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

Abstract

Rare metals like Li, Be, V, Co, Nb, In, Cs, Sn, Ta, and W are considered critical resources and can be significantly enriched in granites and pegmatites. However the mechanisms of their enrichment in granitic magmas 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 their concentration and mobilisation during crustal anatexis. Comprehensive datasets of these elements and their partitioning in metamorphic micas across different metamorphic grades are currently lacking. We present the first extensive in-situ LA-ICP-MS element dataset collected from metasediment hosted muscovite and biotite from three different metamorphic cross-sections traversing sub-greenschist (~400ºC) to granulite-facies conditions (>900ºC). Within the same sample Li, V, Co, Cs, and Ta are more concentrated in biotite, while Be, In, Sn, and W concentrations are higher in muscovite. Sub-solidus micas record only non-systematic compositional variations between samples. Supra-solidus 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 concentrations reach peak concentrations in biotite at 750ºC and 850ºC respectively. Muscovites record systematic enrichment in In and W and depletion in Be, Sn and Cs with increasing metamorphic grade. These distinctive trends appear independent of geological/tectonic setting (i.e. continental collision and crustal thinning). Our dataset highlights the importance of higher-temperature melting (>750ºC) and in particular, biotite breakdown reactions for the release of Li, Be, Sn, Cs and W into crustal melts