INVESTIGATING THE PROVENANCE OF BLACK SAND IN ICELAND: LOCAL VS. DISTAL SOURCES

Major, minor, and trace element geochemistry of individual sand grains is used to investigate the provenance of the volcanic sand at Vik Beach, Iceland, located within the highly active Southern Volcanic Flank Zone (SVFZ). Grains of basaltic volcanic glass, siliceous volcanic glass, clinopyroxene, and plagioclase have been analyzed via EMP and LA-ICPMS, and compared to data collected by other workers from volcanic systems in the SVFZ. The purpose of the study is to determine the efficiency of mixing of material at the surface of the sand deposit based on the contributions made by potential volcanic sources in SVFZ. Surface material that is relatively well mixed might represent the average bedrock composition of the source area, whereas the composition of poorly mixed material might represent a single and probably local volcanic source.

Tholeiitic basaltic glass from Vik Beach has a composition that most closely resembles that of basalts from the Bárðarbunga-Veidivötn volcanic system. Transitional-alkaline basaltic glass is also present and resembles basalts from the Katla-Eldgjá volcanic system. Plagioclase and clinopyroxene from Vik Beach are also compositionally similar to crystals from these systems, but there is some evidence for minor input of clinopyroxene crystals from the Eyjafjallajökull central volcano. All of these sources are in the SFVZ. Most of the siliceous glass has a compositional range similar to rhyolites produced from the Katla central volcano in the SFVZ; however, some of the siliceous glass grains appear to be sourced from volcanoes outside the SFVZ.

The results suggest surficial mixing of the sand at Vik Beach is moderate at best since only nearby (≤ 60 km) Bárðarbunga-Veidivötn and Katla-Eldgjá volcanic systems appear to be the main sources, despite the presence of several other potential volcanic sources in the SFVZ. This is probably related mainly to the large volumes of material that have been produced by these two systems. However, it appears that some of the siliceous glass from Vik Beach was sourced from other, more distal volcanic sources. This could indicate that siliceous volcanic detritus is transported over greater distances compared to the other grain types as a result of relative durability.