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

The common association of microdiamonds with the phase assemblage: phlogopi te, apatite, paragonite and alpha -quartz (containing amorphous Na-Al silic ate inclusions), as inclusions in garnets of quartzofeldspathic rocks from the Saxonian Erzgebirge, Germany, was studied by analytical electron micros copy. The assemblage implies a precedent melt, which coexisted with the mic rodiamonds before and after entrapment in the garnet host, and subsequently crystallized. The formation of microdiamonds in this metamorphic rock coul d be explained by cotectic-induced partial melting of a subducted continent al slab at about 4-6 GPa and 1000 degreesC, as constrained by the occurrenc e of TiO2 with an alpha -PbO2-type structure at the peak metamorphic condit ions, and by the catalytic effect of siderophile and chalcophile elements. (C) 2001 Elsevier Science B.V. All rights reserved.