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INTRODUCTION Molten silicate liquids are the primary transport agents within the deep earth. Their thermodynamic behavior governs a variety of processes including convection, melting, crystallization and mineral-liquid partitioning. Recent studies have shown that the dynamic properties of silicate liquids are significantly different from those of silicate glasses. Therefore it is important in investigating silicate melt behavior to consider data relevant to the molten state. However, at pressures greater than one atmosphere it is

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Mat. Res. Soc. Symp. Proc. Vol. 499 ©1998 Materials Research Society

difficult to make observation in the molten state and a variety of approaches have been adopted to model silicate melt behavior [1]. Recent developments in high temperature calorimetry [2] enable the heat of solution of oxide components in silicate liquids to be measured in situ. This method is particularly well suited to large cations, which result in anomalous melt behavior. Two examples are TiO 2 , La 20 3 . These measurements provide the partial molar enthalpies of the melt components and reflect the molecular scale interactions that govern the melt dynamics. As such these measurements provide a starting point for modeling the high pressure behavior of silicate liquids. At one atmosphere the chemical potential, 0

W i,T, latm

of a melt component is defined by:

0

t

A i,T,latm - Pi,298.15K,atm

+ RT Ina

(1)

Where ai is the activity of the melt component. Alternatively the chemical potential can be written in terms of the partial molar enthalpy and entropy as: PiT,Iatm "'--Pi,298.15K +

AHi

-

TASi

(2)

As pressure is increased it is necessary to introduce a volume term. ,i,°TP =TA298.15K,,at

+ A-I• - T

P .+ f A

V.dP

(3)

1

To define the chemical potential at any temperature and pressure there are two unknowns, the partial molar entropy and the partial molar volume.

Experimental techniques: The solution behavior of oxide components can be studied at high temperature and at one atmosphere by dropping pellets of oxide from room temperature in to a reservoir of molten silicate solvent, maintained at a constant temperature (1760 K). The resultant heat effect is the heat of drop solution, AH sol. Heat of solution measurements are made using a Setaram HT1500 calorimeter. The calorimeter detector comprises Pt-Rh detector with an upper series of thermocouple junctions surrounding an alumina crucible, which contains a platinum crucible containing the silicate solvent. A lower set of junctions surround an identical set of alumina and platinum reference crucibles. The detector is contained within an alumina muffle tube open to the air. Graphite heating elements surround the muffle tube and are continually flushed with argon. The calorimeter is operated at a temperature of 1760 K with temperature calibrated against the melting point of gold (1336.15 K). The thermal gradient with in the vicinity Of the calorimetric detector

186

is 3 K. Nanovolt DC thermopile signals are communicated digitally to an IBM compat