Quantifying frozen melt in crustal rocks: A new melt-o-meter based on zircon rim volumes

Abstract

Quantifying the distribution of granitic melt at all scales in mid- to lower crustal migmatitic terranes is critical to understand crustal melting processes, chemical differentiation of the crust and its rheological behavior during deformation. We propose a new method to determine the fraction of frozen granitic melt on a hand specimen scale based on the relative volumes of newly precipitated to total zircon (FPZ = Fraction of newly Precipitated Zircon) as obtained by image analysis on dated zircon cores and rims. Using the calculated Zr-solubility [Zr]sat in the melt at the inferred melting temperature and the Zr concentration in the bulk sample [Zr]bulk, the fraction of melt Fmelt can be determined through Fmelt = FPZ × [Zr]bulk / [Zr]sat. The such obtained Fmelt corresponds to the melt fraction in the hand specimen at the time the system closed for melt mobility. Thermodynamic modelling further allows estimation of H2O-contents required to maintain the melt fraction obtained from the melt-o-meter in a molten stage. The applicability of this method has been tested on eight migmatitic samples with peak temperatures between 725 and 925 °C. Most of the lower temperature migmatites (<750 °C) retained Fmelt of 0.15–0.25 (±0.03–0.05), a melt fraction below or at the static melt escape threshold. In contrast, most of the higher-temperature migmatites (>800 °C) retained Fmelt of 0.35–0.50 (±0.07–0.10). At these melt fractions, melt extraction and melt migration from and within the source should be efficient. Consequently, these samples are likely open-system migmatites affected by melt accumulation or depletion processes. The melt-o-meter requires that the rock types under consideration produced a granitic melt that remained zircon-saturated and is therefore restricted to migmatitic meta-sediments and meta-granitoids. When applied carefully, this melt-o-meter offers a new and powerful tool to not only quantify melt distribution but also evaluate the extent of melt mobility in migmatites.