E. Buchner & M. Schmieder reply: In their discussion, El Goresy & Schmidt (Reference El Goresy and Schmidt2019) question the analytical methods of Buchner & Schmieder (Reference Buchner and Schmieder2017) that were applied to obtain geochemical results from coatings on shatter cone surfaces from the Steinheim Basin impact structure in Germany. Furthermore, they disagree with some of the interpretations in that article. In our opinion, most points of criticism stated by El Goresy & Schmidt (Reference El Goresy and Schmidt2019) arose from an overinterpretation of the conclusions presented in our article.

The authors criticise that Buchner & Schmieder (Reference Buchner and Schmieder2017) analysed Fe₂O₃-rich coatings on shatter cones from the Opalinus Claystone formation using scanning electron microscopy – energy dispersive X-ray spectroscopy (SEM-EDS) on uncoated shatter cone specimens. In fact, 10 out of 11 shatter cone samples analysed in our study were carbon-coated, which is clearly stated in Figure 3, Table 6, as well as in the main text of the paper. As El Goresy & Schmidt (Reference El Goresy and Schmidt2019) point out correctly, Figure 3f shows a shatter cone sample that is covered by Fe₂O₃, i.e. iron oxide, not by metal. Neither did we claim the shatter cone is covered by metal anywhere in the paper, nor did we report geochemical data for the coating of that shatter cone sample simply because we did not geochemically analyse that shatter cone. We only chose to show the shatter cone as an example of a remarkably well-developed shatter cone from the Opalinus Claystone formation (see Fig. 3 and text in Buchner & Schmieder, Reference Buchner and Schmieder2017). One shatter cone individual (rich in Fe, Ni, Co and Mn oxides) remained purposely uncoated during SEM-EDS analysis under high vacuum conditions to avoid contamination of the shatter cone samples with gold, carbon or other elements potentially derived from the sputter apparatus. The analysis of an uncoated shatter cone was, therefore, only carried out to test the finding of fine gold aggregates dispersed across the surface of other shatter cone samples, and to make sure fine gold aggregates do not represent contamination from the carbon coating process. Indeed, fine gold aggregates do occur on the uncoated shatter cone specimen, as well. Owing to the minor impact of electrical charging of the sample during SEM-EDS analysis, the electrical conductivity of the uncoated shatter cone specimen was seemingly sufficient for SEM imaging. Aside from the uncoated shatter cone specimen, all other analyses reported in our study were carried out on carbon-coated shatter cone specimens. The reproach that our analyses and interpretations are based on ‘a naked eye inspection’ of an oxide layer or patina is, hence, not justified.

El Goresy & Schmidt (Reference El Goresy and Schmidt2019), moreover, questioned how Buchner & Schmieder (Reference Buchner and Schmieder2017) analysed the typically low concentrations of Pt (measured as PtO) using a SEM. In Tables 1, 2 and 3 of Buchner & Schmieder (Reference Buchner and Schmieder2017), three individual SEM-EDS analyses are presented that reflect localized concentrations of Pt in the range of 1.4 to 4.4 wt % PtO, i.e. concentrations significantly above the detection limits of the SEM-EDS analytical technique.

In their fourth paragraph, El Goresy & Schmidt (Reference El Goresy and Schmidt2019) discussed an abstract by Buchner & Schmieder (Reference Buchner and Schmieder2016), which is cited in the introduction of Buchner & Schmieder (Reference Buchner and Schmieder2017), but which is related to findings at the nearby Ries crater. The results presented in that abstract are not directly relevant to the paper of Buchner & Schmieder (Reference Buchner and Schmieder2017). Therefore, we would like to highlight only one particular statement in the comment of El Goresy & Schmidt (Reference El Goresy and Schmidt2019). They stated: ‘Instead, Buchner & Schmieder (Reference Buchner and Schmieder2016) cited Schmidt & Pernicka (Reference Schmidt and Pernicka1994) that “On the basis of rather high concentrations of Ni, Cr, Co, and Ir in a few of the suevite samples, some authors [e.g., 6] favored an achondritic projectile (aubrite)”, but in this paper no high concentrations of Ir in suevite were reported.’ In fact, Buchner & Schmieder (Reference Buchner and Schmieder2016) did not state that Schmidt & Pernicka (Reference Schmidt and Pernicka1994) reported on high Ni, Cr, Co and Ir concentrations, we rather referred to the discussion in Schmidt & Pernicka (Reference Schmidt and Pernicka1994), in which a number of authors reported on the high concentrations of these elements.

