Growth zoning
Characteristics:
Oscillatory zoning – chemical zoning – observed in a mineral;
Concentric/parallel bands with generally variable thicknesses and/or concentrically distributed inclusions.
Origin:
Developed during the growth of a mineral;
Can point to pauses and renewed growth;
In magmatic context: can point to changing conditions in the melt in contact with the growing mineral (e.g., slow diffusion and unavailability of trace elements compatible with the growing crystal, magma replenishment and mixing events). Can also point to an incomplete reaction between the mineral (solid solution mineral) and the melt.
In hydrothermal context: can point to chemical changes in the fluids or in successive fluids entering the system (Tanner et al., 2017, presents a beautiful chalcopyrite example). The zoning can also be related to temperature changes in the fluid (e.g., Ti-in-quartz, Te-in-pyrite) or other physical-chemical variations.
Not to be mistaken for:
Sieve-textured plagioclase: the core is an older and corroded plagioclase, the rim is a neo-plagioclase. This indicates that the conditions promoting plagioclase growth interrupted for a time, before being renewed. This can point to magma mixing or a sudden loss of fluids from the magma system.
Inherited cores: this is common in zircon. The inherited core can lack oscillatory zoning (thermal erosion) and be anhedral (partial dissolution of the grain), when the overgrowth is euhedral and displays delicate oscillatory zoning. The inherited core can be inherited from the same magmatic system (antecryst) or from a distinct rock (xenocryst). It served as nuclei to the neo-zircon.