ZIRCON PRESERVATION IN MAFIC MELTS: A THEORETICAL AND EXPERIMENTAL STUDY

Zircon is one of the most ubiquitous crustal accessory minerals. They provide dates of events and information on conditions at which they crystallized. The improvements in the knowledge of dissolution, growth, and survival processes of zircons have significantly advanced our understanding of crustal evolution. Recently, zircons are becoming increasingly relevant for characterizing the petrogenetic processes in mafic and ultramafic systems. Zircons have been found in mantle and mantle-derived rocks. However, the presence of inherited zircon in these rocks is enigmatic because they must be unstable in these systems. One of the most important unresolved questions is the preservation of zircons in mafic melts. There are two main factors that control the preservation of zircon in a molten system: i) the zircon saturation limit and ii) the rates of Zr diffusion in the melt. The calculated Zr content to saturate zircon in a MORB basalt composition at 1200 °C reaches c. 26000 ppm. On the other hand, Zr diffusion coefficients in any melt composition can be estimated considering the melt viscosity [1]. Our calculations indicate a D_Zr c. 2.3*10^-12 m²/s for a MORB composition at the same temperature, which fits very well with direct experimental data [e.g. 2], where available. Using these parameters, we can estimate the dissolution rate of a spherical zircon suspended in a basalt. The results show that a zircon of 1 mm radius is dissolved in a few days at 1200 °C. Our preliminary experimental results of a zircon cube of 1 mm side suspended in a basalt melt for 24 h at 1200 °C show that the zircon volume was reduced to half and support these calculations. To explain the presence of zircons in these systems we performed some experiments shielding the zircon from the melt in a spinel lherzolite assemblage. The basaltic melt migration through the peridotite dissolved zircon; at 1200 ºC and 24 h, the dissolved volume of zircon was smaller than in the previous case. Therefore, the shielding mechanism can inhibit zircon dissolution to some extent, but not enough to account for long term preservation. Thus, the mechanism of preservation of zircon in a long time petrogenetic processes in mafic systems is still unresolved.

[1] Fanara et al., (2017) J. Non-Cryst Solids, 455: 6-16.

[2] Holycross and Watson, (2016) Contrib Mineral Petrol, 171:80.