The Ligand Chemistry of Crustal Fluids

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A knowledge of element fractionation between aqueous fluids and minerals or melts is essential for the understanding of many geochemical processes. However the dissolved load of an aqueous fluid is primarily dependent on figand concentrations, and hence fluid-mineral and fluid-melt fractionation factors will in general also depend on the ligand content of the specific fluid of interest. Unfortunately, this cannot be deduced ab initio from the chemistry of the solid rock or mineral end product, although good indications of total 'equivalent salinity' can be obtained from fluid inclusion microthermometry. The key to the dissolved load of most crustal fluids is the chloride concentration. For fluids equilibrated with a given plagioclase feldspar, the Ca/Na ratio of the fluid depends on its salinity (Orville, 1972), with more saline fluids having higher proportions of Ca. Many other potentially important ligands form minerals with Ca (e.g. fluorite, apatite, calcite and gypsum or anhydrite), and saturation with these minerals at a given Ca concentration provides an upper limit to the relevant ligand's concentration. Hence they will tend to be less abundant, both absolutely and relative to C1, in more saline fluids. Amounts of other potentially important ligands, including BO3, H2S and Br, are not so readily constrained.