Geology, petrography, shock petrography, and geochemistry of impactites and target rocks from the Kärdla crater, Estonia

Abstract

Abstract-The Kärdla crater is a 4 km-wide impact structure of Late Ordovician age located on Hiiumaa Island, Estonia. The 455 Ma-old buried crater was formed in shallow seawater in Precambrian crystalline target rocks that were covered with sedimentary rocks. Basement and breccia samples from 13 drill cores were studied mineralogically, petrographically, and geochemically. Geochemical analyses of major and trace elements were performed on 90 samples from allochthonous breccias, sub-crater and surrounding basement rocks. The breccia units do not include any melt rocks or suevites. The remarkably poorly mixed sedimentary and crystalline rocks were deposited separately within the allochthonous breccia suites of the crater. The most intensely shockmetamorphosed allochthonous granitoid crystalline-derived breccia layers contain planar deformation features (PDFs) in quartz, indicating shock pressures of 20-35 GPa. An apparent Kenrichment and Ca-Na-depletion of feldspar-and hornblende-bearing rocks in the allochthonous breccia units and sub-crater basement is interpreted to be the result of early stage alteration in an impact-induced hydrothermal system. The chemical composition of the breccias shows no definite sign of an extraterrestrial contamination. By modeling of the different breccia units with HMXmixing, the indigenous component was determined. From the abundances of the siderophile elements (Cr, Co, Ni, Ir, and Au) in the breccia samples, no unambiguous evidence for the incorporation of a meteoritic component above about 0.1 wt% chondrite-equivalent was found.

Density and magnetic susceptibility of rocks from the Lockne and Tvären marine impact structures

“…The vertical lines show the average density contrast for marine craters (1 and 3) and for craters formed on land (2 and 4). than 100 m (Puura et al 2004), whereas Tv‰ren and Lockne formed in a water depth of at least 100 m (Ormˆ and Lindstrˆm 2000). From our observations of drill cores from the K‰rdla structure, we conclude that open fractures here are rather rare, but not as rare as in Lockne and Tv‰ren.…”

Target rocks, impact glasses, and melt rocks from the Lonar impact crater, India: Petrography and geochemistry

“…Only samples L-37X and L-6B have substantially lower Na contents relative to K, similar to the samples reported from melt rocks at Brent (Grieve 1978) and Ames (Koeberl et al 1997) impact structures. Moderately enriched values of K, compared to Na, may be the result of vaporization or post-impact hydrothermal processes, as discussed in detail by Puura et al (2004). In our samples, indications of a post-impact hydrothermal influence are ambiguous because the high Cr and Ni abundances in all analyzed samples are associated with rather low levels of As, elevated values of which might indicate hydrothermal activity.…”

Major and trace element characteristics of impactites from the Yaxcopoil‐1 borehole, Chicxulub structure, Mexico

“…Despite the extensive petrographic observation indicating a Kmetasomatic event, our geochemical data do not reveal any strong substitution of K for Na or Ca. This indicates that K redistribution occurred at a scale of less than sample size, unlike, for example, at the Kärdla crater, where a distinct Kfeldspar enrichment was observed exchanging for Ca and Na feldspars (see Puura et al 2004).…”