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Discovery of a Meteoritic Ejecta Layer Containing Unmelted Impactor Fragments at the Base of Paleocene Lavas, Isle of Skye, Scotland

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Discovery of a Meteoritic Ejecta Layer Containing Unmelted Impactor Fragments at the Base of Paleocene Lavas, Isle of Skye, Scotland View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by NERC Open Research Archive Discovery of a meteoritic ejecta layer containing unmelted impactor fragments at the base of Paleocene lavas, Isle of Skye, Scotland 1 1 2 3 4 5 Simon M. Drake *, Andrew D. Beard , Adrian P. Jones , David J. Brown , A. Dominic Fortes , Ian L. Millar , Andrew Carter1, Jergus Baca1, and Hilary Downes1 1School of Earth and Planetary Sciences, Birkbeck College, University of London, Malet Street, Bloomsbury, London WC1E 7HX, UK 2Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK 3School of Geographical and Earth Sciences, University of Glasgow, Lilybank Gardens, Glasgow G12 8QQ, UK 4ISIS Neutron Facility, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Chilton, Didcot, Oxfordshire OX11 OQX, UK 5British Geological Survey, Natural Environment Research Council, Keyworth, Nottingham NG12 5GC, UK ABSTRACT extensive comparative geochemistry (see the Evidence for meteorite impacts in the geological record may include the presence of GSA Data Repository1). 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 FIELD RELATIONS AND CHEMISTRY first recorded occurrence of vanadium-rich osbornite (TiVN) on Earth, from two sites on Skye, OF METEORITIC EJECTA LAYER northwest Scotland, which are interpreted as part of a meteoritic ejecta layer. TiVN has only DEPOSITS AT SITES 1 AND 2 previously been reported as dust from comet Wild 2, but on Skye it has been identified as an The Isle of Skye forms part of the British unmelted phase. Both ejecta layer sites also contain niobium-rich osbornite (TiNbN), which Palaeogene Igneous Province (BPIP), a volca- has not previously been reported. An extraterrestrial origin for these deposits is strongly nic region that extends from the Inner Hebrides supported by the presence of reidite (a high-pressure zircon polymorph), which is only found of Scotland to Northern Ireland. Igneous activ- naturally at sites of meteorite impact. Barringerite [(Fe,Ni)2P], baddeleyite (ZrO2), alabandite ity on Skye spanned ca. 61–54.5 Ma (Bell and (MnS), and carbon-bearing native iron spherules, together with planar deformation features Williamson, 2002). The BPIP forms part of the and diaplectic glass in quartz, further support this thesis. We demonstrate through field North Atlantic Igneous Province, a 1.3 × 106 relationships and Ar-Ar dating that the meteorite strike occurred during the mid-Paleocene. km2 area (Eldholm and Grue, 1994; Fig. 1A). This is the first recorded mid-Paleocene impact event in the region and is coincident with Site 1 is located at An Carnach on the the onset of magmatism in the British Palaeogene Igneous Province (BPIP). The Skye ejecta Strathaird Peninsula (Figs. 1B, 2A, and 2B), layer deposits provoke important questions regarding their lateral extent at the base of the where a 0.9-m-thick meteoritic ejecta layer over- BPIP and the possibility of their presence elsewhere beneath the much larger North Atlantic lies Middle Jurassic sedimentary rocks (Drake Igneous Province. 1 GSA Data Repository item 2018039, detailed BACKGROUND The methodology employed during our methodologies, geochronology and comparative geo- chemistry of osbornite, barringerite, reidite, alabandite, Meteorite impact deposits are found through- study comprises field observations, petrography, native iron, and additional shocked images, is available out the geological record. However, in this paper electron microprobe analysis, Raman micros- online at http://www.geosociety.org/datarepository we detail the youngest recorded UK meteorite copy, U-Pb and Ar-Ar radiometric dating, and /2018/ or on request from [email protected]. impact event, located beneath mid-Paleocene Onshore Paleogene lava SVALBARD NORTH lavas (Fig. 1A) at 2 sites 7 km apart on what elds and sill complexes Long 6˚5’W Long 5˚50’W Oshore Paleogene lava NORTH is now the Isle of Skye, northwest Scotland elds and sill complexes (Fig. 1B). The only other known meteoritic ABBARENTS SEA ejecta deposit in Scotland is much older (1177 ± 70˚N Lat 57˚35’N 5 Ma) than the Skye deposits and occurs within GREENLAND Precambrian rocks on the Scottish mainland MOHNS RIDGE (Parnell et al., 2011; Reddy et al., 2015). AY YAN U JAN MA NORW We present compelling mineralogical and TEA RIDGE VORING textural evidence within the Skye deposits for ICELAND PLA Portree FA AEGIR RIDGE Lat 57˚20’N impact-derived shock metamorphism at pres- OE-ICER-- Paleogene silicic 60˚N LAND intrusions sures ≥30 GPa. Within the deposits at both sites, Paleogene basic Broadford JANES