Impact Melt Origin Research Articles

Lunar Mantle Composition Based on Spectral and Geologic Analysis of Low‐Ca Pyroxene‐ and Olivine‐Rich Rocks Exposed on the Lunar Surface

AbstractWe studied the lunar mantle composition based on the characteristics of low‐Ca pyroxene (LCP)‐rich and olivine‐rich rocks exposed on the lunar surface. Using data mining with SELENE (Kaguya) hyperspectral data, we found 531 sites with spectra dominated by an ultramafic LCP end‐member mineral, most of which are located in the Imbrium basin and the South Pole‐Aitken (SPA) basin. Stratigraphic analysis of each site revealed that LCP‐rich rock bodies are exposed on fresh geological features that are less affected by space‐weathering, such as steep slopes at peaks, and crater walls and ejecta deposits at smaller craters. We also found that, in the SPA and Imbrium basins, LCP‐rich bodies are more numerous and more widely distributed than olivine‐rich rocks, suggesting that LCP‐rich materials deep in the mantle were excavated during the formation of these huge basins. However, olivine‐rich rocks were abundant, and no LCP‐rich rocks were found in the Moscoviense, Crisium, and Humboldtianum basins, which are known to have almost‐zero crustal thicknesses, indicative of mantle excavation. Thus, our results suggest that the composition of rocks derived from the lunar mantle varies with the impact basin. Such a difference might indicate a layered structure of mantle composition, with the olivine‐rich upper mantle overlying the LCP‐rich mantle, a horizontal heterogeneity in the mantle composition, a regional heterogeneity in early lunar basaltic magmatism, or an impact melt origin. The layered structure or horizontal heterogeneity might have resulted from a mantle overturn caused by gravitational instability in the early stages of the lunar magma ocean.

Petrogenesis of ungrouped enstatite meteorite Zakłodzie: Fabric, texture, and nanostructure analysis for identification of mechanisms responsible for chondrite–achondrite transition

AbstractZakłodzie is an enstatite meteorite of unknown petrogenesis. Chemically, it resembles enstatite chondrites, but displays an achondrite‐like texture. Here we report on fabric and texture analyses of Zakłodzie utilizing X‐ray computed tomography and scanning electron microscopy and combine it with a nanostructural study of striated pyroxene by transmission electron microscopy. With this approach we identify mechanisms that led to formation of the texture and address the petrogenesis of the rock. Zakłodzie experienced a shock event in its early evolution while located at some depth inside a warm parent body. Shock‐related strain inverted pyroxene to the observed mixture of intercalated orthorhombic and monoclinic polymorphs. The heat that dissipated after the peak shock was added to primary, radiogenic‐derived heat and led to a prolonged thermal event. This caused local, equilibrium‐based partial melting of plagioclase and metal‐sulfide. Partial melting was followed by two‐stage cooling. The first phase of annealing (above 500 °C) allowed for crystallization of plagioclase and for textural equilibration of metal and sulfides with silicates. Below 500 °C, cooling was faster and more heterogeneous at cm scale, allowing retention of keilite and quenching of K‐rich feldspathic glass in some parts. Our study indicates that Zakłodzie is neither an impact melt rock nor a primitive achondrite, as suggested in former studies. An impact melt origin is excluded because enstatite in Zakłodzie was never completely melted and partial melting occurred during equilibrium‐based postshock conditions. Texturally, the rock represents a transition of chondrite and achondrite and was formed when early impact heat was added to internal radiogenic heat.

An impact melt origin for Earth’s oldest known evolved rocks

Earth’s oldest evolved (felsic) rocks, the 4.02-billion-year-old Idiwhaa gneisses of the Acasta Gneiss Complex, northwest Canada, have compositions that are distinct from the felsic rocks that typify Earth’s ancient continental nuclei, implying that they formed through a different process. Using phase equilibria and trace element modelling, we show that the Idiwhaa gneisses were produced by partial melting of iron-rich hydrated basaltic rocks (amphibolites) at very low pressures, equating to the uppermost ~3 km of a Hadean crust that was dominantly mafic in composition. The heat required for partial melting at such shallow levels is most easily explained through meteorite impacts. Hydrodynamic impact modelling shows not only that this scenario is physically plausible, but also that the region of shallow partial melting appropriate to formation of the Idiwhaa gneisses would have been widespread. Given the predicted high flux of meteorites in the late Hadean, impact melting may have been the predominant mechanism that generated Hadean felsic rocks. Earth’s oldest known felsic rocks formed by partial melting at low pressures and high temperatures caused by impact melting of mafic Hadean crust, according to phase equilibria and trace element modelling.

