Inspection of rock samples from Holuhraun lava indicates that the erupted magma contains minor amount of plagioclase phenocrysts (<1%) and even scarcer olivine phenocrysts. Chemical analyses show that the magma is slightly olivine-normative tholeiite (0-5 wt% ol).
A few samples have been cut and polished for inspection, photography and chemical analyses with a scanning electron microscope equipped with a backscatter-electron detector and an energy-dispersive spectrometer (SEM-EDS). It is seen that the samples have crystallized to a variable degree, but in general the groundmass mostly comprises subophitic intergrowth of plagioclase and augite (Figure 1), along with smaller amounts of olivine. All samples also contain variable amounts of interstitial glass, whereas Fe-Ti oxides are absent or in very small amounts only. This suggests that the samples had quenched before Fe-Ti oxides could form.
Chemical analyses with the SEM-EDS instrument indicate that the groundmass plagioclase has the composition An75-80, while olivine and augite have Mg number 75-80. The institial glass has become quartz normative. The composition of two plagioclase phenocrysts that were measured was about An84 and An90.
Immiscible sulfides globules, noted in highly glassy samples of the first eruption, persist in later, more crystallized samples of the newer Holuhraun lava (Figure 2). The composition of the globules seems to vary to some degree, but Fe is always the most abundant cation, followed by Cu and Ni and minor amounts of Co. The relative abundance of Cu and Ni is varies significantly, even in a single globule.
Figure 1. SEM-EDS images of sample WM1491-4 from Holuhraun lava showing intergrowth of plagioclase, augite and olivine along with some interstitial glass; a) backscatter-electron image; b) X-ray map for Mg with olivine as the brightest mineral, then augite and plagioclase darkest .
Figure 2. SEM-EDS images of sample KPRW-5 from Holuhraun lava showing intergrowth of plagioclase and augite, and some interstitial glass and a few sulfide globules; a) backscatter-electron image; b) X-ray map for S revealing the presence of sulfide globules as bright spots.