A tale of two magmas

Since the geological expedition of R.W. Bunsen to Iceland in the mid 19^th century, scientists have been puzzled by the frequent co-occurrence of basalt and rhyolite at many volcanoes. Bunsen, who also invented of the Bunsen burner, was the first to describe this phenomenon of “bimodal volcanism”, but these fundamentally different lava types have by now been found together at sites across the planet. Crucially, the mixing of basalt and rhyolite in a volcano’s magma chamber is a major cause of violently explosive eruptions, but in the 160 years since Bunsen’s observations, no consensus has been reached on how bimodal volcanism actually originates. A new article in “Nature Communications” now re-ignites the debate and offers a fresh perspective on bimodal volcanism at continental volcanoes. Using detailed chemical analyses of rocks from the Carlingford Igneous Centre, the roots of a large, extinct volcano in northeast Ireland, an international team of scientists suggests that the key control on bimodal volcanism could, in fact, be the crustal rocks that lie below the erupting volcano.

Sixty million years ago, the North Atlantic Ocean was only beginning to form and America and Europe were slowly breaking apart. This process was exacerbated by an increased flow of molten rock from the Earth’s mantle, known as a mantle plume, which caused extensive volcanism throughout northeast Ireland, Greenland and western Scotland. Fissure-fed basaltic lava, as seen at the Giant’s Causeway in Northern Ireland, was the most common type of activity, but a number of large volcanoes also formed. A key feature of these volcanoes was that they were short-lived and bimodal, producing significant amounts of light-coloured rhyolite and granite, as well as dark basalt. One such volcano was the Carlingford Igneous Centre, Co. Louth, Ireland. As the hot basaltic magma (>1200 °C) beneath Carlingford made its way from the mantle to the surface, it passed through the Earth’s continental crust, which is 30 km thick in this part of Ireland. “Luckily rocks from the crust and rocks from the mantle have characteristic chemical compositions, like geological DNA”, explains Dr Fiona Meade, the principal author of the article, “By using cutting-edge isotope analyses on the volcanic rocks from Carlingford, we can detect that the crust began to melt and that these melts were incorporated into the ascending magmas, transforming the basalt into rhyolite and granite”.

_{The Carlingford Igneous Centre, NE Ireland, erupted 60 million years ago but a new study published in Nature Communications reveals it has much to teach us about currently active volcanoes.}

Significantly, the team’s work has shown that the continental crust was most strongly involved during the early stages of activity at Carlingford. It appears that while a first flush of crustal melt was easy to extract, melting became increasingly difficult and granite formation quickly stalled. This is because not all minerals in crustal rocks melt at the same temperature, and while some components are readily incorporated into the magma, others are left behind and will never melt. “This research suggests that crustal melts are vital for the formation of rhyolite/granite magmas in continental volcanic systems, and that once the crust can no longer produce such melts, the volcanoes rapidly return to producing basalt – forming a bimodal rock suite” added Prof Valentin Troll, the team leader and chair of petrology at Uppsala University (Sweden). “Evidence of basalt-rhyolite magma mixing is preserved at Carlingford, indicating that violent eruptions are likely to have been triggered early in the lifetime of the volcano, and while Carlingford has not posed any danger for 60 million years, it gives us a major insight into the processes that drive currently active volcanoes”, he concludes.

_{Mixing of basalt (dark) and rhyolite (light) magmas, which is preserved in the rocks at Carlingford, can often lead to explosive volcanic eruptions}

This project was initiated at Trinity College Dublin by Prof Valentin Troll and Dr Fiona Meade, who are now based at Uppsala University (Sweden), and was supported by an international team of co-workers from institutions in the UK, Italy and the Netherlands. The research was funded by Science Foundation Ireland (SFI), the Irish Research Council for Science, Engineering and Technology (IRCSET) and the TEKNAT faculty at Uppsala University.

For more information please contact Prof Valentin Troll or Dr Fiona Meade.

Meade, F.C., Troll, V.R., Ellam, R.M., Freda, C., Font, L., Donaldson, C.H., Klonowska, I. (2014) Bimodal magmatism produced by progressively inhibited crustal assimilation. Nature Communications, doi:10.1038/ncomms5199

About the lead authors

Dr Fiona Meade received her PhD from Trinity College Dublin (Ireland) in 2009 and is currently a research associate at the Department of Earth Sciences, Uppsala University, Sweden. Besides academic research, she also runs her own business as a freelance scientist. Her research focuses on the interaction of magmas and the continental crust.

  

Professor Valentin Troll was awarded his PhD from the University of Kiel (Germany) in 2001. He holds the Chair of Petrology at the University of Uppsala and is co-director of the Centre for Experimental Mineralogy, Petrology and Geochemistry (CEMPEG). He is an expert on the mineralogy and chemistry of volcanic rocks and has studied volcanoes from Indonesia to Mars