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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: $$ {\text{F}}_{{{\text{melt}}}} {\left( {{\text{wt}}\% } \right)} = {\left[ {{\left( {{\text{Na}} + {\text{K}}/{\text{Al}}} \right)}{\left\{ {2.1110^{{ - 3}} {\text{T}}{\left( {^\circ C} \right)} + 1.5778} \right\}}} \right]} - 2.789 $$ 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.
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Bruno Scaillet, Ray Macdonald. Fluorite stability in silicic magmas.. Contributions to Mineralogy and Petrology, Springer Verlag, 2004, 147, pp.319-329. ⟨10.1007/s00410-004-0559-1⟩. ⟨hal-00023899⟩



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