5.1 Fluorine in Igneous Rocks
Many papers in the scientific literature have noted the association of higher F content in igneous rocks of high silica content, typically granites, granitoids, and rhyolites. Silicic igneous rocks are formed by one or more of three possible processes: assimilation, fractional melting, and/or fractional crystallization from a melt that is originally mafic. Assimilation is the incorporation of pre-existing silica-rich continental or near-shore rocks (sandstone, shales, greywackes, and their metamorphic equivalents) into magma as it rises from deep to shallower depths in the crust, making the magma more silicic. Fractional (or partial) melting is the melting of minerals with lower temperatures of fusion which would be the more silicic and sodic-rich minerals. Fractional crystallization (or differentiation) is the process by which a deep magma chamber which is usually strongly mafic in composition such as a basalt, begins to crystallize with the more mafic minerals crystallizing first, leaving the residual liquid magma less mafic and more silicic in composition.
Both assimilation and fractional crystallization seem to have occurred for the large rhyolitic strata at Yellowstone National Park (Christiansen, 2001; Hildreth, 1981; Hildreth et al., 1991). During early stages of magma cooling and fractional crystallization, F is “incompatible” in that it cannot enter the lattice structures of the first major minerals solidifying from the melt, so it becomes concentrated in the residual liquid. Regardless of the relative importance of these magmagenetic processes, there is a clear partitioning of incompatible trace elements and isotopes which favors the enrichment of F in the more silicic igneous rock and later hydrothermal fluids. A comparison of the average F content of the Earth’s mantle of 25 mg/kg (Palme and O’Neal, 2014) with the continental crust 553 mg/kg (Rudnick and Gao, 2003), also reflects this partitioning. A striking example of the relation between the F and SiO2 content of silicic rocks during magma evolution is the occurrence of beryllium deposits at Spor Mountain, Utah. After consideration of chemical and isotopic data, Dailey and others (2018) concluded that the rhyolites at Spor Mountain formed from a basaltic magma that intruded into a previously mixed or hybridized crust and then experienced extensive fractional crystallization before eruption. Glass from the less-evolved rhyolitic magma contained 0.7 by weight percent F and glass from the more-evolved rhyolitic magma contained 1.6 by weight percent F.
A complicating factor is that at least two processes can change the F content of an extrusive igneous rock after it crystallizes: devitrification and hydrothermal alteration. Christiansen (2001) published analyses of basalts and rhyolites from Yellowstone National Park that included F determinations. These values are plotted in Figure 3 and show the largest contents of F in rhyolites except for two samples of devitrified tuff. Devitrification allows easily soluble elements to be released more readily by weathering. All these samples have negligible hydrothermal alteration. Normally fluorite is found in rock that was mineralized from hydrothermal alteration, but it has been found also as phenocrysts in a peralkaline rhyolite from Kenya (Marshall et al., 1998).
An investigation on artificial recharge of a fractured and hydrothermally altered breccia pipe in South Africa showed that fluorine-rich apophyllite would release unacceptable amounts of fluoride into the groundwater system (Cavé, 1999).
Figure 3 – Fluorine content of basalts and rhyolites from Yellowstone National Park (data from Christiansen, 2001).