FLUORITE STABILITY IN SILICIC MAGMAS

Abstract

Recent experimental evidence is used to assess the conditions under which fluorite forms an early crystallising phase in silicic magmas. Fluorite solubility primarily depends on the (Na + K)/Al balance in the coexisting silicic melt, reaching a minimum in metaluminous melts. It can display reaction relationships with topaz and titanite, depending on changes in melt composition during crystallisation. An empirical model of fluorite stability in Ca-poor peralkaline rhyolite melts is derived and applied to selected rocks: <formula form="DISPLAY" disc="MATH"><![CDATA[$$ {\text{F}}_{{{\text{melt}}}} {\left( {{\text{wt}}\% } \right)} = {\left[ {{\left( {{\text{Na}} + {\text{K}}/{\text{Al}}} \right)}{\left\{ {2.1110^{{ - 3}} {\text{T}}{\left( {^\circ C} \right)} + 1.5778} \right\}}} \right]} - 2.789 $$ ]]></formula>

At the F contents preserved in most silicic rocks, fluorite should normally appear late in the crystallisation sequence, in agreement with petrographic observations. During fluid-absent crustal anatexis, fluorite should melt at a relatively early stage and restitic fluorite is unlikely to persist during prolonged melting. Fluorite may, however, exert a decisive control on the alkali/alumina balance of sub-aluminous anatectic melts and it can affect the liquid line of descent of silicic magmas once extracted from source.