Cryogenic flow features on Ceres: Implications for crater‐related cryovolcanism

AbstractCraters on Ceres, such as Haulani, Kupalo, Ikapati, and Occator show postimpact modification by the deposition of extended plains material with pits, multiple lobate flows, and widely dispersed deposits that form a diffuse veneer on the preexisting surface. Bright material units in these features have a negative spectral slope in the visible range, making it appear bluish with respect to the grey‐toned overall surface of Ceres. We calculate the drop height‐to‐runout length ratio of several flow features and obtain a coefficient of friction of < 0.1: The results imply higher flow efficiency for flow features on Ceres than for similar features on other planetary bodies with similar gravity, suggesting low‐viscosity material. The special association of flow features with impact craters could either point to an impact melt origin or to an exogenic triggering of cryovolcanic processes.

Global occurrence trend of high-Ca pyroxene on lunar highlands and its implications

We present details of the global distribution of high-Ca pyroxene (HCP)-rich sites in the lunar highlands based on the global data set of hyperspectral reflectance obtained by the SELENE Spectral Profiler. Most HCP-rich sites in the lunar highlands are found at fresh impact craters. In each crater, most of the detection points are distributed on the ejecta, rim, and floor of the impact craters rather than the central peaks, while the central peaks are dominated by purest anorthosite (PAN). This indicates that HCP-rich materials originate from relatively shallower regions of the lunar crust than PAN. In addition, while all ray craters with sizes larger than ∼40 km possess HCP-rich materials, small fresh craters with sizes less than ∼6–10 km do not, indicating that the uppermost mixing layers in the lunar crust are not dominated by HCP. Based on these results, we propose that in the upper lunar crust, a HCP-rich zone overlying the PAN layer exists below the uppermost mixing layer. This HCP-rich zone may originate from interstitial melt during the formation of the flotation anorthositic cumulate, while an impact ejecta origin, impact melt origin, and/or magmatic intrusion into the upper lunar crust may also account for the occurrence of HCP-rich sites in the highlands.

Multiple origins for olivine at Copernicus crater

Multiple origins for olivine-bearing lithologies at Copernicus crater are recognized based on integrated analysis of data from Chandrayaan-1 Moon Mineralogy Mapper (M3), Lunar Reconnaissance Orbiter (LRO) Narrow Angle Camera (NAC) and Kaguya Terrain Camera (TC). We report the diverse morphological and spectral character of previously known olivine-bearing exposures as well as the new olivine occurrences identified in this study. Prominent albedo differences exist between olivine-bearing exposures in the central peaks and a northern wall unit (the latter being ∼40% darker). The low-albedo wall unit occurs as a linear mantling deposit and is interpreted to be of impact melt origin, in contrast with the largely unmodified nature of olivine-bearing peaks. Small and localized occurrences of olivine-bearing lithology have also been identified on the impact melt-rich floor, representing a third geologic setting (apart from crater wall and peaks). Recent remote sensing missions have identified olivine-bearing exposures around lunar basins (e.g. Yamamoto et al., 2010; Pieters et al., 2011; Kramer et al., 2013) and at other craters (e.g. Sun and Li, 2014), renewing strong interest in its origin and provenance. A direct mantle exposure has commonly been suggested in this regard. Our detailed observations of the morphological and spectral diversity in the olivine-bearing exposures at Copernicus have provided critical constraints on their origin and source regions, emphasizing multiple formation mechanisms. These findings directly impact the interpretation of olivine exposures elsewhere on the Moon. Olivine can occur in diverse environments including an impact melt origin, and therefore it is unlikely for all olivine exposures to be direct mantle occurrences as has generally been suggested.

