Komatiites are ultrabasic lavas, erupted at liquidus temperatures approaching 1600°C. With a handful of exceptions, they are not considered to have formed since the Palaeoproterozoic (i.e., earlier than 2 billion years ago). This is attributed to our planet’s heat being significantly higher in early Earth history. The distribution of komatiite lavas preserved in the geological record thus has the potential to yield important insights into the cooling of the Earth. Of particular relevance to the study described here is the existence of komatiites with ages <2 billion years old. Such occurrences present opportunities to elucidate plate tectonics and magmatic activity resulting from anomalously high heat flow in the mantle.
One of the youngest known komatiites was believed to exist in the Shetland archipelago (UK). It was thought to be Neoproterozoic in age (~700 million years old), and was the topic of a lively debate in the 1980s (see the research papers by Flinn & Moffat in 1985 and by Nesbitt & Hartmann in 1986). The rock outcrop occurs within a sequence of submarine-deposited volcaniclastic rocks called the Dunrossness Spilites, which crop out on the Shetland mainland, and considered to be youngest Proterozoic-to-Ordovician in age. The interpretation of these rocks as komatiite was used to suggest dramatic ‘total adiabatic melting’ of the mantle due to rifting of the ancient continent, Laurentia. This model had a significant bearing on the development of models for sedimentary basin formation during deposition of the upper part of a thick stratigraphic succession known as the Dalradian Supergroup, on Shetland.
Our new study, an international collaboration between US and UK scientists and published in the Journal of the Geological Society (London), has attempted to resolve the issue surrounding the interpretation of the Shetland komatiite, using geochemical techniques.
The rocks in question are highly altered by secondary processes (serpentinisation), so we have employed a combined approach of measuring osmium (Os) isotopes and highly siderophile element (HSE; Os, Ir, Ru, Pt, Pt + Re) abundances. An important advantage of the application of the HSE to the study of magmatic processes is that they have siderophile and chalcophile tendencies, and are much less susceptible to silicate melt modification or secondary alteration in the same way as the lithophile elements. Our combined dataset shows that the so-called komatiites are actually samples of the Earth’s mantle, which started out as rocks called peridotites but have since experienced significant seafloor alteration. This result is intriguing because ~50 km to the north, on the islands of Unst and Fetlar, the Shetland Ophiolite Complex (SOC) occurs. It represents oceanic lithosphere formed during closure of the Iapetus Ocean at 492 Ma, and contains a lowermost mantle portion. In light of our new results, known offshore geophysical (gravity, aeromagnetic) anomalies suggest that the ophiolite may extend all the way from Unst and Fetlar to the Dunrossness locality, such that the area covered by the ophiolite is an order of magnitude greater than previously considered. This would make it one of the largest ophiolite masses in Europe.
The full paper citation is:
Day, J.M.D., O’Driscoll, B., Strachan, R.A., Daly, J.S. and Walker, R.J. 2017. Identification of mantle peridotite as a possible Iapetan ophiolite sliver in south Shetland, Scottish Caledonides. Journal of the Geological Society, London, 174, 88-92.
You can also access the paper free online here at the University of Manchester holdings.
Further suggested research paper reading:
Flinn, D. & Moffat, D.T. 1985. A peridotitic komatiite from the Dalradian of Shetland. Geological Journal, 20, 287–292.
Nesbitt, R.W. & Hartmann, L.A. 1986. Comments on ‘A peridotitic komatiite from the Dalradian of Shetland’ by D. Flinn and D. T. Moffat. Geological Journal, 21, 201–205.
O’Driscoll, B., Day, J.M.D., Walker, R.J., Daly, J.S., McDonough, W.F. and Piccoli, P.M. 2012. Chemical heterogeneity in the upper mantle recorded by peridotites and chromitites from the Shetland Ophiolite Complex, Scotland. Earth and Planetary Science Letters, 333-334, 226-237