A review of the geochemical changes occurring during metamorphic devolatilization of metasedimentary rocks

Abstract

Abstract Redistribution of elements by fluids produced by the metamorphic devolatilization (MDV) of sedimentary rocks is one of the major processes responsible for the differentiation of the crust. Comparison of compositions of rocks with similar original composition (protolith) with increasing metamorphic grades is the principal method of observation and quantification of the mass transfer caused by the loss of fluids. This work summarises observations from metamorphic complexes covering a wide range of PT conditions and lithologies that have been studied worldwide for the chemical effects of metamorphism. The metamorphic complexes range from coherent sedimentary sequences where the same stratigraphic horizons have been traced across different metamorphic zones, to folded turbidite piles and accretionary prisms where terranes with different peak PT conditions are divided by tectonic boundaries. The geochemical changes caused by the MDV are different in different complexes. Some elements are depleted in almost all complexes, whereas others are affected only in exceptional cases. The major elements, and the majority of lithophile and siderophile elements are not depleted by metamorphic devolatilization. Mercury is the first element to be lost from sediments at conditions of sub-greenschist metamorphism, which may coincide with the loss of Cd. Arsenic depletion is almost universal in metamorphic rocks, and is correlated with the decomposition or recrystallisation of sedimentary pyrite during greenschist facies metamorphism, where it can occur at similar temperatures both in regional metamorphic rocks and in subduction zone metamorphism. Gold, Sb, Ag, and W depletion are documented to coincide with As loss in regional metamorphic complexes. The content of C and S changes in some cases but remain conserved in others. Base metals including Cu, Pb and Zn are typically conserved during metamorphic devolatilization but exceptions have also been reported, where these metals are progressively depleted during upper greenschist and lower amphibolite facies metamorphism. Chlorine can be extracted by the metamorphic devolatilization in some metamorphic sequences, whereas F is conserved. The majority of lithophile elements show conservative behaviour during metamorphic devolatilization including REE, HFSE, and LILE, with the exceptions of Cs, N, U, and B in some complexes, where the depletion in some cases is associated with dehydration and in others possibly with the beginning of anatexis. The loss of hydrous fluids may be limited during high-pressure metamorphism and in sediments with abundant carbonates; under these conditions depletion of trace elements can be insignificant. Metamorphic devolatilization has the potential to be an efficient and powerful mechanism for the concentration of dispersed elements into ore-grade mineralisation. The connection between devolatilization and the origin of orogenic gold deposits is the most established in this regard, and some other deposit types are likely related to specific types of metamorphic fluids.