Deciphering Magma-Mixing Processes Using Zoned Titanite from the Ross of Mull Granite, Scotland

A textural and geochemical investigation of titanite from the Ross of Mull Granite, Scotland, reveals that zoning in trace elements strongly correlates with field evidence of magma-mixing processes within the pluton. Diorite enclaves contain a range of textures indicative of variable states of interaction with the host granite and suggest that a number of discrete mixing events have occurred in the pluton, both in situ and prior to emplacement at the present level. Single titanites within both the enclaves and the host are compositionally zoned and provide a detailed record of magmatic processes. Zoning includes concentric fine-scale oscillatory types and major discontinuities linked to dissolution events associated with changes in titanite stability. The zones reflect growth during changing melt chemistry and correlate with variations in rare earth element and high field strength element contents. Geothermometry using the Zr content of titanite indicates that titanite crystallization occurs at temperatures of c. 750–700°C. Titanites respond to changes in melt composition, oxygen fugacity and temperature caused by mixing and the introduction of fresh diorite magma into the granite host. Mixing processes recorded by the titanites include complete mixing of crystal-free melts, crystal transfer between partially crystallized magmas, melt segregation and transfer from crystalline enclaves and late-stage diffusive exchange. Titanite chemistry responds to local activity adjacent to sites of mixing but is also influenced by more distal magma-mixing events. In this respect titanite is capable of revealing many otherwise hidden events within the magma chamber.