The Role of Halogens in Terrestrial and Extraterrestrial Geochemical Processes

Abstract

Terrestrial worlds on which H2O-bearing fluids are stable tend to show a higher degree of chemical differentiation than similar worlds on which such fluids are not present or stable. Other than CO2 and S, important components of these H2O-bearing fluids are the halogens, specifically the more common F and Cl and, to a lesser degree, the less common Br and I. When taken together in combination with H2O, F and Cl are potentially powerful ligands with respect to solution-enabled mass transport of most metal cations. This characteristic has indirect implications with respect to the reactivity of halogen-bearing fluids with many mineral species from among the silicates, oxides, sulfides, and orthophosphates and with melts. The low wetting angle of Cl-bearing solutions along grain boundaries, coupled with a high reactivity, allow them to flow easily along and through grain boundaries under mid-crustal to lower crustal/lithospheric mantle P-T conditions, thus aiding both mineral re-equilibration and mass transfer on a local (cm’s) as well as on a large (km’s) scale during metamorphism. The low H2O activity of these brines under high-grade conditions enables them to co-exist with nominally anhydrous Fe-Mg silicate minerals characteristic of the lower crust and the lithospheric mantle, such as orthopyroxene, clinopyroxene, and olivine. Evidence for the presence of these brines is seen in the fluid and melt inclusions found in the minerals from igneous rocks, ore deposits, sedimentary rocks, lower crustal rocks, and the lithospheric mantle. In magmas, halogens play an important role with regard to the solubility of various metal cations as well as affecting crystallization temperatures. During crystallization of plutonic magma bodies, while F tends to be retained in felsic melts, fluorapatite, and micas, Cl is expelled as a NaCl-KCl-CaCl2-bearing fluid into the surrounding country rock taking with it a large variety of other metal cations. These are subse- quently deposited in associated fissures, veins, and pegmatites as ore minerals/ deposits during crystallization of the main magma body. The presence of such fluids has been confirmed by extensive fluid inclusion and experimental studies. Beyond the Earth, halogens play an important role in the geochemistry of other terrestrial worlds in the solar system on which H2O can exist as a fluid whether on or beneath the surface. These include Mars, volatile-rich asteroids, and those ice moons of Jupiter and Saturn believed to contain subsurface oceans. In each of these cases both direct observation and terrestrial analogues allow for a more complete understanding of the role that halogens play in the various H2O-aided geochemical processes present on these worlds. The goal of this volume is to bring together a diverse group of geo- scientists with long-range interests, knowledge, and experience concerning the role that halogens play in or during a variety of geochemical and hydrothermal processes in the crust and mantle of the Earth as well as on volatile-rich asteroids, Mars, and the ice moons of Jupiter and Saturn in a series of review chapters. The range and depth of knowledge contained within these chapters regarding the role of halogens in geo- chemical processes, both terrestrial and extraterrestrial, outline and provide a firm foundation on which to base our current understanding of how halogens contribute to the geochemical/geophysical evolution and stability of liquid H2O-bearing terrestrial worlds in the solar system overall.