Hafnium in zircon from rapakivi granite

The hafnium distribution in zircon crystals extracted from rapakivi granite is studied. Karelia, a typical rapakivi representative was used to determine concentrations of zircon, hafnium and yttrium using a JEOL JCZA-733 Superprobe. The initial ZrO2/HfO2 ratio for central parts of zircon crystals averages 43.7, characteristic of zircon from crustal granite formed without participation of mafic magma. The peripheral parts of zircon crystals are always richer in hafnium than the central parts, and consequently, have a lower ZrO2/HfO2 ratio. Hafnium and yttrium concentrations in the melt fluctuate. The hafnium and yttrium distribution in zircon crystals reflect the normal pattern of crystallization of granite magma.