Compositional variation and mixing of impact melts on microscopic scales

Abstract— We investigated the compositional characteristics of schlieren-rich, holohyaline impact glasses from Ries, Wabar, and Meteor Crater using a Cameca SX 100 scanning electron microprobe. This instrument is capable of producing detailed maps of major elements at spatial resolutions of <10 μm. The objective was to characterize the composition of an unusually large number of individual schlieren and to evaluate details of the process that causes melts of lithologically diverse target rocks to mix on scales of micrometers. The Ries and Meteor Crater impacts involved lithologically heterogeneous targets; whereas, Wabar Crater formed in relatively uniform dune sand. Texturally heterogeneous, schlieren-rich glasses from the Ries Crater illustrate that schlieren of highly variable color can be surprisingly similar in composition, as first detailed by Stähle (1972). Consistent with these earlier findings, most schlieren represent mixtures of diverse rock melts; their compositions deviate only subtly from the average melt and do not resemble monomineralic melts nor binary mixtures of major rock-forming minerals. A specific population of schlieren is enriched in mafic elements (Mg, Fe, and Ca), which suggests incomplete homogenization of an amphibolite progenitor. In the case of Wabar Crater, a compositionally simple melt of dune sand mixed with projectile (IIIA iron meteorite) materials, and specific schlieren are variable mixtures of these two progenitors. The optically homogeneous glass from Meteor Crater is compositionally homogeneous as well, which suggests ideal mixing of such diverse lithologies as platform carbonates, sandstone, and a class IIIA iron meteorite. The mixing of projectile and target melts at Wabar and Meteor Crater unambiguously demonstrates that melts initially produced in distinctly different stratigraphic/structural locations will undergo wholesale mixing, if not homogenization. Also, the projectile melts unquestionably formed relatively early in the cratering process, and their dissemination throughout the prospective melt volume, albeit at variable concentration levels, suggests that the entire mixing process may be an early cratering feature. This also follows from the fact that we investigated ballistic melt ejecta, which thereby eliminates all of those mixing processes that may additionally operate during the pooling and generation of massive melt-ponds following gravitational collapse of large, structurally complex craters. Substantial turbulence ranging from field dimensions to microscopic scales seems inescapable to accomplish the observed degree of mixing, yet this is not readily inferred from current models of macroscopic material motions during hypervelocity impact.     

Impact processing of chondritic planetesimals: Siderophile and volatile element fractionation in the Chico L chondrite

Abstract— A large impact event 500 Ma ago shocked and melted portions of the L‐chondrite parent body. Chico is an impact melt breccia produced by this event. Sawn surfaces of this 105 kg meteorite reveal a dike of fine‐grained, clast‐poor impact melt cutting shocked host chondrite. Coarse (1–2 cm diameter) globules of FeNi metal + sulfide are concentrated along the axis of the dike from metal‐poor regions toward the margins. Refractory lithophile element abundance patterns in the melt rock are parallel to average L chondrites, demonstrating near‐total fusion of the L‐chondrite target by the impact and negligible crystal‐liquid fractionation during emplacement and cooling of the dike. Significant geochemical effects of the impact melting event include fractionation of siderophile and chalcophile elements with increasing metal‐silicate heterogeneity, and mobilization of moderately to highly volatile elements. Siderophile and chalcophile elements ratios such as Ni/Co, Cu/Ga, and Ir/Au vary systematically with decreasing metal content of the melt. Surprisingly small (?10²) effective metal/silicate‐melt distribution coefficients for highly siderophile elements probably reflect inefficient segregation of metal despite the large degrees of melting. Moderately volatile lithophile elements such K and Rb were mobilized and heterogeneously distributed in the L‐chondrite impact breccias whereas highly volatile elements such as Cs and Pb were profoundly depleted in the region of the parent body sampled by Chico. Volatile element variations in Chico and other L chondrites are more consistent with a mechanism related to impact heating rather than condensation from a solar nebula. Impact processing can significantly alter the primary distributions of siderophile and volatile elements in chondritic planetesimals.     

