems with >40% Or, initial crystals are relatively enriched in the potassium feldspar (orthoclase) component. As such crystals continue to separate on cooling, they react continuously with the melt so that crystal composition changes down the solidus as the remaining liquid changes composition down the liquidus, both toward decreasing Or content until all the melt is used up. The result is a rock composed of a feldspar solid solution. For systems with >40% Or, these crystals may be thought of as potassic orthoclase crystals with an albite component in solid solution. All solid solutions between the two end members are stable at high temperatures (and low pressures) after they СКАЧАТЬ