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NASA NTRS1978

Manicouagan impact melt, Quebec. I - Stratigraphy, petrology, and chemistry

A sheet of clast-laden impact melt 230 m thick and 55 km in diameter forms an annular plateau surrounding an uplift of shocked anorthosite within the moderately eroded Manicouagan structure. Three gradational units of the melt sheet are characterized with respect to grain size, inclusions, texture, and mineralogy. The melt rocks as a group are chemically homogeneous with a bulk composition similar to that of latite and with no statistically significant regional chemical variations. The melt is not completely chemically homogeneous as a local mafic variant represented by two samples with poikilitic texture was found. These poikilitic rocks texturally resemble some Apollo 17 impact melt rocks and are inferred to have had a similar origin and thermal history.

Floran, R. J.↗
NASA NTRS1993

The granulite suite: Impact melts and metamorphic breccias of the early lunar crust

The granulite suite consists of two major types of rocks. One is coarse-grained and poikilitic with many euhedral crystals of olivine and plagioclase. These characteristics indicate crystallization from a melt; the poikilitic granulites are impact melt breccias. The other group is finer-grained and granoblastic, with numerous triple junctions; the granoblastic granulites are metamorphic rocks. Compositional groups identified by Lindstrom and Lindstrom contain both textural types. Two pyroxene thermometry indicates that both groups equilibrated at 1000 to 1150 C. Calculations suggest that the granoblastic group, which has an average grain size of about 80 microns, was annealed for less than 6 x 10 exp 4 y at 1000 C, and for less than 2500 y at 1150 C. Similar equilibration temperatures suggest that both groups were physically associated after impact events produced the poikilitic melts. Granulitic impactites hold important information about the pre-Nectarian bombardment history of the Moon, and the composition and thermal evolution of the early lunar crust. Granulitic impactites are widely considered to be an important rock type in the lunar crust, but how they formed is poorly understood. Metal compositions and elevated concentrations of meteoritic siderophile elements suggest that most lunar granulites are impact breccias. Their occurrence as clasts in approximately 3.9 Ga breccias, and Ar-(40-39) ages greater than or = 4.2 Ga for some granulites show that they represent a component of the lunar crust which formed prior to the Nectarian cataclysm. Petrographic characteristics of lunar granulites indicate at least two endmember textural variants which apparently formed in fundamentally different ways. One type has granoblastic textures consisting of equant, polygonal to rounded grains, and abundant triple junctions with small dispersions around 120 degrees indicating a close approach to textural equilibrium. As suggested by many authors, granoblastic granulites probably formed by subsolidus annealing and recrystallization of fragmental or glassy protoliths. Examples of this type include 15418, 78155, and 79215. The other textural type consists of poikilitic to poikiloblastic rocks with euhedral to subhedral plagioclase and olivine enclosed by interstitial pyroxene. In some cases, the texture resembles that of an orthocumulate. Examples of this type include 60035, 67955, and 77017. Rounding of grain edges is common in poikilitic granulites, but the regular crystal shapes and widely dispersed dihedral angles show they are far from textural equilibrium. The textures of poikilitic granulites are more consistent with the formation of these rocks by crystallization from a melt than by subsolidus metamorphism. A few samples have been recognized with textural characteristics transitional between those of the granoblastic and poikiloblastic endmembers (e.g., 72559, 78527). Pyroxene compositions taken from the literature and determined for this study by electron microprobe were used to calculate equilibration temperatures. The Kretz Ca transfer (solvus) thermometer and the Lindsley and Anderson graphical method both give similar temperatures, which range from approximately 1000 to 1150 C. There is no apparent temperature difference between granoblastic and poikilitic varieties, but there is a hint in these data that the more ferroan varieties equilibrated to lower temperatures. Additional studies are in progress to test this possibility.

Cushing, J. A.↗
NASA NTRS2017

The Onset of the Cataclysm: In Situ Dating of the Nectaris Basin Impact Melt Sheet

The impact history of the Moon has significant implications beyond simply excavating the surface of our nearest neighbor. The age distribution of lunar impact breccias inspired the idea of a catastrophic influx of asteroids and comets about 4 billion years ago and motivated new models of planetary dynamics. An epoch of heavy bombardment after planets had atmospheres and continents would have influenced the course of biologic evolution. The story of a cataclysmic bombardment, written in the rocks of the Moon, has far-reaching consequences.

