Abstract
Exsolution of iron oxides appears to account for the clouding of plagioclase in metadolerite dikes in the Bighorn Mountains, Wyoming. An intra- rather than extra-plagioclase source of the iron is indicated by its accumulation along crystallographic planes and by comparative chemical, electron microprobe, oxygen isotopic, and X-ray diffraction studies of the clouded plagioclase and compositionally and mineralogically similar clear plagioclase in younger quartz dolerite dikes of the same area. The emplacement of the metadolerite into a terrane heated by metamorphic processes and the resulting slow subsolidus cooling of the plagioclase are thought to explain the exsolution. Electron microprobe studies indicate that the concentration of iron in clear plagioclase increases with decreasing An content and with proximity to crystal edges. This is interpreted to mean that the incorporation of iron by plagioclase is responsive to the availability of iron to the plagioclase.
