Origin and physical-chemical control of topaz crystallization in felsic igneous rocks: Contrasted effect of temperature on its OH-F substitution

In highly evolved F-rich felsic igneous rocks, topaz Al2SiO4(F,OH)(2) forms in different genetic stages: orthomagmatic, pegmatitic, transitional magmatic-hydrothermal, and hydrothermal. The present study, considers some of the problems involved in deciphering the origin of OH-F variations in topaz. We conclude that topaz compositions in magmatic and post-magmatic hydrothermal stages range from pure fluor-topaz to about Al2SiO4F1.3(OH)(0.7). Magmatic topaz in rhyolites and topazites is extremely enriched in F, and topaz of hydrothermal origin may be more enriched in hydroxyl components. During the transition from magmatic to subsolidus hydrothermal conditions, pegmatitic topaz and vapor-phase topaz found in cavities and vugs are F-rich as well. The OH/(OH + F) ratio of topaz is commonly used as geothermometer for some strongly fractionated F-rich felsic igneous rocks and associated hydrothermal products. It seems to provide reasonable temperatures for transitional magmatic-hydrothermal/hydrothermal topaz, but for magmatic topaz, this geothermometer clearly conflicts with experimental data. Thus, a review of the chemical composition of topaz in relation to its formation temperature is undertaken for different geological provinces. This review indicates two composition fields, the first is that of hydrothermal topaz and topaz in vapor-phase cavities where the F-content increases with temperature, and the second field is that of magmatic topaz where this trend is reversed. Some data from: i) recent experiments in F-rich magmatic systems; ii) the distribution of fluorine among topaz and coexisting mica; and iii) chemical zoning within single grains of topaz at both magmatic and hydrothermal conditions, lend support to the hypothesis that fluorine concentrations in primary magmatic topaz increase with decreasing temperature. In addition, magmatic cooling tends to favor partitioning of fluorine into topaz rather than coexisting mica, whereas transitional magmatic-hydrothermal/hydrothermal processes result in concomitant distributions of fluorine into such F-rich aluminosilicate minerals. These observations have important implications for the genesis and evolution of metaluminous to peraluminous igneous systems, especially for distinguishing primary magmatic versus secondary hydrothermal and vapor-phase topaz.

Automated summary

In highly evolved F-rich felsic igneous rocks, topaz Al2SiO4(F,OH)(2) is formed in different genetic stages including orthomagmatic, pegmatitic, transitional magmatic-hydrothermal, and hydrothermal stages. The composition of topaz varies from pure fluor-topaz to Al2SiO4F1.3(OH)(0.7) in magmatic and post-magmatic hydrothermal stages. The OH/(OH + F) ratio of topaz is commonly used as a geothermometer, providing reasonable temperatures for transitional magmatic-hydrothermal/hydrothermal topaz but conflicting with experimental data for magmatic topaz.