Redistribution of heat-producing elements during partial melting of felsic rocks of the lower continental crust, Ivrea Verbano Zone

The geochemical differentiation of the Earth’s crust is influenced by high-temperature metamorphic processes. Partial melting of the lower continental crust leads to the redistribution of the heat-producing elements (HPEs), which are responsible for ca. 60% of the crustal heat flux. The major carriers of HPEs in typical lower crustal rocks are accessory minerals, such as zircon and monazite. During partial melting, the solubility of these accessory phases in anatectic melts controls the redistribution of the HPEs. The felsic rocks of the lower continental crust exposed in the Ivrea-Verbano Zone (IVZ) host significant amounts of HPEs and they can be used as a natural laboratory to investigate the link between melting reactions, solubility of accessory minerals and migration of HPEs.

This study focuses on the felsic metasediments of boreholes 5071-1A and 5071-1B, which have been recently drilled in the framework of the ICDP-DIVE project. Metapelites and metapsammites show macroscopic and microscopic signs of partial melting, such as the segregation of leucocratic domains. Petrographic observations, geothermobarometric calculations and thermodynamic modeling show that partial melting in 5071-1B rocks occurred at upper amphibolite facies conditions, at P-T conditions of ca. 7 kbar and 750°C and is predominantly controlled by muscovite dehydration melting. Instead, 5071-1A lithologies experienced temperatures in excess of 900°C, associated with extensive anatexis related to biotite dehydration melting.

The spatial distribution of the accessory minerals has been determined with a combination of SEM chemical mapping and BSE imaging, supervised classification of minerals by XMapTools and counting statistics by image analyses techniques. Furthermore, a full trace element budget has been performed by means of LA-ICPMS analyses on major minerals and accessory phases.

Our results indicate that the HPEs budget is high in the amphibolite facies part of the sequence, with values that are around five times greater than average lower crustal values and comparable to values typical for middle and upper crust (Rudnick and Gao, 2014). The higher-grade felsic rocks are relatively depleted and more similar to previous estimates for the lower crust. The primary hosts of U and Th at amphibolite facies are monazite, allanite, and, to a lesser extent, zircon and apatite. The U-Th budget is shared between zircon, monazite and rutile in the higher-grade equivalents.

Our results offer novel insights on the factors controlling the behavior of the accessory minerals during partial melting and permit to investigate the applicability of models and solubility equations in comparison with natural rocks. At UHT conditions, the partial preservation of zircons and monazites (such as inherited zircon cores and metamorphic monazites) proves that these minerals are not fully dissolved in partial melts even at extreme crustal temperatures. A representative migmatite shows that biotite-sillimanite melanocratic domains, interpreted as restites formed after melt loss, are relatively enriched in Th, U, and K compared with the interlayered leucocratic domains that have experienced net melt gain. In particular, monazite is enriched by approximately one order of magnitude in the melanosome, in contradiction with predictions from models, indicating that the redistribution of HPEs in the crust is more complicated than previously thought.