Exsolution of cummingtonite, actinolite and sodic amphibole in hornblende in high-pressure metamorphism

Abstract

Exsolution phenomena in amphiboles indicate the existence of miscibility gaps^1,2. The phase relationships of hornblende, the commonest amphibole in igneous and metamorphic rocks, to the other monoclinic amphiboles such as cummingtonite, actinolite and sodic amphibole has particular petrological importance. In these systems a solvus has been directly confirmed by exsolution in the hornblende–cummingtonite system^3–5. Chemical discontinuities suggesting miscibility gaps have been shown for other systems^6–18. In the hornblende–actinolite system, which was recently confirmed to have a solvus by experimental synthesis¹⁹, fine lamellar texture that is too fine for microprobe resolution is expected to result from exsolution^9,10. However, there has been no report of amphibole exsolution from high-pressure metamorphic rocks. We have found exsolution lamellae of three kinds of clinoamphibole, cummingtonite, actinolite and sodic amphibole, in a zoned hornblende from high-pressure metamorphic rock, and present here direct evidence to confirm the miscibility gaps between hornblende and these three clinoamphiboles in high-pressure metamorphic conditions. The genesis of exsolution lamellae discussed below is based on back-scattered electron-scanning images and microprobe analyses.