Topotaxic relationships between spinel and pyroxene in kelyphite after garnet in mantle-derived peridotites and their implications to reaction mechanism and kinetics

Kelyphite is a reaction product between garnet and olivine, which was formed by subsolidus reactions upon decompression during the ascent of mantle peridotite. We studied crystallographic relationships among constituent (product) phases of kelyphite - orthopyroxene, clinopyroxene, spinel and reactant phases, garnet and olivine, using EBSD and found that, for a relatively high temperature sample (from Czech Moldanubian), spinel and pyroxenes are in a topotaxic relationship in such a way that spinel {111} coincides with pyroxene (100) and spinel {110} coincides with pyroxene (010); while the topotaxy is incomplete or non for a lower-temperature sample (from western Norway). On the basis of the observed microstructural and crystallographic relationships, we propose a hypothesis that the topotaxic relationship may be established at nucleation stages of the onset of the kelyphitization and that the degree of topotaxy may be related to the transformation temperature and the degree of supersaturation of the reaction. The lower the temperature, the higher the supersaturation and, therefore, more rapid the nucleation becomes, resulting in a more disordered state in topotaxic relationship.