Mineralogical and geochemical variations across three basaltic lava flows from the Faeroe Islands

Abstract

Three basaltic lava flows from the Faeroe Islands, one from the lower, one from the middle and one from the upper lava series, have been sampled with a maximum vertical separation between samples of 2 m, and with two parallel sample profiles for each flow. The investigations based on this close sampling have shown that significant mineralogical and chemical variations are to be found within single extrusive bodies. On the basis of 97 chemical analyses, microscopic investigations comprising modal analyses and deter­mination of the oxidation stages of the Fe-Ti-oxides, and electron microprobe analyses of plagioclase, pyroxenes and Fe-Ti-oxides, the mineralogical and geochemical variations across the flows are described. Only averages of the chemical analyses for each flow are presented, but the results of all the 97 analyses can be seen in the figures, which give the variations across the flows for the main oxides, except P2O5 which hardly varies. In a TiO,-K20-P2O5 diagram the samples from the flows from the lower and upper series fall in the oceanic field, whereas the samples from the flow from the middle series cross the border between the oceanic and non-oceanic fields. In a F1-F2 diagram the samples from the lower and upper series fall in the within-plate basalts field, whereas the samples from the middle series fall in the field common to calc-alkali basalts and low­potassium tholeiites. In a F,-F3 diagram the samples from the middle series fall mainly in the field of low-potassium tholeiites; the contact samples fall in the middle of this field, while most of the other samples trend towards the field of ocean-floor basalts, and two samples from the middle of the flow actually reach the field of ocean-floor basalts. Plotted in a total alkalies versus silica diagram the samples all fall in the tholeiitic field well below the dividing line of Macdonald & Katsura ( 1964 ). The variations in the flows from the lower and middle series are interpreted as being due mainly to pre-eruptional processes whereas the variations in the flow from the upper series are believed to be due entirely to post-eruptional processes.