Microdiamond daughter crystals precipitated from supercritical COH plus silicate fluids included in garnet, Erzgebirge, Germany

Microdiamonds associated with phlogopite, quartz, paragonite, phengite, apa tite, and rutile are found as regular constituents of minute polyphase incl usions in garnet of ultrahigh-pressure (P) metamorphic gneiss lenses within migmatites of the gneiss-eclogite unit, Erzgebirge, Germany. These aggrega tes are interpreted to represent original inclusions of a supercritical den se COH fluid rich in K, Na, and SiO2. From this fluid, diamond was precipit ated as a daughter crystal due to cooling at ultrahigh-P conditions. Brittl e failure of the garnet host due to overpressure during release of confinin g pressure is demonstrated by healed radial cracks. During further cooling, the silicate phase assemblage of the inclusions crystallized at reduced in ternal pressures outside the stability field of diamond, as indicated by th e presence of quartz, paragonite, and plagioclase, It is proposed that the mica-dominated mineral assemblage of the inclusions formed by reaction betw een the fluid and the garnet host, the previously formed diamond daughter c rystals being preserved metastably, These diamond-bearing inclusions provid e an unequivocal record of dense supercritical COH fluids rich in alkalies and silica within subducted continental crust during ultrahigh-P metamorphi sm.