Fluorine geochemistry in volcanic rock series: examples from Iceland and Jan Mayen

Abstract

A new analytical procedure has been established in order to determine low fluorine concentrations (30–100 ppm F) in igneous rocks, and the method has also proven successful for higher concentrations (100–4000 ppm F). Fluorine has been measured in a series of olivine tholeiites from the Reykjanes Peninsula, a tholeiite to rhyolitic rock series from Kerlingarfjöll, central Iceland, and an alkaline rock series from Jan Mayen that ranges from ankaramites to trachytes. Fluorine is not appreciably degassed during extrusion and appears to be insensitive to slight weathering. The olivine tholeiites from the Reykjanes Peninsula have F contents of 30–300 ppm and exhibit linear increases proportional to the incompatible elements K, P, and Ti. Such incompatible behaviour for F has been confirmed for the less evolved rocks of the other series. The tholeiites from Kerlingarfjöll (100–2000 ppm F) show a linear increase with K 2O, whereas the F content decreases slightly for the andesitic to rhyolitic rocks. The change from a linear increase with K for the tholeiites to a moderate decrease in F for the more evolved rocks is associated with a sharp decrease in P, suggesting that apatite controls the F distribution for these rocks. When the activity of P is too low to allow for significant control of F by apatite the F abundances increase towards the most K-rich rhyolites. Similar relationships between F, K, and P are also apparent for the alkaline Jan Mayen rock series, but the shifts in F and P behaviour occur at higher K 2O and SiO 2 concentrations.