Abstract

Rods and laths of clinopyroxene apparently exsolved in host bustamite are described from the lode rocks of Broken Hill, New South Wales, Australia. The rods are commonly oriented with c clinopyroxene ∥b bustamite. They are associated with fluid inclusions which may be in contact with the clinopyroxene rods (type A) or not (type B).

Freezing-point depressions recorded for the inclusion fluids range from 3° to 50°C. Although there is a region of overlap, type A inclusion fluids have salinities in the lower part of the range, whereas type B inclusions contain fluids in the higher part of the range. In bustamite samples containing only moderate numbers of clinopyroxene rods where relationships can be clearly observed, the rods are seen to be arranged within curved sheets localized along healed former fractures. Where fluid inclusions of both types are present within any one sheet, they have approximately the same salinity. Homogenization temperatures fall in the range 240–360°C, but the trapping temperatures are unknown owing to uncertain corrections for pressure.

The clinopyroxene rods are believed to have originated by a mechanism of exsolution, in which fluid inclusions act as centres of nucleation and growth in the high-temperature solid-solution phase as it cools below solvus temperatures. The rods propagate either from the solid or liquid inclusion-terminated end, but the preferred mechanism is by solution growth, the fluid inclusion advancing ahead of the rod through the host grain.

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