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

Journal article
Authors C Broman
Erik Sturkell
AE Fallick
Published in GFF
Volume 133
Issue 1-2
Pages 101-107
ISSN 1103-5897
Publication year 2011
Published at Department of Earth Sciences
Pages 101-107
Language en
Links dx.doi.org/10.1080/11035897.2011.59...
Keywords lockne impact structure, oxygen isotopes, cavity-grown quartz crystals
Subject categories Earth and Related Environmental Sciences

Abstract

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 parts per thousand (Vienna Standard Mean Ocean Water) and differ from those of the hosting granite basement rock with delta(18)O quartz between +10.1 and +11.7 parts per thousand. The delta(18)O values of the quartz are more consistent with derivation from a fluid of relatively high delta(18)O 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

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