Petrogenesis of a vitrophyre in the martian meteorite breccia NWA 7034

Northwest Africa (NWA) 7034 and its paired meteorites NWA 7533 and NWA 7475 are the first recognized martian polymict breccia samples. An unusual, large, subrounded clast in NWA 7034 shows a vitrophyric texture, consisting of skeletal pyroxene and olivine with mesostasis. This lithology has not been observed in the paired meteorites. It crystallized under disequilibrium conditions as indicated by its olivine and pyroxene KDFe/Mg partitioning values, as well as reversed order of crystallization and mineral compositions relative to those predicted by MELTS. We report the highest bulk Ni value (1020ppm) measured in any known martian meteorite or martian igneous rock, suggesting an impact melt origin for the vitrophyre. Addition of 5.3–7.7% chondritic material to the target rock would account for the Ni enrichment. The bulk major and trace element abundances of the vitrophyre indicate that the protolith was not the host breccia nor any other martian meteorites. However, the clast is compositionally similar to Humphrey rock in Gusev crater analyzed by the Spirit rover and to a texturally distinct group of clasts in the paired meteorite NWA 7533. Thus, we propose that the target rock was an igneous lithology similar to Gusev basalts, which was subsequently contaminated by a chondritic impactor.

Olivine or impact melt: Nature of the “Orange” material on Vesta from Dawn

NASA’s Dawn mission observed a great variety of colored terrains on asteroid (4) Vesta during its survey with the Framing Camera (FC). Here we present a detailed study of the orange material on Vesta, which was first observed in color ratio images obtained by the FC and presents a red spectral slope. The orange material deposits can be classified into three types: (a) diffuse ejecta deposited by recent medium-size impact craters (such as Oppia), (b) lobate patches with well-defined edges (nicknamed “pumpkin patches”), and (c) ejecta rays from fresh-looking impact craters. The location of the orange diffuse ejecta from Oppia corresponds to the olivine spot nicknamed “Leslie feature” first identified by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130–157) from ground-based spectral observations. The distribution of the orange material in the FC mosaic is concentrated on the equatorial region and almost exclusively outside the Rheasilvia basin. Our in-depth analysis of the composition of this material uses complementary observations from FC, the visible and infrared spectrometer (VIR), and the Gamma Ray and Neutron Detector (GRaND). Several possible options for the composition of the orange material are investigated including, cumulate eucrite layer exposed during impact, metal delivered by impactor, olivine–orthopyroxene mixture and impact melt. Based on our analysis, the orange material on Vesta is unlikely to be metal or olivine (originally proposed by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130–157)). Analysis of the elemental composition of Oppia ejecta blanket with GRaND suggests that its orange material has ∼25% cumulate eucrite component in a howarditic mixture, whereas two other craters with orange material in their ejecta, Octavia and Arruntia, show no sign of cumulate eucrites. Morphology and topography of the orange material in Oppia and Octavia ejecta and orange patches suggests an impact melt origin. A majority of the orange patches appear to be related to the formation of the Rheasilvia basin. Combining the interpretations from the topography, geomorphology, color and spectral parameters, and elemental abundances, the most probable analog for the orange material on Vesta is impact melt.

Crater floor topography and impact melt sheet geometry of the Sudbury impact structure, Canada

Terra Nova, 00, 000–000, 2010 Abstract Based on high-resolution topography data and the geometry of geological contact traces at the surface, we determined the orientation of layer interfaces of the Sudbury Igneous Complex (SIC). Our analysis provides, for the first time, quantitative structural evidence for the thickness variation of its layers, which supports an impact melt origin of the SIC. The present crater floor topography varies up to 400 m over distances of hundreds of metres to a few kilometres, and up to 1500 m over a distance of about 25 km. Crater floor depressions are spatially associated with economically important sulphide mineral deposits, which points to a viable exploration strategy. Finally, observed variations in the width of the thermal aureole imparted by the SIC on its host rocks are due to primary thickness variations of the SIC rather than post-impact deformation.

Spectral properties of simulated impact glasses produced from martian soil analogue JSC Mars-1

