Garnet, a marvellous mineral for deriving P-T paths of metamorphic rocks, but what are the pitfalls and limitations?

Al-garnet is a common constituent of medium- to high-grade metamorphic rocks of sedimentary and basic to acidic igneous protoliths. Due to its compositional variability (main components: almandine, grossular, pyrope, spessartine), this mineral is very important to decipher the pressure-temperature (P-T) evolution of these rocks. In many cases, garnet occurs as compositionally zoned porphyroblast being the result of prograde metamorphism during which hydrous minerals such as chlorite or lawsonite were decomposed. The relevant temperature interval for corresponding mineral reactions is usually in the range 450-650 °C. Thus, garnet is a nearly perfect mineral to decipher P-T conditions experienced by medium-grade metamorphic rocks. In addition, it preserves minerals which were enclosed during its growth. Therefore, P-T conditions can also be derived for a metamorphic stage before garnet growth based on inclusion minerals in garnet cores.

Prograde metamorphism, characterized by slight heating but significant pressure increase at high temperatures, commonly leads to garnet by breakdown of dry minerals such as cordierite and plagioclase (anorthite component). Corresponding reactions mainly occur between 0.5 and 1.8 GPa. Above this pressure range, garnet cannot be used to precisely determine P-T conditions of rocks, which were subjected to metamorphism in the high-temperature eclogite- and high-pressure granulite-facies, unless melting reactions took place, for example, with participating hydrous minerals such as micas, (clino)zoisite, and amphibole resulting in the formation of peritectic garnet. But the derivation of the P-T conditions of peritectic garnet formation, concerning pressures also below 1.8 GPa, requires complex thermodynamic modelling as various parameters such as H₂O content of the rock and possible melt loss have to be considered. In addition, intracrystalline cation diffusion, particularly of Mg, in garnet complicates this derivation by the perceptible change of the original garnet composition at metamorphic peak temperatures above 750-800 °C. As both complex thermodynamic modelling for peritectic garnet, if applicable at all, and modelling of this cation diffusion were very rarely applied in the past, published P-T paths through the realms of the high-temperature eclogite- and granulite-facies are fairly uncertain and sometimes even wrong.

The annoying intracrystalline cation diffusion, however, can be valuable, for example, in the case of a contact of two garnet generations with different chemical compositions within a single grain. Modelling of this contact feature, which is typical of polymetamorphic rocks, can yield a time interval for early cooling at high temperatures. In such rocks characterized by an early medium-temperature and garnet-free mineral assemblage, initial growth of garnet during burial can be delayed due to an energetic barrier deferring the formation of garnet seeds. This so-called “garnet overstepping” concerns a pressure range up to 1 GPa above the garnet-in curve and leads to garnet porphyroblasts with nearly homogeneous chemical composition. Therefore, this type of garnet was, so far, frequently mistaken for a high-temperature garnet homogenized by intracrystalline cation diffusion. Despite such pitfalls and the aforementioned limitations, garnet is essential to deduce the P-T evolution of metamorphic rocks of deep-seated crustal sections and, thus, to better understand geodynamic processes involving the Earth’s crust.