(FORMER) MELT INCLUSIONS: NOVEL MICROSTRUCTURAL INSIGHTS INTO CRUSTAL MELTING
MI are generally isometric and very small in size, mostly ≤15 μm, and occur in clusters. In slowly cooled regional rocks they are crystallized to a cryptocrystalline assemblage of quartz, feldspars and one or two micas depending on the case study, commonly with accessory trapped phases (kyanite, graphite, zircon, apatite, rutile). Due to their bulk chemical composition they have been named nanogranite.
When MI trapped within peritectic minerals (garnet, cordierite, spinel, ilmenite, zircon) display textural features pointing to a primary origin, they demonstrate the growth of their host in the presence of melt. Therefore MI add to other microstructures (e.g., pseudomorphs after melt films or crystal faces of feldspars or cordierite against quartz) as a most reliable microstructural criterion for the presence of melt in a rock. Not only MI indicate that a rock was partially melted, but also they add constraints to the mineral(s) which were in equilibrium with the melt. In the case of hosts like zircon or monazite, the occurrence of MI allows anatectic events to be dated with unprecedented confidence.
Preservation of crystallized MI in relicts of refractory/hard minerals such as garnet allows the former presence of melt to be identified also in those rocks that underwent total structural and mineralogical reworking during the retrograde evolution or polymetamorphism.
The possibility of combining the microstructural information with the chemical characterization (major elements, traces, volatiles) of the anatectic melts will have a big impact on tectonic studies, either by allowing better constraints to natural case studies, or by providing refined input parameters for quantitative modelling.