Chromium isotopes identify the extraterrestrial component in impactites from Dhala impact structure,India

The Dhala structure in north-central India is a confirmed complex impact structure of Paleoproterozoic age. The presence of an extraterrestrial component in impactites from the Dhala structure was recognized by geochemical analyses of highly siderophile elements and Os isotopic compositions; however, the impactor type has remained unidentified. This study uses Cr isotope systematics to identify the type of projectile involved in the formation of the Dhala structure. Unlike the composition of siderophile elements (e.g., Ni, Cr, Co, and platinum group elements) and their inter-element ratios that may get compromised due to the extreme energy generated during an impact, Cr isotopes retain the distinct composition of the impactor. The distinct ε⁵⁴Cr value of −0.31 ± 0.09 for a Dhala impact melt breccia sample (D6-57) indicates inheritance from an impactor originating within the non-carbonaceous reservoir, that is, the inner Solar System. Based on the Ni/Cr ratio, Os abundance, and Cr isotopic composition of the samples, the impactor is constrained to be of ureilite type. Binary mixing calculations also indicate contamination of the target rock by 0.1–0.3 wt% of material from a ureilite-like impactor. Together with the previously identified impactors that formed El'gygytgyn, Zhamanshin, and Lonar impact structures, the Cr isotopic compositions of the Dhala impactites argue for a much more diverse source of the objects that collided with the Earth over its geological history than has been supposed previously.     

Clues to the nature of the impacting bodies from platinum-group elements (rhenium and gold) in borehole samples from the Clearwater East crater (Canada) and the Boltysh impact crater (Ukraine)

Abstract— Seven large (10 g) impact melt rock samples from boreholes from the Boltysh impact crater (Ukraine) and six samples from the East Clearwater crater (Canada) were analyzed for Os, Ir, Ru, Rh, Pd, Re and Au by the nickel sulfide technique in combination with neutron activation. Earlier analyses of Clearwater East impact melt rocks have shown that they are strongly enriched in Ir, Os, Pd and Re. In this work, I confirm earlier findings and demonstrate similarly high enrichments of Rh and Ru. The average Os/Ir, Ru/Ir, Pd/Ir, Rh/Ir and Ru/Rh ratios of the melt rock samples from Clearwater East are CI-chondritic and yield an average Ir content of 25.2 ± 6.5 ng/g relative to an average upper crust concentration of 0.03 ± 0.02 ng/g Ir. The amount of meteoritic component corresponds to 4 to 7% of a nominal CI component for Clearwater East. The impact melt rock samples from a bore hole from Boltysh are low in Ir with an average of 0.2 ± 0.1 ng/g. The CI-normalized abundances increase from the refractory to the more volatile siderophile elements (Os < Ir < Ru < Rh ～ Pd ～ Au ～ Ni ～ Co). Because of the low Ir anomaly and uncertainties in making corrections (correlations are weak) for indigenous siderophile elements, no clear projectile assignment can be made.     

Parent body histories recorded in Rumuruti chondrite sulfides: Implications for the onset of oxidized,sulfur-rich core formation

