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Kenneth Amor, Oxford University (United Kingdom)
Steven Hesselbo, Oxford University (United Kingdom)
Don Porcelli, Oxford University (United Kingdom)
Scott Thackrey, Aberdeen University (United Kingdom)
John Parnell, Aberdeen University (United Kingdom)
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Proximal ejecta blankets surrounding impact craters are rarely preserved on earth due their removal by erosion. We have recently discovered a 1.2 Ga impact layer in a continuous sedimentary sequence in North West Scotland. The presence of shocked quartz with multiple, intersecting, planar deformation features provide unequivocal evidence for an impact origin. Subsequent rapid burial has preserved the ejecta in almost pristine condition and provides an excellent opportunity for studying proximal impactites. Field evidence suggests that the impact occurred in strata with extensive groundwater. The sedimentary environment is analogous to those ascribed to Mars and therefore this impactite may be a terrestrial analogue of Martian fluidized or layered crater ejecta.
Initial analyses show evidence for an enrichment in the platinum group elements (PGE) in the impact layer, relative to surrounding strata. The increased abundance in PGE is most likely caused by incorporation of a meteoritic component which is highly enriched in these elements compared to crustal rocks. Some post-depositional remobilization of the PGEs has altered their relative abundances, and has caused some enrichment in the sediments immediately overlying and underlying the ejecta deposit.
A new method developed for the isolation and concentration of chromium was applied to the impact samples and high precision chromium isotope ratio measurements have been made using a multiple collector, inductively coupled plasma, mass spectrometer (MC-ICP-MS). Preliminary data has found a positive, anomalous enrichment in 53Cr relative to other Cr isotopes. Such Cr isotope variations are not seen in terrestrial materials but are widespread in meteorites due to the decay of 53Mn (t1/2=3.74Ma) in the early solar system, and confirms a meteoritic component in the impact layer. A combination of PGE and 53Cr isotope data, can place constraints on identification of the specific meteorite class of the impactor.
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