Zircon Crystallization in Low-Zr Mafic Magmas: Possible or Impossible?

Zircon is a ubiquitous accessory phase that carries important isotopic and geochemical information. Experimental work indicates that zircon solubility in silicate melts increases exponentially with decreasing silica; accordingly, crystallizing mafic magmas would only precipitate zircon from the last drops of residual melts, likely of granophyric composition. However, this view is inconsistent with the abundance of syn-magmatic zircons in many mafic rocks, in which zircon often occupies textural positions compatible with early crystallization. Given that factors other than temperature and magma composition negligibly affect the solubility of zircon, its precipitation from mafic magmas must involve the formation of small zircon-saturated transient zones. Here we explored that possibility using 2D finite elements to model the crystallization of MORB melts confined in pores. We found that zircon-saturated volumes may form locally at the growing mineral-melt interfaces if the growth rate of a low KdZr mineral (<0.2) is much faster than the diffusion rate of the rejected Zr4+ away into the melt, thus leading to the precipitation of zircon in low-Zr mafic magmas. Local saturation close to interfaces growing in confined pores also explains the wide range of crystallization temperatures displayed by zircons of mafic rocks, the variety of their inclusions and textures, and in the particular case of the Atlantic seafloor, why syn-magmatic zircons are more common in troctolites and olivine gabbros than in the more evolved clinopyroxene or amphibole-rich gabbros. This mechanism can also account for the formation of other accessory phases composed of elements scarcely partitioned in major minerals.