Models of planetary core formation beginning with melting of Fe,Ni metal and troilite are not readily applicable to oxidized and sulfur-rich chondrites containing only trace quantities of metal. Cores formed in these bodies must be dominated by sulfides. Siderophile trace elements used to model metallic core formation could be used to model oxidized, sulfide-dominated core formation and identify related meteorites if their trace element systematics can be quantified. Insufficient information exists regarding the behavior of these core-forming elements among sulfides during metamorphism prior to anatexis. Major, minor, and trace element concentrations of sulfides are reported in this study for petrologic type 3–6 R chondrite materials. Sulfide-dominated core-forming components in such oxidized chondrites (ƒO₂ ≥ iron-wüstite) follow metamorphic evolutionary pathways that are distinct from reduced, metal-bearing counterparts. Most siderophile trace elements partition into pentlandite at approximately 10× chondritic abundances, but Pt, W, Mo, Ga, and Ge are depleted by 1–2 orders of magnitude relative to siderophile elements with similar volatilities. The distribution of siderophile elements is further altered during hydrothermal alteration as pyrrhotite oxidizes to form magnetite. Oxidized, sulfide-dominated core formation differs from metallic core formation models both physically and geochemically. Incongruent melting of pentlandite at 865°C generates melts capable of migrating along solid silicate grains, which can segregate to form a Ni,S-rich core at lower temperatures compared to reduced differentiated parent bodies and with distinct siderophile interelement proportions.     

Chemical modification of projectile residues and target material in a MEMIN cratering experiment

Abstract– In the context of the MEMIN project, a hypervelocity cratering experiment has been performed using a sphere of the iron meteorite Campo del Cielo as projectile accelerated to 4.56 km s^?1, and a block of Seeberger sandstone as target material. The ejecta, collected in a newly designed catcher, are represented by (1) weakly deformed, (2) highly deformed, and (3) highly shocked material. The latter shows shock‐metamorphic features such as planar deformation features (PDF) in quartz, formation of diaplectic quartz glass, partial melting of the sandstone, and partially molten projectile, mixed mechanically and chemically with target melt. During mixing of projectile and target melts, the Fe of the projectile is preferentially partitioned into target melt to a greater degree than Ni and Co yielding a Fe/Ni that is generally higher than Fe/Ni in the projectile. This fractionation results from the differing siderophile properties, specifically from differences in reactivity of Fe, Ni, and Co with oxygen during projectile‐target interaction. Projectile matter was also detected in shocked quartz grains. The average Fe/Ni of quartz with PDF (about 20) and of silica glasses (about 24) are in contrast to the average sandstone ratio (about 422), but resembles the Fe/Ni‐ratio of the projectile (about 14). We briefly discuss possible reasons of projectile melting and vaporization in the experiment, in which the calculated maximum shock pressure does not exceed 55 GPa.     

Petrographic studies of the impact melts from Meteor Crater,Arizona, USA

Abstract— We investigated the ballistically dispersed melts from Meteor Crater, Arizona, USA to determine the stratigraphic extent of its melt zone from the compositional relationship of melts and target rocks. Most melt particles are crystallized, hydrated, and oxidized; pristine glasses are rare. Hydration and oxidation occurred at ambient temperatures long after the impact. The preserved glasses are generally clear and texturally homogeneous, but unlike typical impact melts, they have unusually heterogeneous compositions, both within individual particles and from sample to sample. For example, the average SiO₂ for individual particles ranges from 43 to 65%. The projectile content is unusually high and it is distributed bimodally, with specific samples containing either 5–10% or 20–30% FeO. These compositional heterogeneities most likely reflect the high carbonate content of the target rocks and the release of copious CO₂ that dispersed the melts, thereby terminating melt flow and mixing. The high projectile content and the CO₂ depleted residue of purely sedimentary rocks produced mafic melts that crystallized fine‐grained olivine and pyroxene. The melts fall into three compositional groups reflecting variable proportions of the major target formations, Moenkopi, Kaibab, and Coconino. Least‐square mixing calculations revealed one group to contain 55% Moenkopi, 40% quartz‐rich, upper Kaibab, and 5% meteorite, suggesting a source depth of <30 m from the pre‐impact surface. The other two melt groups have higher contents of meteorite (15–20%) and Kaibab (50–70%) and contain more SiO₂ than average Kaibab. The additional quartz may have been derived from Coconino or the upper Kaibab, implying melt depths >90 m or <30 m, respectively. Additional studies, especially hydrocode calculations, are needed to better understand the source depth of these melts and their exceptionally high projectile content.     

