In the sedimentary record limestone’s are sometimes very pure and because calcite itself is stable under most crustal conditions these rocks may not develop new minerals during metamorphism. However, many limestone’s contain other constituents such as detrital grains or diagenetic dolomite, and these may react extensively with calcite during metamorphism. Marly sediments containing a mixture of carbonate and silicate components are also common, and there is a complete spectrum possible between purely carbonate and purely silicate sediments.

Metamorphic rocks reflect the variability of the sedimentary record, and so also include both pure marbles and a range of metasediments with variable proportions of carbonate. However, it is also not unusual, especially at medium to high grades, to find metasediments that are rich Ca-or Ca-Mg-silicates (such as zoisite, grossular, amphibole or diopside) but which contain little or no carbonate. These rocks are known as calc-silicates, and in many cases are probably the products of metamorphism of originally carbonate. Bearing sediments. We infer this because calcite and dolomite are the major Ca- and constituents of sediments, and the reactions in which they participate typically involve breakdown of carbonates with loss of CO2 in the production of silicates. Skarns are a variety of calc-silicate rock formed by metasomatic interaction between marble and silicate rock. The most spectacular examples result from intrusion of granite into marble.

In practice, therefore, it becomes convenient for the description of metamorphosed calcareous sediments to divide them into two categories: marbles in which carbonates are abundant; and calc-silicates with little or no carbonate. The possible range in mineralogy of calc-silicates is very large, since it depends on the precise mixture of sedimentary components in the original layer as well as being susceptible to metasomatic interactions with adjacent layers. For this reason, no attempt will be made here to provide a comprehensive guide to the mineralogy of calc-silicates, even though they are often sensitive indicators of metamorphic grade. However, some examples of the types of mineralogical zoning most commonly found are outlined later in the chapter. The compositions of the phases discussed in this chapter are listed in the Glossary.

CALCITE MARBLES

The term marble is used for metamorphosed calcareous rocks in with carbonate minerals dominate. Many marbles are composed only of calcite with minor quartz and phyllosilicates, originally of detrital origin. There is sometimes graphite derived from organic debris, and pyrite is also a common accessory. The mineral assemblage in a marble of this type provides few clues as to the conditions of formation, since calcite is stable at all but the highest pressures, and even where aragonite does form during burial, it is likely in most cases to change back completely to calcite during uplift, except at very low temperatures. At very high temperatures and low pressures, calcite may react with any quartz present it produce calcium silicate, wollastonite. Despite the lack if mineralogical reaction in calcite marbles, they are susceptible to extensive textural changes due to recrystallization of calcite to produce a coarser grain size and often a preferred orientation.

The reaction to form wollastonite provides a simple example of one of the most common types of reaction to occur in carbonate rocks, i.e. a decarbonation reaction:

Lie H2O, CO2 forms a supercritical fluid under metamorphic conditions, with a density that is broadly similar to that of supercritical water, though slightly greater under most metamorphic conditions.

Reaction was studied experimentally by Harker and turtle. In their work the pressure of the fluid in the experimental capsule CO2 was equal to the total.

P-T diagram to show the stability limits of calcite quartz. Curves for calcite quartz breakdown are given for various values of XCO2 and for PCO2=1bar. Data of Johannes and puhan…

Pressure applied. This usually the case in high pressure experiments because the noble metal capsules containing the reacting minerals (or experimental charge) are weak and collapse on to the grains in the charge forcing them together until the pressure on the fluid in the remaining interstices is equal to the applied pressure.

Harker and turtle´s results, shown in fi. 5.1, demonstrated that at pressures of more than a couple of kilo bars the temperature required to form wollastonite is beyond the normal range of regional metamorphism. This is consistent with the fact that most wollastonite occurrences are in thermal aureoles formed by contact metamorphism at relatively low pressures. Nevertheless, wollastonite is sometimes found in situations where it apparently formed at significantly higher pressures but without excessive temperatures. An explanation for these occurrences necessitates considering the possibility of metamorphic fluids intermediate in composition between H2O and CO2.

MIXED – VOLATILE FLUIDS

The principle introduced in chapter 2 that higher pressures of CO2 inhibit decarbonation reactions has been understood at least since the time of James Hutton and sir James hall in the eighteenth century. Nevertheless, if it were possible to apply a high pressure to the solid phases in a marble while allowing CO2 o escape at low pressure, then, since the molar volume of wollastonite is loss than of 1 mol quartz+1 mol calcite, we might expect the reaction to take place at lower temperatures than when the total pressure on the system is also low. This effect was discussed in the early 1950s by H. Ramberg and T.F.W Barth. It is difficult to envisage a natural situation that would correspond so such an experiment but in 1962 H.J. Greenwood and P.J. Wyllie independently pointed out that a very similar effect would be produced if the fluid phase in contact with the calcite and quartz were rich in H2O. At the temperatures of the green schist facies and above. H2O and CO2 supercritical fluids are completely miscible (except where the aqueous fluid contains large amounts of dissolved salts). Hence the partial pressure due to the CO2 in a mixed H2O-CO2 fluid may be very much less than the total fluid pressure, even if Pfluid=Plithostatic. Fluid composition is conveniently expressed in terms of the mole fraction of CO2 or XCO2:

Where n denotes the number of molecules of the subscripted species in the system. Partial pressures are then given by:

A major innovation by Greenwood, was to carry out experiments at controlled values of XCO2 in the fluid phase, and hence with PCO2<Ptotal. His work is included on fig. 5.1 and demonstrates the lowering of the temperature for the appearance of wollastonite in the presence of an H2O-rich fluid. These results provide a possible explanation of the occasional occurrences of wollastonite in regionally metamorphosed marbles: i.e. that is occurs where water was able to infiltrate the marble from adjacent schist and, favorable to wollastonite growth.

