Chemical Interaction between Projectile and Sandstone Target
As mentioned above, the partly fused projectile fragments contain metallic melts at their margins (Fig. 6C). Microprobe analyses indicate that these melts represent a mixture of the CDC meteorite components. The dominant contribution is from the kamacite matrix, with an admixture of P-rich phases. We assume that the rhabdites are the main P-donators as they are more homogeneously distributed than schreibersite. The dissemination of projectile material in the sandstone is a highly dynamic and turbulent process. It appears that some portion of the molten rims were separated from the projectile fragments, injected into the low viscosity sandstone melt, and disseminated as small metallic droplets. In these projectile droplets Ni and Co are enriched to a greater degree than Fe, compared with the molten margins of partly fused projectile fragments and the CDC meteorite composition. The positive slope of this trend, shown in Fig. 9, is a result of the removal of Fe (see above). The range of projectile material compositions observed in the ejecta cannot be explained as simple mixtures of taenite and kamacite (Fig. 9). The Fe/Ni and Fe/Co decrease, due to loss of iron, but Ni/Co remains nearly constant. Obviously the Fe depletion in the molten margins of projectile fragments and the metallic droplets is associated with the enrichment of Fe and to a lesser degree of other siderophile elements in the sandstone melt and in shocked quartz.
Summarizing these petrographic and geochemical observations, two coexisting but largely immiscible melts (projectile melt and sandstone melt) exist in the highly shocked ejecta fragments. Several studies of Meteor Crater (e.g., Hörz et al. 2002; Kearsley et al. 2004; Mittlefehldt et al. 2005), Wabar crater (e.g., Gibbons et al. 1976; Hörz et al. 1989), Monturaqui crater (e.g., Bunch and Cassidy 1972; Kearsley et al. 2004), and Kamil crater (D’Orazio et al. 2011) described similar petrographic features in siliceous impactite material. The authors observed numerous metallic droplets with meteoritic origin in vesicular glasses, comparable to our observations shown in Figs. 5A and 6B. In impact melts of Meteor Crater (Arizona, USA), Hörz et al. (2002) and Kearsley et al. (2004) described a decrease in Fe/Ni in meteoritic droplets compared with the composition of the Canyon Diablo meteorite, and a complementary increase in this ratio in the surrounding target melt. Based on microprobe data, Hörz et al. (1989) and Mittlefehldt et al. (1992) demonstrated that impact melt materials from Wabar crater are generally enriched in Fe over Ni compared with the iron meteorite impactor. The results of our study (Fig. 8; Table 4) are consistent with these data. In our study the average Fe/Ni in sandstone melt of highly shocked ejecta fragments is generally above the Fe/Ni in the bulk Campo del Cielo. The Fe/Ni also varies strongly in the sandstone melt between 120 and 1406. Thus, during the formation of Meteor Crater, Wabar crater, and the laboratory impacts of the MEMIN research unit, comparable selective element partitioning processes between the projectile and target occurred.
To quantify such fractionation processes, it is important to understand the behavior of the moderate siderophile elements Fe, Co, and Ni. Partitioning of these elements between silicate and metallic melts has been studied extensively in laboratory experiments at varying conditions of pressure (P), temperature (T), oxygen fugacity (fO2), as well as metal and silicate composition. All experimental data demonstrated that independent of P, T, fO2, and compositional variations the sequence is always DNimet/sil >> DComet/sil > DFemet/sil (e.g., Schmitt et al. 1989; Walker et al. 1993; Hillgren et al. 1994, 1996; Ohtani et al. 1997; Righter et al. 1997), where Dxmet/sil is the metal/silicate partitioning coefficient of element x. The experimental results clearly indicate that the partitioning coefficient of Fe is always lower than the coefficient for Ni. Therefore, iron partitions preferentially into the silicate melt compared with Ni and Co. The siderophile character of Fe, Ni, and Co is linked to the selective oxidation behavior of these elements (White 2011) which is based on differences in the affinity to form an oxide. The Gibbs free energies (ΔG) for oxidation of pure Fe, Ni, and Co are as follows: ΔG°Fe/FeO << ΔG°Co/CoO < ΔG°Ni/NiO for all temperatures. Thus, Fe oxidizes preferentially over Co and Ni. An iron meteorite can be considered as a metallic alloy whose oxidation is more complex than oxidation of pure metals. When oxidation of alloys (e.g., Fe-Ni-Co) proceeds with selective oxidation of a less noble metal like Fe, a depletion zone of this less noble metal is formed in the underlying alloy (Seo and Sato 1983). In our experiment, the molten margins of larger projectile fragments, which are enriched in Ni, could be considered as such a depletion zone, caused by an oxidation process. We suggest that the projectile material, especially the material that later forms droplets, is injected into the sandstone melt and immediately reacts as described above. During this process, the projectile material was slightly oxidized (cf. Tables 1 and 4). We consider gas trapped/originally present in the pore space of the sandstone, and the dissociation of H2O (Brett 1967) originating from the phyllosilicate-bearing matrix as potential sources for oxygen. This view is supported by the highly vesicular sandstone melt (Figs. 5 and 6), which implies vaporization of certain amounts of H2O, which in turn, may have been dissociated. Brett (1967), Gibbons et al. (1976), Kearsley et al. (2004), Kelly et al. (1974), and Mittlefehldt et al. (2005) considered selective oxidation as the main reason for enrichment of Ni (and Co) observed in metallic droplets in impactites from the Wabar, Monturaqui, Henbury, and Meteor Craters. D’Orazio et al. (2011) described similar reaction margins in the ablation crust of Gebel Kamil, the iron meteorite that formed the Kamil crater in Egypt.
