Análisis de supervivencia de datos

As a concluding discussion on the theoretical constraints on carbonate melt structure I will summarise above experimental and theoretical restrictions in order to outline theories for carbonate melt structure and their implications for the theological properties of carbonate melts.

2.3.3.a Carbonates as Ionic Melts.

Any model for melt structure must reflect the ionic character of carbonate melts. However, it is clear, from the above considerations, that carbonate melts can neither be treated as ideal solutions of a COj^ solvent with metal solutes, nor as real solutions with

independent ionic groups and metal cations. Hence, any realistic model must incorporate considerations of the metal spéciation of the melt, in terms of temperature, pressure and composition.

As outlined above the spéciation of the alkaline earths and alkali metals is related to electronegativity due to the ionic nature of interactions between and these elements. The spéciation of the transition metals, however, is not a function of electronegativity, due to their tendency to form coordinate bonds with

Metal spéciation is a function of both fOj and Pco2» with increased metal-carbonate spéciation at either high fOj’s or high Pco2- The presence of other volatiles, such as, H, S or P may effect the dependence of spéciation on fOj and Pco2 through coupled redox reactions, such as reactions 2.8. A model by which melt structure is controlled by fOz and Pco2 is however, only appropriate in open systems in which the activities of these gases are externally buffered. Most geologically applicable melts, however, demonstrate internal buffering and exist as closed systems for much of their evolution, the fugacities of their gas phases being controlled by temperature, pressure, composition of the melt phase and the buffering capacity of equilibrium crystalline phases.

If a carbonate melt exists as a closed system, the fO; and Pco2 will be controlled by the temperature, pressure and the composition of the melt by dissociation of C O ^ and

any coupled redox reactions. In view of the above discussions it seems likely that for anhydrous carbonate melts at least that the lowest oxygen fugacities will be demonstrated by melts with high concentrations of alkali metals, as dissociation will be minimised, similarly such melts should demonstrate low partial pressures of COj, it also seems probable that both fOj and Pco2 will increase with K"^:Na^ ratio. Melts dominated by the alkaline earths will demonstrate higher fOj’s and Prog's, increasing in the series B a^ >

> M g^, which would favour the development of a coexisting vapour phase.

In terms of melt structure it seems likely that carbonate melts will consist of populations of isolated ionic groups, metal cations and complexed species. Alkaline earths such as Ca^, Ba^^ and Sr^^ wül demonstrate the largest populations of carbonate complexes, whereas the alkali metals, such as, Na^ and will exist essentially as isolated cations, the spéciation of the transition metals and the rare earth elements are, however, open to speculation.

Pressure and temperature effects on carbonate melt structure, will be related to both the dissociation of and the activation energies for bond breaking of metal carbonate bonds. Increases in the equilibrium constant for dissociation suggest that spéciation of metals as carbonate complexes will decrease with temperature, in response to decreases in the activities of whereas increases in pressure which reduce the equilibrium constant will produce increases in carbonate complexation.

2.3.3.b Rheological Properties.

The structure of a melt and its energetics will effectively control the macroscopic properties of the melt. Rheological data on carbonate melts are, however, limited and viscosity and electrical conductivity data exists only for the alkali metal carbonate melts.

Electrical conductivity, as described above, decreases with increasing electronegativity of the alkali metal cation, in response to the stronger metal-carbonate, metal-oxygen interactions and subsequent reductions in electrolyte mobility in the melt. Similarly, the presence of large soluble molecules, such as, metal-carbonate complexes would be expected to reduce electrical conductivity, therefore, it seems probable that the electrical conductivity of a carbonate melt will decrease with Ca^, B a^ and Sr^ content.

system LijCOj - NajCOj - K2CO3, demonstrate viscosities between 1-5 mPa s over the temperature range 900 - 700°C (fig.2.19), such viscosities are approximately 2-3 orders of magnitude smaller than silicate melts. Since viscosity is related to flow by bond breaking under an external shear stress, the low viscosities of the alkali carbonate melts must relate to the low metal-carbonate cohesive forces. Disagreement of experimental results from different studies of an order of magnitude (Janz 1963, Ejima et al 1987) demonstrate the technical difficulties of accurate measurement of such low viscosities. It is clear, however, that viscosity decreases in the series Rb^<K^<Na^<Li^ occur relating to increasing bond strength in this series (fig.2.16). Likewise, in systems with significant populations of metal-carbonate and metal-oxide complexes that higher viscosities would occur both due to increasing bond strengths and also the presence of larger molecular groups, it also seems possible, that since, carbonate complexes are larger than oxide spéciation molecules, the viscosities of systems with high Ca^, Ba^, Sr^, contents will be larger than those with the other alkali metals.

No experimental data is available on the effects of water content upon the viscosity of carbonate melts, however, coupled redox reactions (reaction 2.8), reduce the activities of in the melt by formation of OH . Hence, water content would be expected to reduce viscosity.