The partitioning of samarium and thulium between garnets and melts in the systems Mg3Al2-Si3O12-H2O and Ca3Al2Si3O12-H2O has been studied as a function of REE concentration in the garnets at 30 kbar pressure. Synthesis experiments of variable time under constant P, T conditions indicate that garnet initially crystallizes rapidly to produce apparent values of D
Sm (D
Sm=concentration of Sm in garnet/concentration of Sm in liquid) which are too large in the case of pyrope and too small in the case of grossular. As the experiment proceeds, Sm diffuses out of or into the garnet and the equilibrium value of D
Sm is approached. Approximate values of diffusion coefficients for Sm in pyrope garnet obtained by this method are 6 × 10−13 cm2 s−1 at 1,300 ° C and 2 × 10−12 cm2 s−1 at 1,500 ° C, and for grossular, 8.3 × 10−12 cm2 s−1 at 1,200 ° C and 4.6 × 10−11 cm2 s−1 at 1,300 ° C. The equilibrium values of D
Sm have been reversed by experiments with Sm-free pyrope and Sm-bearing glass, and with Sm-bearing grossular and Sm-free glass. Between 12 ppm and 1,000 ppm Sm in pyrope at 1,300 ° C and between 80 ppm and >2 wt.% Tm in pyrope at 1,500 ° C, partition coefficients are constant and independent of REE concentration. Above 100 ppm of Sm in garnet at 1,500 ° C, partition coefficients are independent of Sm concentration. At lower concentrations, however, D
Sm is dependent upon the Sm content of the garnet. The two regions may be interpreted in terms of charge-balanced substitution of Sm3Al5O12 in the garnet at high Sm concentrations and defect equilibria involving cation vacancies at low concentrations. At very low REE concentrations (< 1 ppm Tm in grossular at 1,300 ° C) DREE garnet/liquid again becomes constant with an apparent Henry's Law value greater than that at high concentrations. This may be interpreted in terms of a large abundance of cation vacancies relative to the number of REE ions. The importance of defects in the low concentration region has been confirmed by adding other REE (at 80 ppm level) to the system Mg3Al2Si3O12-H2O at low Sm concentrations. These change D
Sm in the defect region, demonstrating their role in the production of vacancies. Experiments on a natural pyropic garnet indicate that defect equilibria are of importance to REE partitioning within the concentration ranges found in nature.

An experimental investigation of the partitioning of REE between garnet and liquid with reference to the role of defect equilibria

Experiments were performed on amorphous silica at temperatures of 1000°–2850°C and pressures of 9–14 GPa using a 2000-t uniaxial split-sphere multianvil apparatus (USSA-2000). The melting curve of coesite was extended from 8 to 13.7 GPa, and the melting temperatures indicate a slope that is almost independent of pressure. Melting of stishovite was observed at 2850°C and 14 GPa. The triple point for coexisting coesite, stishovite, and melt was located at 2800°C and 13.7 GPa. The coesite-stishovite phase transition was determined between 2000° and 2800°C and is described by the equation P (GPa) = 5.8(4) + 0.0028(2)T(°C). The resulting dP/dT slope is more than twice that determined in the in situ X ray study of Yagi and Akimoto (1976). The present coesite-stishovite boundary cannot be predicted by existing thermodynamic data (e.g., Fei et al., 1990), which suggests some uncertainties in the extrapolation of heat capacities of coesite and stishovite at high temperatures. Thermodynamic extrapolations of the metastable melting curves of quartz and coesite show a strongly negative slope and thus are consistent with the proposal that the room temperature amorphization of quartz and coesite results from crossing of the metastable extension of their melting curves.

Melting and subsolidus relations of SiO2 at 9–14 GPa

A method is described for the determination of silicate melting points to 2000°C and 50 kb, using tungsten | rhenium thermocouples in piston-cylinder apparatus. Pressures are corrected for frictional losses and temperatures for the effect of pressure on the emf of the thermocouple. The melting curve of diopside has been measured to 50 kb; agreement with F. R. Boyd and J. L. England's 1963 data is good to 30 kb, but our results lie at substantially higher temperatures than theirs at the highest pressure. The difference in the two curves amounts to 88°C at 50 kb. Finally, the melting relationship proposed by G. C. Kennedy is examined for these new data.

Melting curve of diopside to 50 kilobars

Automated identification of mass spectra by the reverse search

The self-diffusion of 14C in TiC0.67 was studied in the temperature range 1745°–2720°C. The diffusivity, which exhibits a discontinuity at 2080°C, can be represented by two expressions: D0.67*=(2.85±0.20)×10−4exp[−(49 600±300)/RT] cm2/sec above 2080°C and D0.67*=(1.14±0.66)×102exp[−(109,900±2000)/RT] cm2/sec below this temperature. No discontinuity was found in carbon saturated TiC0.97 and the self-diffusion coefficient for this composition can be described to a temperature of 2720°C by the single expression D*0.97=(6.98±1.24) exp[−(95 300±700) / RT] cm2/sec.

Anomalous Diffusion of 14C in TiC0.67

Tungsten‐rhenium thermocouple systems were evaluated for use in measuring temperatures between 1600 and 3000°C. Temperature‐millivolt relationships were extended from 2300 to 3000°C for bare‐wire W3Re/W25Re and W5Re/W26Re thermocouples in vacuum. The performance of W26Re sheathed, high‐fired beryllia‐insulated thermocouples was limited only by the melting point of the beryllia insulation (∼2550°C). The thermoelectric output of high‐fired thoria‐insulated thermocouples sheathed in W26Re was determined and found to be reliable up to at least 2850°C, when corrected for electrical shunting through the insulation. Stability of high temperature thermocouples using thoria insulation was within ±0.3 mV (±40°C) when held at 2425°C for 148 h.

P. E. Novak

Papers

Tungsten‐Rhenium Thermocouples for Use at High Temperatures