  Zircon is the most frequently used mineral for dating terrestrial and
extraterrestrial rocks. However, the system of zircon in mafic/ultramafic melts
has been rarely explored experimentally and most existing models based on the
felsic, intermediate and/or synthetic systems are probably not applicable for
prediction of zircon survival in terrestrial shallow asthenosphere. In order to
determine the zircon stability in such natural systems, we have performed
high-temperature experiments of zircon dissolution in natural mid-ocean ridge
basaltic and synthetic haplobasaltic melts coupled with in situ electron probe
microanalyses of the experimental products at high current.
  Taking into account the secondary fluorescence effect in zircon glass pairs
during electron microprobe analysis, we have calculated zirconium diffusion
coefficient necessary to predict zircon survival in asthenospheric melts of
tholeiitic basalt composition. The data imply that typical 100 micron zircons
dissolve rapidly (in 10 hours) and congruently upon the reaction with basaltic
melt at mantle pressures. We observed incongruent (to crystal ZrO2 and SiO2 in
melt) dissolution of zircon in natural mid-ocean ridge basaltic melt at low
pressures and in haplobasaltic melt at elevated pressure. Our experimental data
raise questions about the origin of zircons in mafic and ultramafic rocks, in
particular, in shallow oceanic asthenosphere and deep lithosphere, as well as
the meaning of the zircon-based ages estimated from the composition of these
minerals. Large size zircon megacrysts in kimberlites, peridotites, alkali
basalts and other magmas suggest the fast transport and short interaction
between zircon and melt.The origin of zircon megacrysts is likely related to
metasomatic addition of Zr into mantle as any mantle melting episode should
obliterate them.