To simulate the formation of impact glasses on Mars, an analogue of martian bright soil (altered volcanic soil JSC Mars-1) was melted at relevant oxygen fugacities using a pulsed laser and a resistance furnace. Reduction of Fe3+ to Fe2+ and in some cases formation of nanophase Fe0 in the glasses were documented by Mössbauer spectroscopy and TEM studies. Reflectance spectra for several size fractions of the JSC Mars-1 sample and the glasses were acquired between 0.3 and 25 μm. The glasses produced from the JSC Mars-1 soil show significant spectral variability depending on the method of production and the cooling rate. In general, they are dark and less red in the visible compared to the original JSC Mars-1 soil. Their spectra do not have absorption bands due to bound water and structural OH, have positive spectral slopes in the near-infrared range, and show two broad bands centered near 1.05 and 1.9 μm, typical of glasses rich in ferrous iron. The latter bands and low albedo partly mimic the spectral properties of martian dark regions, and may easily be confused with mafic materials containing olivine and low-Ca pyroxene. Due to their disordered structures and vesicular textures, the glasses show relatively weak absorption features from the visible to the thermal infrared. These weak absorption bands may be masked by the stronger bands of mafic minerals. Positive near-infrared spectral slopes typical of fresh iron-bearing impact or volcanic glasses may be masked either by oxide/dust coatings or by aerosols in the Mars' atmosphere. As a result, impact glasses may be present on the surface of Mars in significant quantities that have been either misidentified as other phases or masked by phases with stronger infrared features. Spectrometers with sufficient spatial resolution and wavelength coverage may detect impact glasses at certain locations, e.g., in the vicinity of fresh impact craters. Such dark materials are usually interpreted as accumulations of mafic volcanic sand, but the possibility of an impact melt origin of such materials also should be considered. In addition, our data suggest that high contents of feldspars or zeolites are not necessary to produce the transparency feature at 12.1 μm typical of martian dust spectra.

Mineralogy and petrology of the Dar al Gani 476 martian meteorite: Implications for its cooling history and relationship to other shergottites

Abstract— Dar al Gani 476, the 13th martian meteorite, was recovered from the Sahara in 1998. It is a basaltic shergottitic rock composed of olivine megacrysts reaching 5 mm (24 vol%) set in a finegrained groundmass of pyroxene (59 vol%) and maskelynitized plagioclase (12 vol%) with minor amounts of accessory phases (spinel, merrillite, ilmenite). Dar al Gani 476 is similar to lithology A of Elephant Moraine A79001 (EETA79001) in petrography and mineralogy, but is distinct in several aspects. Low‐Ca pyroxenes in the Dar al Gani 476 groundmass are more magnesian (En76Fs21 Wo3∼En58Fs30Wo12) than those in lithology A of EETA79001 (En73Fs22Wo5∼En45Fs43Wo12), rather similar to pyroxenes in lherzolitic martian meteorites (En76Fs21 Wo3∼En63Fs22Wo15). Dar al Gani 476 olivine is less magnesian and shows a narrower compositional range (Fo76‐58) than EETA79001 olivine (Fo81‐53), and is also similar to olivines in lherzolitic martian meteorites (Fo74‐65). The orthopyroxene‐olivine‐chromite xenolith typical in the lithology A of EETA79001 is absent in Dar al Gani 476. It seems that Dar al Gani 476 crystallized from a slightly more primitive mafic magma than lithology A of EETA79001 and several phases (olivine, pyroxene, chromite, and ilmenite) in Dar al Gani 476 may have petrogenetic similarities to those of lherzolitic martian meteorites. Olivine megacrysts in Dar al Gani 476 are in disequilibrium with the bulk composition. The presence of fractured olivine grains in which the most Mg‐rich parts are in contact with the groundmass suggests that little diffusive modification of original olivine compositions occurred during cooling. This observation enabled us to estimate the cooling rates of Dar al Gani 476 and EETA79001 olivines, giving similar cooling rates of 0.03‐3 °C/h for Dar al Gani 476 and 0.05‐5 °C/h for EETA79001. This suggests that they were cooled near the surface (burial depth shallower than about 3 m at most), probably in lava flows during crystallization of groundmass. As is proposed for lithology A of EETA79001, it may be possible to consider that Dar al Gani 476 has an impact melt origin, a mixture of martian lherzolite and other martian rock (Queen Alexandra Range 94201, nakhlites?).

