Volcanic impact

Materials World magazine
,
1 Feb 2018

Mineral deposits on the Isle of Skye indicate a meteorite impact that could have caused the island’s earliest volcanic activity. Ellis Davies reports.

Minerals discovered on Scotland’s Isle of Skye, UK, show that the earliest volcanic activity there was enhanced by meteorite impact around 60 million years ago. Geologists at Birkbeck University London, UK, made the discovery while conducting research on silicic pyroclastic volcanism on the island and its place within the volcanological evolution. The discovery is the first ever reported on Earth for these mineral forms – unmelted vanadium-rich (TiVN) and niobium-rich osbornite (TiNbN).

Simon Drake, Associate Lecturer of Geology at Birkbeck and lead author of the paper, Discovery of a meteoritic ejecta layer containing unmelted impactor fragments at the base of Paleocene lavas, Isle of Skye, Scotland, told Materials World, ‘Skye is extremely important geologically. Several concepts regarding igneous petrology were formulated on the island.’ The island is famously well traversed by geologists and forms part of the British Palaeogene Igneous Province (BPIP), a volcanic region that extends from the Inner Hebrides of Scotland to Northern Ireland. 

The only previous recordings of the minerals were taken by NASA’s Stardust Comet Sample Return Mission – launched in 1999 – as space dust in the wake of the Wild 2 comet suggesting extra-terrestrial origin. The TiVN is unmelted, which points to it being an original piece of a meteorite. Pure osbornite (TiN) is very rare, and has only been found on Earth as trace amounts within carbonado diamonds – which may actually be extra-terrestrial in origin – and as inclusions within coesite in Tibetan ophiolite and coesite-bearing eclogite in the Dabie Mountains, China.

The team also discovered reidite, a form of zircon that is associated with impacts. ‘An extra-terrestrial origin for these mineral deposits is strongly supported by the presence of reidite, which is only found naturally at sites of meteorite impact,’ writes Drake. ‘Barringerite, baddeleyite, alabandite and carbon-bearing native iron spherules, together with planar deformation features and diaplectic glass in quartz, further support this thesis.’

A double discovery

Discoveries were made at two sites – An Carnach on the Strathaird Peninsula and on route B8083, 1.5km south-southwest of Broadford. The meteoritic ejecta layer, in which the minerals were found, at the An Canarch site has a composition of 47wt% silicon dioxide (SiO2), high aluminium oxide (Al2O3) and iron(II) oxide (FeO), and resembles volcanic ash altered to potassium rich clay. The second site contains heterolithic – sedimentary structure made up of interbedded deposits of sand and mud – components, including sporadic outsized blocks of basalt. The osbornite here is likely a carbonitride, as it contains 11wt% carbon according to the research.

The geologists placed the time of impact between 60–61.4 million years ago (Ma), constrained by a 60 Ma radiometric age for the overlying lava flow, and 61.4 Ma for a basalt clast embedded within the ejecta layer. The layers were analysed using an electron microprobe. 

‘We have already investigated further sites on Skye and have more evidence. We have also targeted other areas in the BPIP,’ said Drake. He adds that the age of the Skye meteoritic ejecta layer deposits, and their presence at the base of mid-Paleocene lavas, provoke important questions regarding the relationship between impact and early BPIP volcanism. The team will investigate the ejecta to find out if it extends further throughout the BPIP and possibly the wider North Atlantic Igneous Province.