Brett (1967) suggested that a combined oxidation-fractionation process occurred in a very short time interval prior to the projectile melt injecting into the target melt. This implicates formation of an iron-oxide layer in the projectile prior to the incorporation of FeO into the sandstone melt. In this model, an FeO “skin” around each metallic droplet partitions into the target glass, enriching it in iron, and causing an FeO-free surface of the droplet. In our samples, we did not observe a distinct oxide layer on projectile material. Small droplets have relatively greater surface areas for oxidation, thus possibly explaining the Ni- (and Co-) enrichment with decreasing size (Gibbons et al. 1976), as shown in Fig. 10.
It is apparent from Fig. 11 that projectile droplets at terrestrial craters differ from the respective original meteorite composition. For example, some spheres of meteoritic origin from the Wabar crater have Ni contents of about 90 wt.% (e.g., Gibbons et al. 1976). As previously mentioned, Fe oxidizes more preferentially than Co, which in turn oxidizes more preferentially than Ni. We did not observe this chemical behavior in the projectile material of our study. For most droplets, Ni/Co ranges between the ratios for the bulk CDC and kamacite. Only the most fractionated (Fe-poor) meteoritic droplets show some Ni/Co fractionation (Bunch and Cassidy 1972; Gibbons et al. 1976; Kearsley et al. 2004). At Fe/Co < 15 it is clearly shown that the Ni/Co-ratio of Wabar, Monturaqui, and Meteor Crater spheres differ significantly from their initial meteorite ratio (Fig. 11). In these cases Ni is on average more enriched than Co resulting in a higher Ni/Co. Another process causing selective oxidation of iron meteorite material occurs by interaction of the incoming projectile with air (Mittlefehldt et al. 2005). These authors show that ballistically dispersed metallic spherules from Meteor Crater are generally enriched in Ni and Co over Fe. Projectile droplets from the Meteor Crater that directly interacted with molten target material, however, show a much more pronounced fractionation between Fe and Ni + Co (Kearsley et al. 2004) than the spheres selectively oxidized in air (Fig. 11). Although the projectile droplets in our experiment are surrounded by a sandstone melt, we do not observe such an extreme inter-element fractionation as recorded in samples from Meteor Crater (Kearsley et al. 2004). We suggest that the difference in the intensity of interelement fractionation between nature and experiment is due to differences in physical conditions like maximum pressure, temperature, and time. Especially differences in cooling rates may influence the intensity of chemical interactions between projectile and target. In the case of the MEMIN experiments, composition and thus structure of the siliceous sandstone melt affect the amount of projectile elements dissolved. This melt varies considerably in Al content as a result of different proportions of molten quartz and Al silicates (e.g., phyllosilicates). The Al-rich melts are considerably more depolymerized than Si-rich and Al-poor melts (Mysen and Richet 2005). The relative enrichment of Fe in Al-rich melts is facilitated by the charge compensation of Al3+ by divalent cations.
We have documented in our highly shocked ejecta fragments a slight enrichment of meteoritic Fe and Ni in quartz with PDF and silica glass (Fig. 8; Table 3). This corresponds to an addition of 0.1–0.7% meteorite components to shocked quartz. In contrast to Fe/Ni in sandstone melts, the average Fe/Ni of these shocked SiO2 phases lies close to the ratio for the original Campo del Cielo (Tables 3 and 4). This result is unusual, and rather unexpected. Superficial contamination of the quartz by matter from adjacent projectile droplets as well as partly fused projectile fragments during polishing of the thin sections cannot be completely excluded. We intend to apply other sensitive preparation and analytical methods to test whether this observation can be confirmed.
