Discovery of a meteoritic ejecta layer containing unmelted impactor fragments at the base of Paleocene lavas, Isle of Skye, Scotland

Abstract

Evidence for meteorite impacts in the geological record may include the presence of shocked minerals, spherule layers, and geochemical anomalies. However, it is highly unusual to find unmelted crystals from the actual impactor within an ejecta layer. Here we detail the first recorded occurrence of vanadium-rich osbornite (TiVN) on Earth, from two sites on Skye, northwest Scotland, which are interpreted as part of a meteoritic ejecta layer. TiVN has only previously been reported as dust from comet Wild 2, but on Skye it has been identified as an unmelted phase. Both ejecta layer sites also contain niobium-rich osbornite (TiNbN), which has not previously been reported. An extraterrestrial origin for these deposits is strongly supported by the presence of reidite (a high-pressure zircon polymorph), which is only found naturally at sites of meteorite impact. Barringerite [(Fe,Ni) 2 P], baddeleyite (ZrO 2 ), alabandite (MnS), and carbon-bearing native iron spherules, together with planar deformation features and diaplectic glass in quartz, further support this thesis. We demonstrate through field relationships and Ar-Ar dating that the meteorite strike occurred during the mid-Paleocene. This is the first recorded mid-Paleocene impact event in the region and is coincident with the onset of magmatism in the British Palaeogene Igneous Province (BPIP). The Skye ejecta layer deposits provoke important questions regarding their lateral extent at the base of the BPIP and the possibility of their presence elsewhere beneath the much larger North Atlantic Igneous Province. BACKGROUNDMeteorite impact deposits are found throughout the geological record. However, in this paper we detail the youngest recorded UK meteorite impact event, located beneath mid-Paleocene lavas (Fig. 1A) at 2 sites 7 km apart on what is now the Isle of Skye, northwest Scotland (Fig. 1B). The only other known meteoritic ejecta deposit in Scotland is much older (1177 ± 5 Ma) than the Skye deposits and occurs within Precambrian rocks on the Scottish mainland (Parnell et al., 2011;Reddy et al., 2015).We present compelling mineralogical and textural evidence within the Skye deposits for impact-derived shock metamorphism at pressures ≥30 GPa. Within the deposits at both sites, unmelted vanadium-rich osbornite (TiVN) and niobium-rich osbornite (TiNbN) are preserved as part of the actual impactor. Recognition of actual unmelted impactor mineralogy has previously only been from the Chicxulub crater on the Yucatan Peninsula (Kyte, 1998).

The Mesoproterozoic Stac Fada Member, NW Scotland: an impact origin confirmed but refined

“…The number of confirmed meteorite impact craters on Earth stands at approximately 200 (Osinski & Grieve 2019; see also www.impactearth.com for an up-to-date inventory). It is notable that there remains no confirmed impact crater in the British Isles, although the existence of both distal and proximal impact ejecta layers have been proposed from SW England (Walkden et al 2002) and the Scottish Highlands (Amor et al 2008;Drake et al 2017). Of these, the Stac Fada Member, part of the Torridonian Supergroup, has received most attention to date and is the focus of this study.…”

Constraints on the origins of iron silicide spherules in ultrahigh‐temperature distal impact ejecta

“…Meanwhile, a similar mineral complex with iron silicide microspherules of small diameter (3-100 μm) was found in Ordovician siltstone beds of Manitanird ridge in the Polar Urals (Nikulova et al 2011), in Permian red shale of Fore-Sudetic monocline at the Polkowice-Sieroszowice mine in SW Poland (Muszer 2014), in Miocene gypsum at a depth of 290 m at Koshava mine in Bulgaria (Yanev et al 2016), in lower Miocene limestone at a depth of 64 m in Yevpatoria district of Crimea (Tishchenko et al 2016), in mid-Paleocene impact ejecta layer of the Isle of Sky in Scotland (Drake et al 2017), and in hard sediments from various locations in Hungary (Szöőr et al 2001). All authors of these reports in geological literature had considered their finds as "extraterrestrial" minerals or markers of meteoritic impact.…”

On the track of a Scottish impact structure: a detrital zircon and apatite provenance study of the Stac Fada Member and wider Stoer Group, NW Scotland

“…The geological record of Britain and Ireland describes a rich history of events spanning almost 3 Gyr but no unequivocal impact structures have yet been identified. Evidence for impact events has been proposed at three stratigraphic levels: the Mesoproterozoic Stac Fada Member impact ejecta unit in the Stoer Group, northwestern Scotland (Amor et al 2008), which contains, for example, the high-pressure polymorph of zircon, reidite (Reddy et al 2015); a deposit of reworked microtektites in the Triassic Mercia Mudstone Group of southwestern England, from which multiple orientations of planar deformation features (PDFs) in shocked quartz have been documented and measured (Walkden et al 2002;Kirkham, 2003); and a purported Palaeogene impact ejecta layer on the Isle of Skye, Scotland (Drake et al 2017) that awaits confirmation with unequivocal documentation of shock features. Only the microtektite-bearing Triassic deposit has been linked to a known impact structurethe approximately 100 km diameter Manicouagan impact structure, Quebec, Canada (Thackrey et al 2009).…”