Finally, El Goresy & Schmidt (Reference El Goresy and Schmidt2019) criticised the analysis and interpretation of an unusual, botryoidal oxide flake found on the surface of a limestone shatter cone that exhibits elevated concentrations of Fe, Ni, Co, Pt, Ga and Ge. Given its exotic geochemical composition, Buchner & Schmieder (Reference Buchner and Schmieder2017) argued that the flake may perhaps represent some sort of (altered) meteoritic matter, stating that the oxide flake ‘probably does not reflect the primary, but likely altered, element ratios’. We stress that the Fe/Ni and Ni/Co ratios generally do not rule out a possible iron meteoritic origin for the oxide particle, despite the rare nature of meteoritic matter found in terrestrial rocks. In their Figure 1, El Goresy & Schmidt (Reference El Goresy and Schmidt2019) show that the Ni/Co ratio of the exotic Steinheim oxide flake plots in the field for mafic, ultramafic and amphibolitic crystalline-metamorphic basement rocks, although Kracher et al. (Reference Kracher, Willis and Wasson1980) demonstrated that some overlap may exist between the Ni/Co ratio of different classes and types of meteorites and those of Earth mantle lithologies. In our opinion, plotting element ratios of an oxide flake that does not reflect primary element ratios in the first place (Buchner & Schmieder, Reference Buchner and Schmieder2017) is confusing. Furthermore, El Goresy & Schmidt (Reference El Goresy and Schmidt2019) state: ‘Therefore, it is not appropriate to discuss an iron meteorite as the possible source for the Ni/Co ratios without discussing the Ni/Co ratios of the crystalline basement.’ El Goresy & Schmidt (Reference El Goresy and Schmidt2019) should be aware that the Steinheim impact did not affect the rocks of the crystalline basement, which is situated several hundred metres below the structural crater floor of the Steinheim Basin (e.g. Reiff, Reference Reiff, Roddy, Pepin and Merrill1977, Reference Reiff2004; Buchner & Schmieder, Reference Buchner and Schmieder2010). Rocks of the crystalline basement do not occur anywhere near the Steinheim crater, and were not incorporated into the impact breccia. Hence, we see no geologically plausible explanation that favours a basement source for the Fe-, Ni-, Co-, Pt-, Ga- and Ge-rich particle. It seems, therefore, appropriate to discuss a possible meteoritic origin for the exotic flake adherent to the shatter cone surface.

El Goresy & Schmidt (Reference El Goresy and Schmidt2019) suggested that weathering from interaction with surface water and mobilization of elements by water–rock interaction (such as during post-impact hydrothermal activity) may be responsible for elevated concentrations of Fe, Ni, Co, Mn and Cu in the shatter cone coatings. This is exactly one of the explanations for elevated element concentrations in the shatter cone coatings depicted and discussed by Buchner & Schmieder (Reference Buchner and Schmieder2017) in their sections ‘Contamination from the Mesozoic sedimentary target rock?’ and ‘Contamination from the Palaeozoic crystalline-metamorphic basement?’. We certainly do not discount this possible scenario; however, we also cannot exclude the possibility that the shatter cone coatings might represent redistributed impactor material, potentially remobilized in a short-lived hydrothermal system that developed in response to the Steinheim impact, which occurred in a water-saturated palaeoenvironment (Buchner & Schmieder, Reference Buchner and Schmieder2013a,b). Previous studies of rocks from other terrestrial impact sites have shown that shatter cone surfaces can be contaminated by particulate impactor material. Schmieder et al. (Reference Schmieder, Chennaoui Aoudjehane, Buchner and Tohver2015), for instance, reported brecciated schreibersite (a Fe, Ni phosphide mineral common in iron meteorites but very rare on Earth) and Fe–Ni oxide flakes on the surface of a limestone shatter cone from the Agoudal impact site in Morocco. Similarly, Gibson & Spray (Reference Gibson and Spray1998) investigated the composition of exotic melting- and vaporization-related microfeatures adherent to shatter cones from the large Sudbury impact structure in Canada. An earlier study reported exotic ‘spherules’ attached to shatter cone surfaces from the large, eroded Vredefort impact structure in South Africa (Gay et al. Reference Gay, Comins and Simpson1978).

In our opinion, El Goresy & Schmidt (Reference El Goresy and Schmidt2019) did not provide any game-changing arguments that would convincingly disprove the presence of possible meteoritic contamination of the target rocks from the Steinheim asteroid, and likely the subsequent redistribution and alteration of the meteoritic matter on shatter cone surfaces. Instead, they argue that platinum group elements (PGEs) are the most reliable indicators for the identification of projectile contamination, because of their low abundance in the Earth’s crust and their weak mobility during water–rock interaction (El Goresy & Schmidt, Reference El Goresy and Schmidt2019). This may generally be the case, however, as demonstrated in several cases, PGEs may also be mobilized and resultant PGE patterns affected during alteration (e.g. Wallace et al. Reference Wallace, Gostin and Keays1990; Goderis et al. Reference Goderis, Tagle, Belza, Smit, Montanari, Vanhaecke, Erzinger and Claeys2013). Platinum group elements have the tendency to fractionate particularly in near-surface and sulfide-rich (reducing) hydrothermal environments (e.g. Neal et al. Reference Neal, Burney, Kring, Schmieder, Tikoo, Riller, Simpson, Osinski, Cockell, Coolen and Gulick2018), such as the sulfide-bearing post-impact alteration system hosted by the Steinheim Basin (Buchner & Schmieder, Reference Buchner and Schmieder2010). Therefore, any additional clues that could possibly help resolve the nature of the Steinheim asteroid, including (altered) ‘exotic’ coatings on shatter cone surfaces, are worth studying and discussing in some detail. A multi-technique analysis on less-altered, partly ‘primary’, meteoritic material recently discovered at the Steinheim Basin is currently underway (Buchner et al. Reference Buchner, Hölzel, Schmieder, Ferrière, Koeberl, Rasser, Fietzke, Frische, Meier, Busemann, Maden and Kutterolf2018). Future results will help to interpret the results gained from the shatter cone coatings described by Buchner & Schmieder (Reference Buchner and Schmieder2017).