Geochemical evidence of an extraterrestrial component in impact melt breccia from the Paleoproterozoic Dhala impact structure,India

The Paleoproterozoic Dhala structure with an estimated diameter of ~11 km is a confirmed complex impact structure located in the central Indian state of Madhya Pradesh in predominantly granitic basement (2.65 Ga), in the northwestern part of the Archean Bundelkhand craton. The target lithology is granitic in composition but includes a variety of meta‐supracrustal rock types. The impactites and target rocks are overlain by ~1.7 Ga sediments of the Dhala Group and the Vindhyan Supergroup. The area was cored in more than 70 locations and the subsurface lithology shows pseudotachylitic breccia, impact melt breccia, suevite, lithic breccias, and postimpact sediments. Despite extensive erosion, the Dhala structure is well preserved and displays nearly all the diagnostic microscopic shock metamorphic features. This study is aimed at identifying the presence of an impactor component in impact melt rock by analyzing the siderophile element concentrations and rhenium‐osmium isotopic compositions of four samples of impactites (three melt breccias and one lithic breccia) and two samples of target rock (a biotite granite and a mafic intrusive rock). The impact melt breccias are of granitic composition. In some samples, the siderophile elements and HREE enrichment observed are comparable to the target rock abundances. The Cr versus Ir concentrations indicate the probable admixture of approximately 0.3 wt.% of an extraterrestrial component to the impact melt breccia. The Re and Os abundances and the ¹⁸⁷Os/¹⁸⁸Os ratio of 0.133 of one melt breccia specimen confirm the presence of an extraterrestrial component, although the impactor type characterization still remains inconclusive.     

Platinum group elements in impactites of the ICDP Chicxulub drill core Yaxcopoil‐1: Are there traces of the projectile?

Abstract— This study presents results of platinum group element (PGE) analyses of impactites from the Yaxcopoil‐1 (Yax‐1) and Yucatán 6 drill cores of the 180 km‐diameter Chicxulub crater. These are the main elements used for projectile identification. They were determined by nickel sulfide fire assay combined with inductively coupled plasma mass spectrometry. The concentration of PGE in the samples are low. The concentration patterns of the suevite samples resemble the pattern of the continental crust. We conclude that any meteoritic fraction in these samples is below 0.05%. A syn‐ and post‐impact modification of the PGE pattern from meteoritic toward a continental crust pattern is very unlikely. The globally distributed fallout at the Cretaceous‐Tertiary (K/T) boundary, however, has high PGE concentrations. Therefore, the lack of a significant meteoritic PGE signature in the crater is not an argument for a PGE‐poor impactor. Taking the results of three‐dimensional numerical simulations of the Chicxulub event into account, the following conclusions are drawn: 1) The main fraction of the impactor was ejected into and beyond the stratosphere, distributed globally, and deposited in the K/T boundary clay; and 2) the low amount of projectile contamination in the Yax‐1 lithologies may reflect an oblique impact. However, the role of volatiles in the mixing process between projectile and target is not well‐understood and may also have played a fundamental role.     

Origin of lunar fragmental matrix breccias—Highly siderophile element constraints

Ejecta at North Ray crater (Apollo 16) sampled a unique section of the lunar highlands not accessible at most other landing sites and provide important constraints on the composition of late accreted materials. New data on multiple aliquots of four fragmental matrix breccias and a fragment‐laden melt breccia from this site display a variety of highly siderophile element patterns which may represent the signatures of volatile element‐depleted carbonaceous chondrite‐like material, primitive achondrite, differentiated metal, and an impactor component that cannot be related to known meteoritic material. The latter component is prevalent in these rocks besides characterized by depletions in Re and Os compared to Ir, Ru and Pt, chondritic Re/Os, and a gradual depletion of Pd and Au. The observed characteristics are more consistent with fractionations by nebular processes, like incomplete condensation or evaporation, than with lunar crustal processes, like partial melting or volatilization. The impactor signature preserved in these breccias may stem from primitive meteorites with a refractory element composition moderately different from known chondrites. The presence of distinct impactor components within the North Ray crater breccias together with observed correlations of characteristic element ratios (e.g., Re/Os, Ru/Pt, Pd/Ir) in different impact lithologies of four Apollo landing sites constrains physical mixing processes ranging from the scale of gram‐sized samples to the area covered by the Apollo missions.     

