The provenance of Icelandic volcanic sand: Significance of black sand origin and implications for sampling Mars regolith

About one-fifth of Iceland’s surface is characterized by basaltic volcanic sand deposits. The provenance of the sands, the factors that lead to enrichments from one volcanic source or another, and whether the detritus is representative of the local or regional bedrock is not well understood. Also, basaltic sand regolith is widespread on Mars, and has the potential to provide important information about Mars’ regional volcanic geology, with a more detailed understanding of the sampling and analysis strategies that might be most effective, based on the Iceland sands results.

This study investigates the provenance of ten Icelandic volcanic sand surface deposits collected from beach, dune, river levee and glacial outwash settings across the island. This is achieved by analyzing individual grains of sand, and comparing their chemical compositions to samples from the various volcanic systems on Iceland using published and unpublished data sets. The dominant grain types in each sand deposit are basaltic volcanic glass, plagioclase, clinopyroxene, olivine and siliceous volcanic glass. Some or all of these grain varieties have been analyzed from each sample for major and minor elements via electron microprobe analyses (EMPA). The majority of the grains analyzed by EMPA have been analyzed further for trace element concentrations via laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). All ten Icelandic sand samples have also been analyzed in bulk for major, minor, and trace elements via solution inductively coupled plasma optical emission spectrometry (ICP-OES), or inductively coupled plasma mass spectrometry (ICP-MS), depending on the element.

The results of the study show that the individual sand grains are commonly linked to two or more relatively proximal (within ~100km) volcanic sources. The volcanic glasses appear to be particularly distinctive provenance indicators. Individual mineral grains are found to be much less distinctive, at least in terms of the major element composition, and generally cannot be linked to a specific volcanic source unambiguously. However, mineral grains that have a chemistry that is anomalous in the context of Iceland can be more reliable indicators of provenance.

The composition of the Icelandic sand surface deposits appears to be influenced by several factors. The volcanic sources that have contributed to each sand deposit are almost always explosive volcanic systems, or otherwise have been the site of eruptions having a significant explosive component. Fluvial transport has also had an important effect on the composition of the Icelandic sands, such that when a direct fluvial transport route exists between a given volcanic source and a sample location, volcanic debris from that source is commonly present in the sample. At least in some cases, other factors such as large eruption volume and high eruption frequency have also had an effect on the composition of Iceland’s surface sand.

Most of the sands samples under study were found to have compositions that reflect the volcanic geology of the local or regional “volcanic zone” where the sample was collected. When all ten samples are viewed collectively, they provide information about a significant portion of Icelandic regional geology. Implementation of a similar sampling and analysis strategy for the Mars regolith would likely provide a similar level of information about the regional geology of Mars, while limiting collection sites to a few specific dune fields and eliminating the need for bedrock drilling and transportation of heavy cores.