RIDGE 2 K intrusions unmelted vanadium-rich osbornite (TiVN) and Y DENMARK Paleogene basic lavas 1 Torrin RE niobium-rich osbornite (TiNbN) are preserved NORTH Paleogene silicic ignimbrites CHARLIE- SEA Pre- Paleogene rocks as part of the actual impactor. Recognition of GIBBS FR Meteoritic ejecta deposit sites actual unmelted impactor mineralogy has previ- 50˚N ACTURE 30˚W 15˚W 0˚ 0 300 Lat 57˚05’N 0 km 10 ously only been from the Chicxulub crater on km the Yucatan Peninsula (Kyte, 1998). Figure 1. A: North Atlantic Igneous Province (modified from Saunders et al., 1997). B: Simpli- fied geological map of the Isle of Skye, Scotland, showing the location of meteoritic ejecta *E-mail: [email protected] layer deposits at sites 1 and 2. GEOLOGY, February 2018; v. 46; no. 2; p. 171–174 | GSA Data Repository item 2018039 | https://doi.org/10.1130/G39452.1 | Published online 12 December 2017 ©GEOLOGY 2017 The Authors.| Volume Gold 46 |Open Number Access: 2 | www.gsapubs.orgThis paper is published under the terms of the CC-BY license. 171 c and Beard, 2012) and underlies 70 m of mid- A B base of PalaeogeneLowest basalt An Carnach basalt Blaven ow Paleocene basaltic lavas (Fig. 2A; Fig. DR2 in Meteoritic ejecta the Data Repository). The meteoritic ejecta layer deposit 0.9m thick impactoclastite comprises ≥95 vol% matrix, with a composition Paleocene basalt of 47 wt% SiO and high Al O and FeO, and Mid-Jurassic sedimentary rocks 2 2 3 basalt sill resembles volcanic ash altered to potassium- rich clay. It has a matrix fabric similar to that basalt sill of a welded ignimbrite deposited from a low- concentration pyroclastic density current. An Carnach site 1 Mid-Jurassic sandstone Site 2 is located on route B8083 (Figs. 1B, 2C, and 2D), 1.5 km south-southwest of Broadford. C D Here the meteoritic ejecta layer is 2.1 m thick, comprising a crudely stratified, 0.9-m-thick lower pumiceous-like subunit reminiscent of an unwelded terrestrial ignimbrite (Figs. 2C, 2D; site 2 Fig. DR2). This lower unit crops out unconform- upper unit ably on Cambrian–Ordovician dolostone and basaltic clast within Meteoritic ejecta deposit varies laterally in thickness from 0.25 to 0.90 m. dated as 61.54 ± 0.42 Ma Ar-Ar. This lower unit grades upward to a 1.2-m-thick, B8083 > coarser upper subunit that is largely clast sup- site 2 lower unit Torrin site 2 top of lower unit ported with variable amounts of matrix. The upper subunit contains heterolithic components Figure 2. Field relationships at both meteoritic ejecta layer sites. A: Location of site 1 including sporadic outsized blocks of basalt as (grid reference NG 55371 21112 at the base of An Carnach plateau lavas. B: Site 1 deposit at the base of the plateau lavas (NG 55371 21112). Basaltic sill has chilled margins and large as to 1.15 × 0.7 m (Fig. 2D). One basal- intruded at the earlier contact between meteoritic ejecta layer deposit and Middle Juras- tic block has been dated as 61.54 ± 0.42 Ma sic Valtos sandstone. C: Location of site 2 lower unit of layer (NG 62626 21860). This unit using the 40Ar-39Ar system (Figs. DR1 and DR3; crops out on top of Cambrian–Ordovician dolostone 50 m north-northeast (NG 62730 Table DR1). 21955). D: Site 2 lower unit (NG 62627 21858) grading into an upper unit (NG 62627 21890) that contains a basaltic clast dated as 61.54 ± 0.41 Ma (Data Repository; see footnote 1]). Hammer shaft in B and D is 35 cm length. Field of view in A is 1.5 km across skyline. MINERALOGY OF METEORITIC EJECTA LAYER DEPOSITS AT SITES 1 AND 2 f b ABshock lamellae C The site 1 layer has a very fine grained matrix external Fe(Si) comprising quartz, orthoclase, and clay-rich fabric FeO streaked domains, which are deflected around quartz crystals and granitic and basaltic lithic Barringerite lapilli. Undeformed bubble wall shards are evi- Fe(Si) Osbornite (TiVN) dent in pressure shadows. Common accessory Zircon Osbornite (TiVN) phases are rutile, monazite, and zircon, together FeO FeO with rare chromite. The site 2 lower layer is also fine grained with a matrix of undeformed glass 50µm shards, quartz, and K-feldspar. Within the matrix 25µm are arkosic sandstone and gneiss lithic lapilli, ≤5 cm. The upper unit at site 2 is largely clast 100µm supported and contains subrounded lapilli and DEF blocks of quartzite, arkosic sandstone, and basalt. native Fe Reidite is present and occurs sporadically native Fe Lithic within matrix zircons at both sites (Figs. 3A native iron Lithic and 4A; Fig. DR4; Table DR5). Individual zir- FeO native cons frequently contain reidite shock lamel- iron lae (Fig. 4A). This study has employed both Nb-rich osbornite FeO FeO FeO Barringerite Raman microscopy and electron microprobe native Fe techniques to detect reidite. Our sample bands Barringerite Lithic and peaks accord well with all known natural 50µm Lithic native iron reidite Raman bands (Fig. DR4; Table DR5).
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