Aragonite in California glaucophane schists, and the kinetics of the aragonite-calcite transformation

Coleman & Lee's discovery that aragonite is a widespread metamorphic mineral in Cali-fornia glaucophane schists is confirmed and amplified. Methods of microscopic distinction between aragonite, calcite, and dolomite, including a universal-stage technique, are described. Further data are recorded regarding the paragenesis of aragonite-bearing gJaucophane and lawsonite schists. Carbonates in the greenschist facies are found to be exclusively calcite and dolomite. In many Californian metamorphic aragonites partial inversion to calcite has been observed. This appears to be an equal-volume replacement in which an a axis and an edge [/:/] of _ calcite—both directions of Closest spacing of Ca"1"1 " ions—commonly are parallel to Jwoof the crystal axes a, b, and c of aragonite. The problem of survival of metamorphic aragonite through a necessarily long period of post-metamorphic unloading is approached by experimental exploration of the kinetics of the aragonite-> calcite transformation. It is found that Californian aragonite could survive unloading from a depth of 20 km if the linear temperature gradient were 10 ° C per km, but not appreciably higher. Available experimental data are consistent with crystallization of aragonite, jadeite, and lawsonite at depths of 20-30 km if a gradient of 10 ° C per km is assumed. The corresponding conditions of the glaucophane-schist facies (T = 200°-300 ° C, P = 6000-9000 bars) are attributed essentially to deep burial in regions (subsiding geosyn-clines) of unusually low temperature gradient (10 ° C per km)