“New” lunar meteorites: Impact melt and regolith breccias and large‐scale heterogeneities of the upper lunar crust

Abstract— We have analyzed nine highland lunar meteorites (lunaites) using mainly INAA. Several of these rocks are difficult to classify. Dhofar 081 is basically a fragmental breccia, but much of its groundmass features a glassy‐fluidized texture that is indicative of localized shock melting. Also, much of the matrix glass is swirly‐brown, suggesting a possible regolith derivation. We interpret Dar al Gani (DaG) 400 as an extremely immature regolith breccia consisting mainly of impact‐melt breccia clasts; we interpret Dhofar 026 as an unusually complex anorthositic impact‐melt breccia with scattered ovoid globules that formed as clasts of mafic, subophitic impact melt. The presence of mafic crystalline globules in a lunar material, even one so clearly impact‐heated, suggests that it may have originated as a regolith. Our new data and a synthesis of literature data suggest a contrast in Al₂O₃‐incompatible element systematics between impact melts from the central nearside highlands, where Apollo sampling occurred, and those from the general highland surface of the Moon. Impact melts from the general highland surface tend to have systematically lower incompatible element concentration at any given Al₂O₃ concentration than those from Apollo 16. In the case of Dhofar 026, both the bulk rock and a comparatively Al‐poor composition (14 wt% Al₂O₃, 7 μg/g Sm) extrapolated for the globules, manifest incompatible element contents well below the Apollo 16 trend. Impact melts from Luna 20 (57°E) distribute more along the general highland trend than along the Apollo 16 trend. Siderophile elements also show a distinctive composition for Apollo 16 impact melts: Ni/Ir averaging ?1.8x chondritic. In contrast, lunaite impact‐melt breccias have consistently chondritic Ni/Ir. Impact melts from Luna 20 and other Apollo sites show average Ni/Ir almost as high as those from Apollo 16. The prevalence of this distinctive Ni/Ir ratio at such widely separated nearside sites suggests that debris from one extraordinarily large impact may dominate the megaregolith siderophile component of a nearside region 2300 km or more across. Highland polymict breccia lunaites and other KREEP‐poor highland regolith samples manifest a strong anticorrelation between Al₂O₃ and mg. The magnesian component probably represents the chemical signature of the Mg‐suite of pristine nonmare rocks in its most “pure” form, unaltered by the major KREEP‐assimilation that is so common among Apollo Mg‐suite samples. The average composition of the ferroan anorthositic component is now well constrained at Al₂O₃ ?29–30 wt% (implying about 17–19 wt% modal mafic silicates), in good agreement with the composition predicted for flotation crust over a “ferroan” magma ocean (Warren 1990).     

The Canyon Diablo impact event: 2. Projectile fate and target melting upon impact

