The fluorine content of Icelandic tholeiitic and alkaline basalts matches values found in similar rocks from other areas. Covariation between fluorine and incompatible minor elements such as potassium or phosphorus is found in evolved tholeiites and alkali basalts. Lack of such covariation in primitive olivine tholeiites indicates that fluorine and other incompatible minor and trace elements are not controlled by minerals such as amphibole, mica or apatite in the mantle residue, and that the covariation between these elements in the evolved basalts cannot be inherited from the mantle. Model calculations on rocks from the Langj6kull area show that olivine tholeiite suites are, if derived by simple fractional crystallization, enriched in incompatible elements much in excess of the increase due to crystal removal. These observations are taken to indicate that the well documented covariation between fluorine and other incompatible elements is not established until evolution of the basaltic magma has started in crustal holding chambers. The constancy of element ratios and enrichment in excess of what can be accounted for by crystal fractionation or incremental addition of new batches of primitive magmas does indicate (1) mineral control involving amphibole, mica or apatite and (2) addition of fluorine, potassium and phosphorous from an external source. It is argued that this source is the crustal envelope of the holding chamber. behaviour of fluorine in subaerial basaltic lavas the first part of the paper discusses fluorine abundances in well documented samples from recent volcanic eruptions in Iceland. The second part describes the fluorine abundances in selected suites of rocks from Iceland in order to detect differences between different genetic groups and, if possible, to resolve to what extent the rocks reflect the halogen abundance in the mantle beneath Iceland.
Analytical procedure
Fluorine was determined by using the method of Ingram (1970). The sample was melted with ZnO and Na2CO3, and the cakes leached with water. Fluorine is determined with a ion-specific-electrode (ISE) in the leachate which was maintained at pH 5.25 with TISAB. The following international reference samples were used for calibration: GSP, (GSP+G2)/2, G2, BCR, GM, G-2/10, GM/ t0, BCR/10. Table 1 lists duplicate determinations of fluorine in a few samples. Potassium was determined using XRF (Rose et al. 1963) and atomic absorption spectrometry (Jeffery 1975). Good consistency was obtained between the methods and special care was taken with samples containing less than 0.1 percent K20.
Fluorine in closely related volcanic products
The distribution of fluorine among related volcanic products (e.g. lava flow, tephra layer) were studied. We analysed samples from recent lava flows in Iceland and older lavas and tephras from several volcanoes (Fig. 1).