Critical metal enrichment in crustal melts: The role of metamorphic mica

Metals such as Li, Be, V, Co, Nb, In, Cs, Sn, Ta, and W are considered resources that are critical for modern economies. They can be significantly enriched in granites and pegmatites, but the mechanisms of enrichment remain poorly understood. Many metal-enriched granitic magmas form through mica dehydration reactions during high-grade metamorphism. The preferential incorporation of these metals into micas provides a mechanism for concentra-tion and mobilization during crustal melting. Comprehensive data sets of these elements and their partitioning in metamorphic micas across different metamorphic grades are currently lacking. We present the first extensive in situ laser ablation-inductively coupled plasma-mass spectrometry element data set collected from metasediment-hosted muscovite and biotite from three different metamorphic cross sections traversing sub-greenschist-(similar to 400 degrees C) to granulite-facies conditions (>900 degrees C). Within the same sample, Li, V, Co, Cs, and Ta concentrations are higher in biotite, whereas Be, In, Sn, and W concentrations are higher in muscovite. Subsolidus micas record only nonsystematic compositional variations between samples. Su-prasolidus biotites show systematic depletion in Li, Be, Sn, and Cs and enrichment in V and Co with increasing temperature in the highest-grade (muscovite-absent) samples. Indium and W reach peak concentrations in biotite at 750 degrees C and 850 degrees C, respectively. Muscovites record systematic enrichment in In and W and depletion in Be, Sn, and Cs with increasing metamorphic grade. These distinctive trends appear to be independent of tectonic setting (i.e., continental collision and crustal thinning). Our data set highlights the importance of higher-temperature melting (>750 degrees C), in particular, biotite breakdown reactions, for the release of Li, Be, Sn, Cs, and W into crustal melts.

Automated summary

This study investigates the enrichment mechanism of metals such as Li, Be, V, Co, Nb, In, Cs, Sn, Ta, and W in metamorphic processes. By analyzing muscovite and biotite from metasediment-hosted samples, the study reveals the preferential incorporation of these metals into mica minerals and their release during crustal melting. The findings highlight the importance of higher-temperature melting, particularly biotite breakdown reactions, for the mobilization of these critical metals.