P–T-Conditions During the Impact
Poelchau et al. (2011, 2013) calculated a maximum shock pressure of 55 GPa for our experiment using the planar impact approximation and material parameters for Coconino sandstone and steel. Such a maximum shock pressure, however, is insufficient to cause melting let alone vaporization of the meteoritic projectile. Therefore it is necessary to consider spatially localized factors that may boost local heating beyond the bulk temperatures calculated here to levels sufficient to melt projectile and target material. For instance, the melting point of an iron meteorite is about 1497 °C, its boiling point is about 3235 °C (Remo 1994), which is similar to pure iron with melting and boiling points of 1536 and 2860 °C, respectively. Several observations that are at odds with the calculated peak pressures need to be explained.
Highly shocked ejecta fragments such as illustrated in Fig. 5A display different shock-metamorphic features on a very small scale: melting of quartz seems to start at the quartz grain rims and along the PDF. Hence, it is possible that a transformation from diaplectic quartz to fused quartz (silica glass) has occurred within the PDF. This ongoing transformation is well illustrated in Qtz-B and Qtz-C (Fig. 5A). Vesicles within the Qtz-C and the schlieren-like rims indicate a predominantly molten state of this grain (fused Qtz). The PDF are isotropic glass lamellae along defined crystallographic planes in crystalline quartz; these PDF form by the transformation of quartz in the solid state in the shock pressure interval of 13–35 GPa. The width of the amorphous lamellae increases with pressure to finally merge at a shock pressure > 35 GPa into patches of diaplectic glass that finally replace the entire Qtz grain in a pseudomorphic manner (e.g., Langenhorst and Deutsch 1994; Langenhorst 2002). All these shock features occur at a distance of about 20 μm (Fig. 5) next to fused silica glass, substantiating a very heterogeneous P–T history.
Concerning the projectile fragments, tiny vesicles in the molten rim of a partly fused projectile fragment (Fig. 6C) may indicate boiling of the projectile. This finding concurs with data by Kenkmann et al. (2007, 2013) for the MEMIN pilot study (P1-2808) with a steel projectile and a target of Seeberger Sandstein (impact velocity 5.3 km s−1; calculated peak shock pressure of 50–60 GPa). These authors report droplets with vesicles on the surface of recovered pieces of the projectile and foamy textures of projectile spheroids, indicative of temperatures near the liquid-vapor transition of steel.
In the numerical modeling of the formation of Meteor Crater, Artemieva and Pierazzo (2011) used the ANEOS for pure Fe as equation of state for the Canyon Diablo IAB iron meteorite projectile. They report an onset of melting at about 162 GPa, and of vaporization at about 320 GPa. The peak pressure of 55 GPa as calculated for our experiment is obviously far too low to explain the observed phase changes in the CDC projectile. To overcome this problem, we suggest the formation of locally restricted areas/volumes of significantly enhanced pressure and/or temperature conditions; such areas may be at or near the contact between projectile and sandstone target. Three principal mechanisms are envisaged.
(1) As suggested by Kenkmann et al. (2007, 2013) friction and deformation (i.e., plastic work) of the projectile may play an important role in the increase in temperature.
(2) The effect of shock heating in porous material may be the other process for substantial temperature increase on a macroscopic and microscopic level. Macroscopically, in porous targets, the kinetic energy of the projectile is transferred more effectively into target heating (Ahrens and Gregson 1964). The amount of plastic work during compaction and compression of a porous material is large compared to processes in a shock front passing through nonporous material, leading to higher temperatures after the shock wave has passed (Wünnemann et al. 2006). On the microscopic scale, “pore collapse” results in local pressure and temperature peaks. Recent developments of new material models for use in hydrocodes treat the collapse of pore space during the impact process (Güldemeister et al. 2013). These models provide a good approximation to understand the P–T differences documented by mineralogical changes. Güldemeister et al. (2011, 2013) showed that despite a general decrease in the average shock pressure due to porosity, a significant local increase in shock pressure (up to four times of the initial pressure) occurs in the vicinity of pores. Therefore, we suggest enhanced post shock temperatures (cp., e.g., Stöffler and Langenhorst 1994). This pore collapse mechanism can explain our observations in the highly shocked ejecta fragments, with low and high shock-metamorphic features occurring very close to each other.
(3) The target chamber was evacuated to about 0.3 bar during the impact experiment. The residual atmosphere of the chamber is shocked and compressed in front of the accelerated projectile. The shock compression leads to heating of the air and consequently also of the projectile surface. Berry et al. (2007) measured surface temperatures of up to 1800 °C during a free flight experiment of a hemispherical graphite projectile, accelerated to 4.5 km s−1 at 0.32 bar experimental gas pressure. As the experimental conditions of Berry et al. (2007) are similar to our experiments we also expect some heating and maybe melting of the projectile front prior to impact.