Ti in garnet: complex substitutions and their implications for understanding crustal metamorphism

Abstract:

The nature of Ti incorporation into garnet (substitution mechanisms, site occupancy) has been a matter of interest for decades. Most crustal garnets contain minor to trace quantities of Ti, yet crystallographically-aligned rutile needles in some high grade crustal and mantle garnets suggest Ti solubility can reach major-element concentrations (>1%TiO₂) under certain conditions. Understanding Ti incorporation into garnet holds promise for evaluating and interpreting the history and evolution of metamorphic systems. In this study we will demonstrate that Ti is incorporated into garnets via at least three substitution mechanisms on multiple crystallographic sites.

Garnets were grown in piston cylinder apparatuses at eclogite and granulite facies conditions in multiple bulk compositions. Chemical trends in the experimentally-grown garnets suggest Ti incorporation occurs on the octahedral crystallographic site (^VITi) via at least two substitution mechanisms. Furthermore, Ti partitioning between garnets and their corresponding melts increases with decreasing temperature and increasing melt polymerization.

Ti K_α X-ray Absorption Near Edge Struture (XANES) pre-edge analysis was used to observe Ti coordination in experimental and natural garnets (contact metamorphic grade up to eclogite and granulite facies). XANES analyses confirm the observation of ^VITi in experimental garnets. However, natural garnets contain both ^VITi and ^IVTi (from ~90% ^IVTi to 100% ^VITi). Microprobe analyses of Ti in garnet were combined with the XANES analyses to determine ^VITi and ^IVTi concentrations. Increasing ^VITi is strongly correlated with increasing Ca content in garnet, while ^IVTi behaves similar to ^IVTi in other silicate minerals (e.g. quartz, zircon).

The complex nature of Ti incorporation into garnet diminishes the utility of a single-mineral Ti-in-garnet thermobarometer, but partitioning of Ti between garnet, clinopyroxene and melt could be useful for the development of novel empirical thermobarometers. Also, given this insight into the substitution mechanisms governing Ti incorporation, Ti depletion halos around exsolved rutile needles could be used to determine exhumation rates for high-grade metamorphic systems.