Aragonite in California Glaucophane Schists, and the Kinetics of the Aragonite--Calcite Transformation

Coleman & Lee's discovery that aragonite is a widespreadmetamorphic mineral in California glaucophane schists is confirmedand amplified. Methods of microscopic distinction between aragonite,calcite, and dolomite, including a universal-stage technique,are described. Further data are recorded regarding the paragenesisof aragonite-bearing glaucophane and lawsonite schists. Carbonatesin the greenschist facies are found to be exclusively calciteand dolomite. In many Californian metamorphic aragonites partialinversion to calcite has been observed. This appears to be anequal-volume replacement in which an a axis and an edge [f:f] of calcite—both directions of Closest spacing of Ca⁺⁺ions—commonlyare parallel to of the crystal axes a, b, and c of aragonite. The problem of survival of metamorphic aragonite through a necessarilylong period of post-metamorphic unloading is approached by experimentalexploration of the kinetics of the aragonite calcite transformation.It is found that Californian aragonite could survive unloadingfrom a depth of 20 km if the linear temperature gradient were10? C per km, but not appreciably higher. Available experimentaldata 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-schistfacies (T = 2OO?-3OO? C, P = 6000-9000 bars) are attributedessentially to deep burial in regions (subsiding geosyn-clines)of unusually low temperature gradient (10? C per km).