Abstract– Despite its centennial exploration history, there are still unresolved questions about Meteor Crater, the first recognized impact crater on Earth. This theoretical study addresses some of these questions by comparing model results with field and laboratory studies of Meteor Crater. Our results indicate that Meteor Crater was formed by a high‐velocity impact of a fragmented projectile, ruling out a highly dispersed swarm as well as a very low impact velocity. Projectile fragmentation caused many fragments to fall separately from the main body of the impactor, making up the bulk of the Canyon Diablo meteorites; most of these fragments were engulfed in the expansion plume as they approached the surface without suffering high shock compression, and were redistributed randomly around the crater. Thus, the distribution of Canyon Diablo meteorites is not representative of projectile trajectory, as is usual for impactor fragments in smaller strewn fields. At least 50% of the main impactor was ejected from the crater during crater excavation and was dispersed mostly downrange of the crater as molten particles (spheroids) and highly shocked solid fragments (shrapnel). When compared with the known distribution, model results suggest an impactor from the SW. Overall, every model case produced much higher amounts of pure projectile material than observed. The projectile‐target mixing was not considered in the models; however, this process could be the main sink of projectile melt, as all analyzed melt particles have high concentrations of projectile material. The fate of the solid projectile fragments is still not completely resolved. Model results suggest that the depth of melting in the target can reach the Coconino sandstone formation. However, most of the ejected melt originates from 30–40 m depth and, thus, is limited to Moenkopi and upper Kaibab material. Some melt remains in the target; based on the estimated volume of the breccia lens at Meteor Crater, our models suggest at most a 2% content of melt in the breccia. Finally, a high water table at the time of impact could have aided strong dispersion of target and projectile melt.     

Indigenous abundances of siderophile elements in the lunar highlands: Implications for the origin of the moon

Substantial indigenous abundances of siderophile elements have been found to be present in the lunar highlands. The abundances of 13 siderophile elements in the parental magma of the highlands crust were estimated by using a simple model whereby the Apollo 16 highlands were regarded as being a mixture of three components (i.e. cumulus plagioclase + intercumulus magma that was parentel to the highlands crust + meteoritic contamination by ordinary chondrites). The parental magma of the highlands was found to possess abundances of siderophile elements that were generally similar to the abundances of the unequivocally indigenous siderophile elements in primitive, low-Ti mare basalts. This striking similarity implies that these estimated abundances in the parental highlands magma are truly indigenous, and also supports the basic validity of our simple model.It is shown that metal/silicate fractionation within the Moon cannot have been the cause of the siderophile element abundances in the parental highlands magma and primitive, low-Ti mare basalts. The relative abundances of the indigenous siderophile elements in highland and mare samples seem, instead, to be the result of complex processes which operatedprior to the Moon's accretion.The abundances of the relatively involatile, siderophile elements in the parental highlands magma are strikingly similar to the abundances observed in terrestrial oceanic tholeiites. Furthermore, the abundances of the relatively volatile, siderophile elements in the parental highlands magma are also systematically related to the corresponding abundances in terrestrial oceanic tholeiites. In fact, the parental magma of the lunar highlands can be essentially regarded as having been a volatile-depleted, terrestrial oceanic tholeiite.The complex, siderophile element fractionations in the Earth's upper mantle are thought to be the result of core segregation. However, it is well-known that the siderophile element abundances do not correspond to expectations based solely upon equilibration of metal/silicate at low-pressures, as evidenced by the over-abundances of Au, Re, Ni, Co and Cu. Ringwood (1977a) has suggested that the siderophile element abundances in the Earth's upper mantle are the product of equilibration at very high-pressures between the mantle and a segregating core that contained substantial quantities of an element with a low atomic weight, such as oxygen. Comparable processes cannot have operated within the Moon due to its small internal pressures and the very small size of its possible core. Therefore, the fact that the Moon exhibits a systematic resemblance to the Earth's upper mantle is highly significant.The origin of the Moon is discussed in the context of these results. The possibility that depletion of siderophile elements occurred in an earlier generation of differentiated planetesimals similar to those which formed the basaltic achondrites, stony-irons, and irons is examined but can be dismissed on several grounds. It seems that the uniquely terrestrial siderophile signature within the Moon can be explained only if the Moon was derived from the Earth's mantle subsequent to core-formation.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May, 1978.     

