RATES OF RETROGRADE METAMORPHISM AND VEIN FORMATION

Retrograde alteration affects virtually all crystalline rocks that have returned to the surface. If retrograde reactions are slow, the alteration may document extended periods when the rocks were water-weakened, but if the reactions are fast, water was present only briefly and the crust was mainly dry and strong.We performed experiments on rock cores of fine grained basic hornfels. Cores 4mm in diameter had a central 2mm diameter hole and were loaded with pure water or 3M NaCl solution and reacted at 400^oC for 10 weeks at 3000bars. The starting material was dominated by plagioclase with orthopyroxene, clinopyroxene and Ti-magnetite in a granoblastic polygonal texture. Grain sizes are typically less than 100 microns.

Significant alteration of orthopyroxene was observed in both experiments and near the edges of the cores it was effectively complete. Mg-Al Saponite replaced orthopyroxene, and to a lesser degree plagioclase. The central holes drilled into the cores became lined with saponite, forming much coarser crystals than those replacing matrix minerals, although compositions showed little variation. Growth was more extensive in the presence of NaCl solution.

These experiments confirm that retrograde reactions can proceed rapidly under mid-crustal conditions. The reactions take place far from equilibrium and are dictated by kinetic considerations with clinopyroxene and titanomagnetite effectively inert on the timescale of the experiments. Fluid penetrated though the entire thickness of the rock cores in 10 weeks, but most reaction was near the edges. The experiments provide an insight into the formation of segregation veins, with retrograde products growing as coarse crystals in the open space provided. There was significant mobility of Si, Al and Mg, especially in the presence of NaCl. These results suggest that vein chemistry reflects the stable mineralogy during vein formation rather than the solution chemistry of the metamorphic fluid.