St. John's 2012
Technical Program

SS13:  Planetary Geology:  Canadian contributions to space science research  (Poster) 
Organizers / Organisateurs:  Paul Sylvester and Penny Morrill
Room / Salle:  Salon A

Date:  28/05/2012
Presenter:  Denise Anders

Geochemical signatures of carbonate impact melts from the Steinheim impact crater, Germany

Anders, D.,, Osinski, G.R., Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street, London, ON, N6A 5B7, Grieve, R.A.F., Earth Sciences Sector, Natural Resources Canada, 601 Booth Street, Ottawa, ON, K1A 0E8, Schmieder M., The University of Western Australia, 35 Stirling Highway, Crawley, 6009 WA - Perth, Australia, Kegler, P., Christian-Albrechts-Universität zu Kiel, Ludewig-Meyn-Str. 10, Kiel, 24118, Germany, and Buchner, E., Universität Stuttgart, Herdweg 51, Stuttgart, 70174, Germany

The 3.8 km in diameter Steinheim impact crater counts among the best-preserved complex impact structures with a central uplift on Earth.  It lies within a sequence of flat-lying Triassic to Upper Jurassic sediments of the Swabian Alb plateau (SW Germany).  Previous studies revealed the existence of silicate and carbonate melt lithologies within rocks of the B-26 core drilled in the western central basin of the Steinheim structure.  Here we present a reinvestigation of these results to further evaluate geochemical features and signatures of the carbonate melt.

At a depth of 78-79 m, closely beneath the structural crater floor of the Steinheim Basin, the B-26 drill core consists of Upper Jurassic limestone containing macroscopic fractures that are filled with veins of Ca-Mg-carbonates.  These carbonates exhibit an MgO content of up to ~20 wt% and were previously described as “impact-induced dolomitic melt” generated from the Steinheim target rocks and subsequently injected into host limestone fractures.  The incorporation of detectable amounts of Si, Al and K into the Ca-Mg-carbonate groundmass is consistent with carbonate melt from other terrestrial impact craters such as Haughton crater, Canada.  In general, the major elemental compositions of impact carbonate melts resemble those of carbonatite igneous rocks.  The accumulation of Si, Al and Na in carbonatites is assumed to be attributed to chemical exchange with their wall rocks.  Geochemical plots of the Steinheim carbonate melt show direct correlations of (Ca2+, Mg2+, Fe2+, Mn2+) and Al3+ + (K+, Na+) as well as (Ca2+, Mg2+, Fe2+, Mn2+) and (K+, Na+) + Si4+ suggesting a chemical exchange of cations.

So far, the following substitution processes can be determined that might lead to the accumulation of Si, Al and K within the dolomitic melt assemblage:

2 (Ca2+, Mg2+, Fe2+, Mn2+)  ↔  Al3+ + (K+, Na+)
(Ca2+, Mg2+, Fe2+, Mn2+) + Al3+  ↔  (K+, Na+) + Si4+
C4+  ↔  Si4+

Further investigation and bulk composition analyses of the dolomite melt assemblage are required to confirm this assumption and to explain the incorporation of Si, Al, and K into the dolomitic melt, their source and possible scenarios of the origin of the Steinheim carbonate melt.