gems, garnet, almandine, spessartine, analytical, provenance, microprobe, origin

This paper deals with the different types of zoning shown by metamorphic and igneous garnets. A tentative relationship between zoning and physical conditions of the host-rock during and after crystallization of garnets is inferred. On the basis of the data presented, it is suggested that rocks whose garnets show normal zoning (i.e. a progressive decrease in Mn-content from core to rim) were subjected to no further thermodynamic activity after the crystallization of this mineral. On the other hand, garnets with Mn-enriched rims and reverse zoning (i.e. a progressive enrichment in Mn from core to rim) indicate an important change in the P and/or T conditions after the initial crystallization of garnets. It is suggested that garnets with Mn-enriched rims were subjected to the new thermodynamic activity for a relatively short space of time. Reverse zoning suggests activity over a longer period with temperatures high enough to allow significant internal diffusion.