Zoning

The zoning is a compositional variations found in the crystals. Minerals belonging to a continuous solid solution such as: plagioclase, olivine, clinopyroxene, amphibole, typically develop a compositional zoning during growth, manifested as changes in physical properties (optical) of crystals.

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Figure 1:summary diagram of the various types of Zoning



Compositionally the zoning is divided into normal and reverse, while according to the variation of composition within the crystal can be divided into concentric, which can be continuous, intermittent or oscillating;
The shape of the various concentric zones may follow the shape of the crystal, and thus be generally euhedral. If it shows more or less regular patterns with embayment, in which case this is convoluted.

Sometimes the compositional zoning, rather than concentric, may be located in areas of generally triangular shape, and when the areas are slightly rounded triangular, the zoning is called "hourglass zoning" or sector Zoning.

Normal Zoning
Normal zoning is a form of chemical zoning in which a crystals composition varies continuously from a core representing a high temperature composition of the mineral to a rim with a lower temperature composition of the mineral. Zoning forms in minerals that form a solid solution series in which mineral composition changes with temperature (and pressure). Crystals exhibiting normal zoning are said to be normally zoned.

Reverse zoning
Reverse zoning is a form of chemical zoning in which a crystals composition varies continuously from a core representing a low temperature composition of the mineral to a rim with a higher temperature composition of the mineral. Zoning forms in minerals that form a solid solution series in which mineral composition changes with temperature (and pressure). Crystals exhibiting reverse zoning are said to be reverse zoned. Reverse zoning can occur for a number of reasons such as an increase in magma temperature, or increases in pressure.

Discontinuous zoning
Discontinuous zoning is a form of chemical zoning in which chemical zones show abrupt changes in composition. Crystals containing a homogeneous core with a zoned rim exhibit discontinuous zoning. Discontinuous zoning arises through abrupt changes in temperature and pressure, or periods of arrested crystal growth.

Convolute zoning
Convolute zoning is a form of chemical zoning in which chemical zones are irregular in width and define re-entrant outlines. Convolute zoning often indicates resorption of a crystal by the melt or crystallisation of melt inclusions.

Sector zoning
Sector zoning is a form of chemical zoning in which the chemical zones form four triangular shaped sectors across the crystal. In three dimensions the zones have pyramidal shapes with apexes at the centre of the crystal. Sector zoning is easily mistaken for sector twinning, however, in sector twins the crystals have different optical orientation. Often crystals with sector zoning also have normal or oscillatory zoning. Sector zoning probably arises due to the different surface energies of crystal faces.

Oscillatory zoning
Oscillatory zoning is a form of chemical zoning in which the chemical zones oscillate in a regular fashion through a crystal. Chemical zones repeat at regular distance in oscillatory zoning, although there may be an overall normal or reverse sense to the change in composition. Chemical zoning exhibiting irregularly spaced repetitions is sometimes distinguished with the term multiple zoning. Oscillatory zoning is usually occurs due to growth of a crystal within convection currents in a magma chamber such that circulation causes quasi-regular changes in temperature and pressure. Oscillatory zoning is particularly common in plagioclase phenocrysts within dacites and andesites.

Development of Zoning

To better understand consider the plagioclase zoning, in which the zoning is particularly common and conspicuous. It is common because equilibration of plagioclase crystals with a melt would involve the difficult interchange of Si and Al; it is conspicuous because plagioclase is triclinic, and optical directions rapidly change position relative to crystal axes with changes of composition.

consider the NaAlSi3O8 (albite or Ab) – CaAl2Si2O8 (anorthite or An) plagioclase system shown in Figure 2:

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Figure 2:plagioclase system (Ab-An) at 1bar (modified y Bowen,1913)



The phase relations in the plagioclase system are shown in Figure 2 at constant pressure (1 bar). the upper curve is called the liquidus and the lower curve is called the solidus. At temperatures above the liquidus everything is liquid, below the solidus everything is solid (crystals of plagioclase solid solution). At temperatures between the solidus and liquidus crystals of plagioclase solid solution coexist in equilibrium with liquid.

The crystallization history of composition "start", which is An40 - Ab60. Composition "start", is completely liquid above the liquidus (above 1410°C). Cooling to the liquidus at point A results in the crystallization of a small amount of plagioclase solid solution. The composition of this plagioclase can be found by drawing an isotherm (line of constant temperature, a horizontal line in this diagram) through the temperature 1410°C (Line A-B). Where this isotherm intersects the solidus (at point B), the composition of the solid can be found by drawing a vertical line to the base of the diagram.

Thus it is seen that the first crystals precipitated from composition "start", will have the composition Ab20An80. Note that in this diagram crystals that are in equilibrium with liquid will always be enriched in anorthite component relative to the liquid.
As crystallization continues with lowering of temperature the composition of the plagioclase will change along the solidus, continually reacting with the liquid to produce crystals more enriched in the Ab component. Meanwhile, the composition of the liquid will change along the liquidus, thus also becoming more enriched in the Ab component. At a temperature of 1395°C the liquid composition will be at point A1, while the solid composition will be at point B1.

Crystallization proceeds until a temperature of about 1220°C, at which point the last remaining liquid will have a composition at C, and the solid will have a composition D.

During crystallization the proportion of the solid continually increases while that of the liquid continually decreases. Thus as the composition of the liquid becomes more sodic, approaching D, its volume steadily decreases.
If at any point during the crystallization we wish to determine the amount of solid and liquid, we can apply the lever rule. As an example, we will determine the proportions of liquid and solid in the system at a temperature of 1395°C. At this point, we measure the distances A1- B1. The percentages of liquid and solid are then given as follows:

% solid (with composition B1) = [x/(x + y)] x 100
% liquid (with composition A1) = [y/(x + y)] x 100

We will distinguish between three contrasting conditions:

• Equilibrium crystallization, the crystals remain suspended in the melt, and cooling and crystallization are slow enough to allow continuous, complete reaction between crystals and melt. The early formed crystals will, on cooling, react with the melt continuously and thereby gradually change their composition along the solidus from B to D, while simultaneously the liquid changes from A to C. In such circumstances the crystals will not change composition beyond E, and the end product is a homogeneous mixed crystal (solid solution) having the same composition as the initial melt.