Absence of shock-metamorphic effects in the Bushveld Complex, South Africa: results of an intensive search

An extensive search was made for distinctive shock-metamorphic effects in order to test the theory that the Bushveld Complex (BC) was formed by the impact of large extraterrestrial objects. Field and petrographie studies concentrated on two geological environments in which current impact cratering theories predict the formation and preservation of such shock-metamorphic effects as shatter cones, unusual fragmental or melt breccias, and shock lamellae in quartz. These are: (1) pre-BC quartzite xenoliths in the Rooiberg Group, a thick sequence of acid lavas (rhyolites and rhyodacites) and related rocks, proposed by earlier workers as a possible crater-filling impact melt; and (2) the locally observable contact between the Rooiberg Group and the underlying pre-BC basement (Pretoria Group), a contact possibly corresponding to the original crater floor in established impact structures. More than 320 samples of pre-BC quartzite were collected and examined petrographically for shock-metamorphic effects. No megascopic shock-metamorphic effects (shatter cones or unusual breccias) were observed in any outcrop. In thin section, no petrographic shock effects (e.g., shock lamellae hi quartz and kink bands in biotite) were observed in any sample. The quartzites from both the basement (80 samples) and the Rooiberg xenoliths (245 samples) are virtually undeformed. Cataclastic textures are rare, and individual quartz grains show only normal metamorphic deformation lamellae (Böhm lamellae) and single or multiple sets of fluid inclusions arranged along irregular planes which appear to be healed fractures. Although some of these parallel sets superficially resemble so-called “decorated” shock lamellae, universal stage measurements of their orientation demonstrate that they are totally different. The absence of both megascopic and microscopic shock-metamorphic effects in a large suite of carefully selected samples strongly indicates that impact models for the BC are not correct and that the BC is not an impact structure. This conclusion also agrees with: (1)the complex stratigraphy now recognized in the lower Rooiberg Group, which is not consistent with an impact melt origin, (2) the absence in the Rooiberg Group of any of the distinctive melt-rich fragmentai breccias which dominate the crater-filling deposits in established large impact structures, and (3) the undeformed character of the basement rocks below the Rooiberg Group and the apparently concordant contact between them.

Igneous History of the Aubrite Parent Asteroid: Evidence from the Norton County Enstatite Achondrite

Abstract— We studied numerous specimens of the Norton County enstatite achondrite (aubrite) by optical microscopy, electron microprobe, and neutron‐activation analysis. Our main conclusions are the following: 1. Norton County is a fragmental impact breccia, consisting of a clastic matrix made mostly of crushed enstatite, into which are embedded a variety of mineral and lithic clasts of both igneous and impact melt origin. 2. The Norton County precursor materials were igneous rocks, mostly plutonic orthopyroxenites, not grains formed by condensation from the solar nebula. 3. The Mg‐silicate‐rich aubrite parent body experienced extensive melting and igneous differentiation, causing formation of diverse lithologies, some of which have not been described previously. These lithologies include dunites (represented by forsterite crystals), plutonic orthopyroxenites (represented by most enstatite crystals in the matrix), plutonic pyroxenites (the pyroxenitic clasts), and plagioclase‐silica rocks (like the feldspathic clasts). Presence of impact melt breccias (the microporphyritic clasts and the diopside‐plagioclase‐silica clast) of still different compositions further attest to the lithologic diversity of the aubrite parent body.

Rock magnetic signature of the Cretaceous‐Tertiary boundary

Black magnetic microspherules, commonly found in Cretaceous‐Tertiary (K‐T) boundary sediments and believed to be of an impact melt origin, were found to be responsible for a strong magnetic anomaly at the K‐T boundary in Petriccio, Italy, and in almost all Deep Sea Drilling Project (DSDP) sediment cores investigated in which the K‐T boundary was present. The enriched concentrations of magnetic minerals have been detected by measurements of the magnetic susceptibility which is in a good approximation proportional to the magnetic microspherule content. A rock magnetic characterization of extracted microspherules by hysteresis, Isothermal Remanent Magnetization acquisition, demagnetization and thermomagnetic measurements describes the magnetic minerals in the microspherules as ferrites with a near magnetite composition and ‘soft’, i.e. multidomain, magnetic behavior, in agreement with earlier nonmagnetic studies. On the contrary, volcanic ash from Hawaii has distinctively ‘harder’ magnetic properties, an important finding with regard to the proposed alternative, a volcanic origin of the well‐known Ir‐anomaly at the K‐T boundary. It is suggested here that from magnetic susceptibility scanning of numerous cross‐sections of the K‐T boundary a distinct worldwide pattern might evolve that can potentially reveal the impact/volcano site because of a presumed higher microspherule concentration near the source. The advantage of mapping the microspherule distribution over the previous unsuccessful attempt using Ir‐concentrations lies in the large size (hundreds of microns) of the microspherules with relatively short airborn life times while the iridium might have adhered to small dust particles that circled the globe several times before settling.