Petrology and geochemistry of a silicate clast from the Mount Padbury mesosiderite: Implications for metal‐silicate mixing events of mesosiderite

Abstract— Petrological and bulk geochemical studies were performed on a large silicate clast from the Mount Padbury mesosiderite. The silicate clast is composed mainly of pyroxene and plagioclase with minor amounts of ilmenite, spinel, and other accessory minerals, and it shows subophitic texture. Pyroxenes in the clast are similar to those in type 5 eucrites and could have experienced prolonged thermal metamorphism after rapid crystallization from a near‐surface melt. Ilmenite and spinel vary chemically, indicating growth under disequilibrium conditions. The clast seems to have experienced an episode of rapid reheating and cooling, possibly as a result of metal‐silicate mixing. Abundances of siderophile elements are obviously higher than in eucrites, although the clast is also extremely depleted in highly siderophile elements. The fractionated pattern can be explained by injection of Fe‐FeS melts generated by partial melting of metallic portions during metal‐silicate mixing. The silicate clast had a complex petrogenesis that could have included: 1) rapid crystallization from magma in a lava flow or a shallow intrusion; 2) prolonged thermal metamorphism to equilibrate the mineral compositions of pyroxene and plagioclase after primary crystallization; 3) metal‐silicate mixing probably caused by the impact of solid metal bodies on the surface of the mesosiderite parent body; and 4) partial melting of metal and sulfide portions (and silicate in some cases) caused by the collisional heating, which produced Fe‐FeS melts with highly fractionated siderophile elements that were injected into silicate portions along cracks and fractures.     

Fractionation of refractory siderophile elements in metal from the Rose City meteorite

Abstract— An ～4 × 9 × 12-mm concentration of metal (dubbed RC1) situated between silicate melt and a relict chondritic clast in the Rose City H5 impact-melt breccia is compositionally heterogeneous. Approximately 65 wt% of RC1 is enriched in the refractory siderophile elements, Os and Ir, by 30–40% relative to bulk H chondrite metal; ～20 wt% is depleted in these elements by 31–35%; and 15 wt% is depleted by a considerably greater amount (75%). Common and volatile siderophile elements are essentially unfractionated in all three regions; W is fractionated to only a moderate degree. The compositions of the different regions of RC1 are similar to those of previously analyzed metal nodules and veins in shocked but unmelted ordinary chondrites. All of these objects probably formed by a complex process involving vaporization of chondritic material, rapidly followed by oxidation of W to form volatile oxides, fractional condensation of refractory siderophile elements, transport of the residual vapor (containing common and volatile siderophile elements as well as W oxide) and condensation of this vapor in fractures and voids or on metallic liquid substrates. The common occurrence of vugs in shock-heated chondrites and the pervasiveness of vaporization effects recorded in metal masses and veins underscores the important role of superheating in the formation of impact breccias.     

Early fracturing and impact residue emplacement: Can modelling help to predict their location in major craters?

Abstract— Understanding the nature and composition of larger extraterrestrial bodies that may collide with the Earth is important. One source of information lies in the record of ancient impact craters, some of which have yielded chemical information as to the impacting body. Many deeply eroded craters have no remaining melt sheet or ejecta yet may contain impactor residue within basement fractures. The emplacement mechanism for fractionated siderophile residues is likely to be gaseous, although, melt droplets and some solid materials may survive. For breccia‐ and melt‐filled fractures to contain extraterrestrial material, they must form very early in the impact process. Most current numerical models do not dwell on the formation and location of early major fractures, although, fractures in and around small craters on brittle glass exposed to hypervelocity impact in low Earth orbit have been successfully simulated. Modelling of fracture development associated with larger craters may help locate impact residues and test the models themselves.     