• • Assume that the crystals are continuously removed from the melt, by sinking or some natural filtering process. Reaction of crystals with the melt is prevented, and the composition of the liquid will continue to change along the liquidus curve toward the sodic feldspar component. The only limit to this change of composition of the liquid is the composition of the pure Na feldspar, but the relative amount of very sodic liquid would be very small. As the liquid phase changed composition with continuing removal of crystals, the successively formed crystals would become continuously more sodic; the final product would be pure albite, but it would constitute a very small proportion of the initial amount.

• • • If the crystals remain suspended in the liquid, but relatively rapid crystallization does not allow complete reaction between crystals and liquid, the effect will be somewhat different. In effect, failure to react completely partially removes the already formed crystals from the system. The melt becomes increasingly more sodic, and earlier formed more calcic crystals serve as nuclei on which increasingly more sodic feldspar crystallizes. The resulting crystal contain zones of differing composition; the inner zones being more calcic, and the outer zones more sodic. The bulk (average) composition of the zoned crystal is that of the initial system.

Oscillatory zoning: a particular case

The compositional changes across a plagioclase crystal are seldom perfectly regular, and in igneous rocks are frequently oscillatory in fashion, perhaps involving many tens of reversals or abrupt changes in An %.

The two main features that characterize oscillatory zoning are:

1): Abrupt changes which usually mark a sudden outwards increase in An % and which may feature corrosion of the underlying crystal surface; gradual changes of An % (either reverse or normal) in the plagioclase deposited between successive sharp breaks. The An % change at sharp boundaries varies from small to large; the gradual changes are always small (less than 10%).

2): Abrupt compositional changes require abrupt changes in the dynamic conditions of crystallization, and most re researchers now invocate magma mixing. Thus Nixon and Pearce (1987) show that repeated injection of fresh , hot, basic magma into a chamber of already differentiated and cooled magma repeatedly caused resorption n of already crystallized plagioclase.

The more gradual changes in oscillatory zoning are best interpreted in terms or local effects of disequilibrium crystallization. In this context it is important to understand that liquidus- solidus curves for pure anhydrous plagioclase (Figure 3) are depressed by the presence of other components. Water alone can depress the curves by several hundreds of degrees, and the shape of the curves changes too.

Consider then the local environment of a crystal during magma cooling. As growth proceeds, the components not required by the plagioclase crystal increase in concentration immediately adjacent to the crystal. These components will include H20. The solidus and liquidus curves for the immediate environs of the crystal become depressed (Figure 3), and at a fixed temperature (TI) the crystallizing plagioclase composition shifts from A to B. lf, on the other hand, undercooling increases (TI to T2), perhaps as the result of an increased cooling rate, composition moves from A co C. Reverse zoning forms if undercooling decreases or residual components adjacent to the crystal face increase; normal zoning forms if undercooling increases or the residual components decrease.

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figure 3: Ab-An Diagram in the presence of H2O





Bibliography



• E. W. M. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc
• David Shelley (1983): Igneous and metamorphic rocks under the microscope. Campman & Hall editori.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press
• Shelley D (1992): Igneous and Metamorphic Rocks under the Microscope: Classification, textures, microstructures and mineral preferred orientation
• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• M. J. Hibbard (1994): Petrography to Petrogenesis. Prentice Hall editore

Photo
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Zoned plagioclase crystal in a rhyolite. XPL image, 10x (Field of view = 2mm)
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Zoned plagioclase crystal in a rhyolite. XPL image, 2x (Field of view = 7mm)
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Zoned plagioclase crystal in a rhyolite. XPL image, 10x (Field of view = 2mm)
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Zoned plagioclase crystal in a rhyolite. XPL image, 20x (Field of view = 1mm)
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Zoned plagioclase crystal in a rhyolite. XPL image, 20x (Field of view = 1mm)
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Zoned plagioclase crystal in a rhyolite. XPL image, 2x (Field of view = 7mm)
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Zoned plagioclase crystal in a rhyolite. XPL image, 2x (Field of view = 7mm)
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Zoned plagioclase crystal in a rhyolite. XPL image, 2x (Field of view = 7mm)
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Zoned plagioclase crystal in a rhyolite. XPL image, 2x (Field of view = 7mm)
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Zoned hornblende crystals in a andesite. PPL image, 2x (Field of view = 7mm)
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Zoned hornblende crystal in a andesite. PPL image, 10x (Field of view = 2mm)
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Zoned hornblende crystal in a andesite. PPL image, 10x (Field of view = 2mm)
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Zoned green-hornblende crystal in a rhyolite. PPL image, 2x (Field of view = 7mm)
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Zoned green-hornblende crystal in a rhyolite. PPL image, 10x (Field of view = 2mm)
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Zoned green-hornblende crystal in a rhyolite. PPL image, 10x (Field of view = 2mm)
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Zoned green-hornblende crystal in a rhyolite. PPL image, 10x (Field of view = 2mm)
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Zoned olivine crystal in a basalt. XPL image, 2x (Field of view = 7mm)
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Zoned melanite (garnet) crystal in a Tephrite from Vesuvius, Italy. PPL image, 10x (Field of view = 2mm)
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Zoned melanite (garnet) crystal in a Tephrite from Vesuvius, Italy. PPL image, 10x (Field of view = 2mm)