Mineralogical and geochemical studies of impact melt products from the Chesapeake Bay impact structure
Zusammenfassung der Projektergebnisse
Glass originating in impact processes occurs in different geological settings, and has rather peculiar properties. Neglecting high-pressure impact melt glasses (black veins in meteorites, pseudotachylytes) as well as shock-related mineral glass, four types of glass are formed in terrestrial impact processes: (i) tektites, and (ii) microtektites, both occurring exclusively in geographically defined distal deposits – “strewn fields”. Glassy materials inside the crater or in the continuous ejecta deposits comprise (iii) shards, and bombs in suevitic and melt breccias, and (iv) aerodynamically shaped fallback particles, thought to rain out from the collapsing ejecta plume. Our project was aimed at the origin and evolution of the above-mentioned impact glasses using samples from (a) the Lake Bosumtwi impact structure (1.07 Ma old), Ghana, and (b) the Chesapeake Bay impact structure (35 Ma), Virginia. Both well-preserved complex impact craters were target of an ICDP drilling campaign, both are associated with tektite strewn fields, i.e., (a) the Ivory coast tektite (IVC) with the widespread offshore IVC microtektite (IVC-MT) strewn field (Equatorial Atlantic), and (b) the North American tektite (NAT) strewn field with related microspherules (Carribean). Suevite with glass shards occur in or around both craters. Our comparative geochemical study included tektites (IVC; NAT), microtektites (IVC-MT), suevite glass (Bosumtwi BOT 12), and fallback glass particles (Bosumtwi FBG). Analytical techniques applied ranged from ● electron microprobe (major elements), ● laser ablation mass spectrometry (trace elements), ● thermal ionization mass spectrometry (Sr, Nd isotope composition), ● thermobalance in combination with mass spectrometer (volatile content, esp., H2O, CO, CO2), ● electron energy loss spectroscopy and energy-dispersive X-ray microanalysis on a transmission electron microscope, ● X-ray absorption spectroscopy at the synchrotron source at Karlsruhe (valence states of transition metals) to ● calorimetry (glass transformation temperature). Compared to the average composition of the upper continental crust (UCC), the four groups of glass (IVC, IVC-MT, BOT 12, FBG) show variations in MgO and Na2O, with marked calcium depletion in IVC and IVC-MT, most probably inherited from the target. The precursor lithologies for the IVC were constrained by Sr-Nd systematics to meta-greywacke, the most common target rock. Some greywacke samples have very high contents of Ni, Co, and Cr, reflecting contributions of the ore-bearing meta-volcanics in the Bosumtwi target region. The IVC, IVC-MT, and BOT 12, yet not the FBG group have similar high, yet scattering contents of “meteoritic tracer elements”, obviously inherited from the target; this component masks any possible contribution of the projectile to the analyzed impact glasses. Trace element distribution patterns show little variations within each group of glass, but significant differences between the groups exist. Only the IVC show a minor positive Ce anomaly, pointing to alteration in the precursor material, which is constrained as local soil. Compared to UCC, moderately volatile elements are depleted in the glass to different degrees (in decreasing order Pb, Zn, As, Sb – Cu). IVC-MT, and IVC display these effects drastically, followed by FBG, while they are absent in glass BOT 12. Concerning volatiles, suevite glass contains ~3 % H2O, released < 300° C from clay minerals; other species detected are CO2 (0.28 %) and SOx. In contrast, IVC are extremely dry; only H2O (< 100 ppm), CO2 and CO were detected. The release of 96 % CO and 4 % CO2 from one bubble substantiates the formation of IVC tektites under very reducing conditions. The same holds for bediasites (NAT). The BOT 12 glass contains ≤ 40 % Fe3+, FBG ≤ 20 % Fe3+, while IVC do not contain any detectable ferric iron. Other transition metals in IVC are, in part, reduced as well, i.e., Mn and Cr occur in part in the 2+ state, while Ti is largely in the 4+ state. The glass transformation temperature of one IVC was determined at 758° C, with a rather slow cooling rate of 10 ± 5 K around this T. Compiling the data, and given that modelling predicts only minor vaporization of target material in a “Bosumtwi-type” impact event, we conclude: FBG and suevite glass have meta-greywackes as precursor; IVC and IVC-MT originated from similar, yet Ca-depleted and highly porous material. The dry IVC-MT and IVC homogenized at the highest T, evolved in a reducing environment, and followed a T path causing substantial loss of trace elements, which is less pronounced in FBG, either due to lower ambient T or faster cooling. Suevite glass originates at much lower T. Prime distinctive features of volcanic lapilli to tektites are the dry nature and lack of Fe3+ in the latter.
Projektbezogene Publikationen (Auswahl)
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(2006) Establishing the link between the Chesapeake Bay impact structure and the North American tektite strewn field: The Sr-Nd isotopic evidence. Meteoritics & Planet. Sci. 41, 689-703
Deutsch, A., Koeberl, C.
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(2007) The ICDP Lake Bosumtwi impact crater scientific drilling project (Ghana): Core LB-08A litho-log, related ejecta, and shock recovery experiments. Meteoritics & Planet. Sci. 42, 635-654
Deutsch, A., Luetke, S., Heinrich, V.
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(2009) Geochemical characteristics of target rocks and suevitic glasses from the Eyreville B drill core, Chesapeake Bay impact structure. Geological Society of America Special Paper 458, 435-445
Skála, R., Langenhorst, F., Deutsch, A.
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(2014) Impact metamorphism in terrestrial and experimental cratering events. In Lee, M., Leroux, H. (eds.) EMU Notes in Mineralogy, Vol. 15, Chapter 6, 1–39
Deutsch, A., Poelchau, M. H., Kenkmann, T.
