Oxygen isotopes and implications for the cavity-grown quartz crystals in the Lockne impact structure, Sweden

Abstract

Well-developed quartz crystals are found in open cavities in the intensely fractured crystalline basement of the marine-target impact structure at Lockne in central Sweden. The 458 Ma impact structure has a well-preserved crater in Precambrian granitic basement rock that is covered by resurge deposits composed of breccias and fine-grained sedimentary units of mixed Ordovician limestone, Cambrian black bituminous shales and the basement granite. Directly after the impact, the resurge deposits formed when the seawater rushed back into the crater. The residual heat from the impact facilitated a short-lived hydrothermal system accompanied by the inflowing seawater, which resulted in mineral growth in fractures and open cavities of the granite basement. The oxygen isotope values of the first-precipitated minerals, the cavity-grown quartz crystals, range from +15.2 to +16.2‰ (Vienna Standard Mean Ocean Water) and differ from those of the hosting granite basement rock with δ¹⁸O quartz between +10.1 and +11.7‰. The δ¹⁸O values of the quartz are more consistent with derivation from a fluid of relatively high δ¹⁸O probably attributable to oxygen isotope exchange during seawater–rock interactions in the resurge deposits. The occurrence of organic matter in association with the cavity-grown quartz strongly indicates a relationship to the black bituminous shale in the matrix of the breccia that rests on the crater floor. Comparing the results with previously obtained oxygen data on fracture-grown calcite from Lockne shows that oxygen isotope composition of the cavity-grown quartz crystals is less variable and probably more accurately reflects the original fluid source.

Keywords:

• Lockne impact structure
• oxygen isotopes
• cavity-grown quartz crystals

Acknowledgements

This study is a part of the VR-financed project d.no. 2009–4492: ‘Concentric Palaeozoic astroblems in Sweden – the Lockne and Siljan craters.’ SUERC is financially supported by the Scottish Universities. We are grateful to Kjell Billström and Carl Alwmark for important review comments.