The Role of Halogens During Regional and Contact Metamorphism

Abstract

Halogens are important elements for a range of geological processes during metamorphism from stabilizing mineral phases to being important ligands for mass transfer. Halogens are highly incompatible in most minerals, which makes it difficult to unravel their presence in the past. Minerals useful for understanding halogen behaviour during metamorphism include: scapolite, apatite, titanite, biotite, and amphibole. However, their ability to incorporate halogens depends on parameters such as bulk rock composition, fluid properties, and water-rock ratios. Comprehensive studies of halogens in regional metamorphic rocks and minerals, such as the Clearwater Region, Idaho, USA or the Mary Kathleen Fold Belt, Mt Isa Inlier, Australia, show that halogen contents are highly variable on a bulk rock- and rock layer-scale, reflecting protolith variations. Where low fluid-rock ratios occurred during regional metamorphism, pre-exisiting variations in halogen compositions and ratios across individual layers were not eliminated, resulting in large differences between halogen concentrations on a mineral- and rock-layer scale. Research on F and Cl in apatite in siliceous marbles from five classic aureoles highlights the use of this mineral regarding rock or fluid buffering, and in establishing fluid sources. Chlorine enrichment in biotite and amphibole, associated with regional albitization observed in Cloncurry, Australia or the Bamble Sector Norway, demonstrate advection of saline fluids during albitization and K-feldspar metasomatism that occur in association with regional mineralization. Chlorine-bearing fluids are capable of mobilizing large amounts of metals during large-scale metamorphism on a regional, whole rock, and mineral scale. Consequently, fluid flow could be an essential prerequisite to actively discharge metals from the metamorphic rocks. Recent analytical advancements allow for more routine analyses of halogen contents in minerals and fluid inclusions. For instance, in situ LA-ICP-MS analyses of Cl and Br allow for the reconstruction of the interaction of halogen-bearing fluids with crustal rocks in complex geological settings that have undergone multiple hydrothermal events. In such cases, scapolite can be used as an archive for fluid properties during metamorphism. For example, within the Mount Isa Inlier, it was found that the fluids, which interacted with calc-silicates in the Mary Kathleen Fold Belt, were of bittern brine derivation contrasting with the Cloncurry Region, where the fluids show evidence of dissolved halite. Magmatic fluid interaction with calc-silicate rocks was found to be localized.