A database of chondrite analyses including platinum group elements,Ni, Co,Au, and Cr: Implications for the identification of chondritic projectiles

Abstract— Siderophile elements have been used to constrain projectile compositions in terrestrial and lunar impact melt rocks. To obtain a better knowledge of compositional differences between potential chondritic projectile types, meteorite analyses of the elements Ru, Rh, Pd, Os, Ir, Pt, Cr, Co, Ni, and Au were gathered into a database. The presented compilation comprises 806 analyses of 278 chondrites including new ICP‐MS analyses of Allende and two ordinary chondrites. Each data set was evaluated by comparing element ratios of meteorites from the same chondrite group. Characteristic element abundances and ratios were determined for each group. Features observed in the element abundance patterns can be linked directly to the presence of certain components, such as the abundance of refractory elements Os, Ir, and Ru correlating with the occurrence of refractory inclusions in CV, CO, CK, and CM chondrites. The refined characteristic element ratios appear to be representative not only for meteorites, but also for related asteroidal bodies. Chondrite element ratios were compared to previously published values from impact melt rocks of the Popigai and Morokweng impact structures confirming that an identification of the specific type of projectile (L and LL chondrite, respectively) is possible. The assessment for Morokweng is supported by the recent discovery of an LL chondrite fragment in the impact melt rocks. Ultimately, the database provides valuable information for understanding processes in the solar nebula as they are recorded in chondrites. A new type of complementarity between element patterns of CK and EH chondrites is suggested to be the result of condensation, redox, and transportation processes in the solar nebula.     

Traces of an H chondrite in the impact‐melt rocks from the Lappajärvi impact structure,Finland

Abstract— Here we present the results of a geochemical study of the projectile component in impactmelt rocks from the Lappajärvi impact structure, Finland. Main‐ and trace‐element analyses, including platinum group elements (PGEs), were carried out on twenty impact‐melt rock samples from different locations and on two shocked granite fragments. The results clearly illustrate that all the impact melt rocks are contaminated with an extraterrestrial component. An identification of the projectile type was performed by determining the projectile elemental ratios and comparing the corresponding element ratios in chondrites. The projectile elemental ratios suggest an H chondrite as the most likely projectile type for the Lappajärvi impact structure. The PGE composition of the highly diluted projectile component (?0.05 and 0.7 wt% in the impact‐melt rocks) is similar to the recent meteorite population of H chondrites reaching Earth. The relative abundance of ordinary chondrites, including H, L, and LL chondrites, as projectiles at terrestrial impact structures is most likely related to the position of their parent bodies relative to the main resonance positions. This relative abundance of ordinary chondrites suggests a strong bias of the impactor population toward inner Main Belt objects.     

Meteoritic material in a Boulder from the Apollo 17 Site: Implications for its origin

Sixteen samples of Boulder 1 from Station 2 at the Apollo 17 site were analyzed by radiochemical neutron activation analysis for Ag, Au, Bi, Br, Cd, Cs, Ge, Ir, Ni, Rb, Re, Sb, Te, Tl, U, and Zn. Two clast samples contam no meteoritic material and appear to consist of relatively pristine igneous rocks: an unusual, KREEP-rich pigeonite basalt of very high Ge content, and an alkali-poor coarse norite. Nine grey or black breccia samples contain a unique, Group 3 meteoritic component of Ir/Au ratio 0.65–0.82, which appears to separate into subgroups 3H and 3L on the basis of Ni, Ge, and Re content. It is quite distinct from the Group 2 component (Ir/Au - 0.46–0.54) that dominates at the Apollo 17 site.The unique black-rimmed clasts from this boulder show striking compositional zoning. The cores of anorthositic breccia are very low in Rb, Cs, and U, and have a distinctive 5L meteoritic component (Ir/Au1.1). The black rinds are 5- to 10-fold richer in Rb, Cs, and U and have a Group 3 meteoritic component. The cores may represent breccias formed in an earlier impact that became coated with alkali-rich ejecta during the event that produced the boulder.Because of the rarity of the Group 3 meteoritic component at the Apollo 17 site, this boulder cannot represent ordinary Serenitatis ejecta, with their characteristic admixture of the Group 2 Serenitatis projectile. It may represent pre-Serenitatis material excavated from the fringes of the crater during late stages of the Serenitatis impact, but only lightly shocked and hence uncontaminated by the Serenitatis projectile.