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

M. Mintrone (1) , A. Galli (1) , Oscar Laurent (2, 1) , C. Chelle-Michou (1) , M.W. Schmidt (1)

1 ETH Zürich - Eidgenössische Technische Hochschule - Swiss Federal Institute of Technology [Zürich]
2 GET - Géosciences Environnement Toulouse

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 F melt can be determined through F melt = FPZ × [Zr] bulk / [Zr] sat. The such obtained F melt corresponds to the melt fraction in the hand specimen at the time the system closed for melt mobility. Thermodynamic modelling further allows estimation of H 2 O-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 F melt 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 F melt 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.

Mots clés: Zircon; Melt fraction; Migmatites; Crustal anatexis

Domaines: Géochimie

M. Mintrone, A. Galli, Oscar Laurent, C. Chelle-Michou, M.W. Schmidt. Quantifying frozen melt in crustal rocks: A new melt-o-meter based on zircon rim volumes. Chemical Geology, 2020, 551, pp.119755. ⟨10.1016/j.chemgeo.2020.119755⟩. ⟨hal-02934816⟩