Genesis of microdiamonds from melt and associated multiphase inclusions in garnet of ultrahigh-pressure gneiss from Erzgebirge, Germany

The common association of microdiamonds with the phase assemblage: phlogopite, apatite, paragonite and alpha-quartz (containing amorphous Na¯Al silicate inclusions), as inclusions in garnets of quartzofeldspathic rocks from the Saxonian Erzgebirge, Germany, was studied by analytical electron microscopy. The assemblage implies a precedent melt, which coexisted with the microdiamonds before and after entrapment in the garnet host, and subsequently crystallized. The formation of microdiamonds in this metamorphic rock could be explained by cotectic-induced partial melting of a subducted continental slab at about 4¯6 GPa and 1000° C, as constrained by the occurrence of TiO2 with an alpha-PbO2-type structure at the peak metamorphic conditions, and by the catalytic effect of siderophile and chalcophile elements.