The observed effect of adding H2O to experiments on the equilibrium between calcite, quartz, wollastonite and fluid accord with the phase rule. In the H2O. Absent system there are four phases and three components (CaO, SiO2, CO2), and hence one degree of freedom, i.e. the full assemblage can occur stably only along a univariant curve on a P-T diagram. Adding H2O increases the number of components by one, but does not change the number of phases if it miscible with CO2. Hence there are now two degrees of freedom when calcite, quartz, wollastonite and fluid coexist. Here, fluid composition is a variable in addition to T and P, and by specifying one of these three variables, the equilibrium conditions can be represented by a univariant curve on a plot with the other two variables as axes. For example on fig. 5.1 a plot of P versus T versus XCO2, constructed for some specified constant value of total pressure of 2 kbar, is shown in fig. 5.2 (as an exercise, construct a similar curve for a pressure of 1 kbar from the curves in fig. 5.1. Plots of this type are known as isobaric T-XCO2 diagrams (or simply T-XCO2 diagrams). Divariant equilibria, such as reaction 5.1 taking place in the presence of H2O, plot on such a diagram as a line known as an isobaric univariant curve, i.e. when P is fixed, on degree of freedom remains.

DOLOMITIC MARBLES

The number of phases that can form from limestones composed only of CaCO3+quartz is clearly limited, as shown by fig. 5.1 Only at exceptionally high temperatures and low pressures do other phases such as spurrite and larnite appear, and since the classic locality, Scawt Hill in Northern Ireland, where C.E. Tilley described this extreme type of metamorphism, results from the chance heating of chalk by basalt lava in the immediate vicinity of the surface, they cannot be considered as of widespread geological importance. For this reason the relationships of these, and other associated phases, are not considered further here, even though the paper in which they were interpreted by Bowen 1940 still stands as one of the classics of the metamorphic literature.

Limestone that contain dolomite provide much more useful indicators of metamorphic grade because a range of Ca-Mg-silicates can form in the more usual P-T conditions of metamorphism, for example talc, tremolita and diopside. The general sequence of mineral zonation in dolomitic marble was first described by Eskola (1922) and subsequently refined by Bowen (1940) and Tilley (1951), why first recognized the importance of talc at the lowest grades. The sequence of mineral-appearance isograds in regionally metamorphosed dolomitic limestones appears to be:

- Tal (not always present)- Tremolita- Diopside or forsterite- Diopside + forsterite

Most earlier studies reported the appearance of forsterite before diopside, but the precise conditions for the growth of either mineral are dependent on rock composition, and since both appear at very similar temperatures, chance variation in lithology can dictate the relative order of appearance. For this reason these two minerals have been grouped together here, although it is clear that higher temperatures are needed for them to coexist than for one or the order (according to rock composition) to occur.

The mineral assemblages of impure dolomitic marbles can be conveniently represented in a triangular diagram with CaO SiO2 and MgO at the apices CO2 and H2O are treated as being available in excess to produce carbonate or hydrous phases. The locations of the common phases of metamorphosed marbles, plotted on such a diagram, are shown on fig. 5.4.

In addition to the Ca-Mg silicates and carbonates, and quartz, impure can contain additional phases such as mica, feldspar, garnet, etc. which involve further components, but many marbles nevertheless have compositions that can be modelled very closely in the system Ca-Mg-SiO2. Minor amounts of other phases do not substantially change the reactions among the Ca-Mg-silicates

One of the most extensive of the regional metamorphism of marbles under medium pressure conditions is the classic work by Trommsdorf in the central Alps fig. 5.3 is a map showing the metamorphic grade increases southwards from low grade rocks with talc through tremolita marbles to diopside and forsterite-bearing rocks. The diagnostic three-phase assemblages found by Trommsdorf are represented graphically in fig. 5.4, and from this diagram it is possible to suggest reactions to describe the changes between the zones, in the basic of the shift of the pattern of tie-lines.

Fig. 5.4 (a) represents the original sedimentary assemblages of dolomite+calcite+quartz. The change between this diagram and fig. 5.4 (b), representing the low grade rocks shown in the northern part of the region on 5.3 is the replacement of the dolomite-quartz tie-line by a talc-calcite tie-line, and this change can be represented by the reaction:

Note that rocks containing all four of these solid phases are not uncommon. They might be expected where insufficient water was added to the marble to convert all the available reactants to talc. The appearance of tremolita leads to a more complex situation. In some rocks, talc persists with either dolomite or quartz and calcite, but in other tremolita occurs without talc. Fig 5.4 represents the main paragenesis found when tremolita first reacts in. The composition of tremolita plots within the talc-calcite-quartz triangle of fug. 5.4 (b), from which we can deduce that the reaction is: