Suevites from the Rochechouart Impact Crater, France, and the Lake Mien Impact Crater, Sweden: The Search for Robust Carbon Minerals

Details Suevites from the Rochechouart Impact Crater, France, and the Lake Mien Impact Crater, Sweden: The Search for Robust Carbon Minerals Suevites from the Rochechouart Impact Crater, France, and the Lake Mien Impact Crater, Sweden: The Search for Robust Carbon Minerals

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.521h&link_type=abstract

Meteoritics, vol. 30, no. 5, page 521

Other

Diamonds, Impact, Lake Mien, Rochechouart, Silicon Carbide

Scientific paper

Suevite from the 24km Ries crater in southern Germany has recently been shown to contain diamond, lonsdaleite and silicon carbide (1). These minerals are of impact, not meteoritic, origin and probably formed by a combination of shock and plasma processes with the contribution due to each mechanism yet to be resolved (1). Diamonds are increasingly becoming an accepted impact signature and have been reported for several Ukranian impact craters including Zapadnaya (2) and also for the Popigai impact crater in Siberia (3). We have also found diamonds associated with the K/T boundary event 65 Ma ago in North America (4) and Mexico (5). The genesis of suevite at the Ries is thought to be within a fireball at very high temperatures and travelling at high velocities. Shocked minerals are associated with the suevite as are the high pressure polymorphs of quartz namely coesite and stishovite (6). Such an extreme temperature and pressure history for the suevite make it an ideal rock type to search for impact diamonds, we are therefore endeavouring to study other suevites from further impact craters, including Rochechouart and Mien. The 165-200 Ma Rochechouart impact crater in the Massif Central, France, is thought to have originally been some 20 km across and therefore is very similar in size to the Ries (7). The basement material of gneisses and granites is also akin to the Ries and as such makes it an ideal candidate for diamond poltypes and possibly silicon carbide. Lake Mien in Sweden has no outcrops of suevite breccia but does have glacial erratic blocks of suevite. The impact crater is only some 7-9 km in size and has an age of 120 Ma (8). It affords a useful method of investigating the importance of size as a controlling factor in formation of carbon impact minerals. The samples were treated with an acid demineralization procedure first developed for the extraction of diamond from meteorites and modified for the particular needs of studying terrestrial samples. Combinations of hydrofluorie, hydrochloric, chromie, perchloric and sulphuric acids are used to destroy carbonates, silicates, organics, zircon, rutile and graphitic carbon leaving only resistant forms like diamond and silicon carbide. The acid resistant residues were then examined by scanning electron microscope and transmission electron microscope to resolve the mineral forms present and aliquots analyzed for carbon content and carbon isotope composition using a stepped heating technique combined with static mass spectrometry (9). The study is intended to investigate the presence or absence of robust phases within the suevite samples from impact craters and thus establish how widespread the phenomena may be. The increasing number of confirmed terrestrial impact craters will allow us to investigate the nature of the impact signature and yield more information on their mode and conditions of formation. References: [1] Gilmour I. et al. (1995) LPS XXVI, 463. [2] Gurov E. P., personal communication. [3] Masaitis V. L. (1993) Reg. Geol. Metall., 1, 121-134. [4] Gilmour I et al. (1992) Science, 258, 1621. [5] Hough R. M. et al. (1995) LPS XXVI, 629. [6] Engelhardt W. v. (1990) Tectonophys., 171, 259-273. [7] Lambert P. (1982) GSA Spec. Pap. 190, 57-68. [8] Henkel H. and Pesonen L. J. (1992) Tectonophys., 216, 31-40. [9] Wright I. P. and Pillinger C. T. (1989) USGS Bull., 1890, 9.

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