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

Abstract

Abstract 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 concentration 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- (~400 °C) to granulite-facies conditions (>900 °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. Suprasolidus 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 °C and 850 °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 °C), in particular, biotite breakdown reactions, for the release of Li, Be, Sn, Cs, and W into crustal melts.