Cohen, B. A.↗
NASA NTRS2011

Eucrite Impact Melt NWA 5218 - Evidence for a Large Crater on Vesta

Northwest Africa (NWA) 5218 is a 76 g achondrite that is classified as a eucrite [1]. However, an initial classification [2] describes it as a "eucrite shock-melt breccia...(in which) large, partially melted cumulate basalt clasts are set in a shock melt flow...". We explore the petrology of this clast-bearing impact melt rock (Fig. 1), which could be a characteristic lithology at large impact craters on asteroid Vesta [3]. Methods: Optical microscopy, scanning electronmicroscopy, and Raman spectroscopy were used on a thin section (Fig. 1) for petrographic characterization. The impact melt composition was determined by 20 m diameter defocused-beam analyses with a Cameca SX-100 electron microprobe. The data from 97 spots were corrected for mineral density effects [4]. Constituent mineral phases were analyzed with a focusedbeam. Bidirectonal visible and near-infrared (VNIR) and biconical FT-IR reflectance spectra were measured on the surface of a sample slab on its central melt area and on an eucrite clast, and from 125-500 m and <125 m powders of melt. Results: General petrography: The sample specimen is a coherent, medium dark-grey (N4), melt rock. The thin section captures a central, subophitic-textured melt that contains ~1 cm to tens of m-size subangular to rounded, variably-shocked eucrite clasts. Clasts >100 m are coarse-grained with equigranular ~1 mm size plagioclase, quartz, and clinopyroxene (Fig. 1). Single crystals of chromite, ilmenite, zircon, Ca-Mg phosphate, Fe-metal, and troilite are embedded in the melt. Polymineralic clasts are mostly compositionally similar to the above mentioned larger clasts but scarce granulitic fragments are observed as well.

Wittmann, Axel↗
NASA NTRS2023

Evidence for Extrusive Mg-Suite Magmatism on the Moon? Fine-Grained Magnesian Clasts in an Apollo 16 Impact Melt Breccia

The magnesian suite (Mg-suite) of rocks record some of the earliest intrusive magmatism on the Moon. Studies of these Mg-suite rocks find they are plutonic or hypabyssal, formed typically kilometers under the lunar surface. Several models exist to explain the formation and evolution of the Mg-suite but distinguishing between hypotheses can be difficult given the limited sample availability. The global extent of Mg-suite magmatism remains in debate and is key to constraining models of early secondary crust building. In this study, we present magnesian clasts within Apollo impact melt rock 68815. These clasts contain olivine, plagioclase, with minor amounts of Mg-Al-spinel and pyroxene similar to spinel troctolites of the Mg-suite, but they lack plutonic textures. We provide evidence that some of the clasts may be of extrusive volcanic origin akin to terrestrial komatiites while others might represent crystalline impact melts. There exists a large breadth of evidence for Mg-suite intrusives, whereas here, we present possible evidence of Mg-rich volcanic counterparts. If valid, this would broaden the known diversity of lunar volcanism during the initial stages of secondary crust building. We anticipate this finding to provide a greater constraint onto models of Mg-suite ascent and emplacement, which only currently consider intrusive magmatism, as well as renewed motivation to examine impact melt breccias for rare and understudied lithologies. Future trace element studies or radiometric dating could be used to further interrogate the connections of these clasts to the Mg-suite.

Moon↗
NASA NTRS1979

Origin of impact melt rocks in the Bununu howardite

The Bununu howardite is a polymict regolith breccia which contains impact melt that is largely restricted to a 1-cm thick intrusion containing residual glass. As in Malvern, the melt rock contains melt with meteoritic Ni-Co contents. The cooling rate, interpreted for forming glass from this composition, is a few tenths of a degree per minute. The intrusive melts rock, which is a feature unique to Bununu, may indicate that Bununu was consolidated at the time of impact melting.

Klein, L. C.↗
NASA NTRS2003

Osmium Isotope Constraints on the Proportion of Bolide Component in Chicxulub Impact Melts

Recent drilling has produced the samples of impact melt breccias examined here. All samples examined here are from the YAX-1 borehole [7]. Two main types of impact breccias have been studied. The first type is a green altered impact meltrock found in the lower portion of the impact sequence. The texture of the rock is microcrystalline and is composed of pyroxene, plagioclase, and alkali feldspars. Its composition is consistent with continental margin rocks. It is generally massive with some flow structure. The rock was brecciated and altered after solidfication and contains small amounts of both shocked and unshocked clasts of the impacted lithologies. These lithologies include lithic quartzite, and isolated feldspar crystals. The compositions of these rocks are similar to those seen in meltrocks sampled by the Yucatan-6 borehole [8-9]. Our study includes samples YAX-1_861.4, YAX-1_863.51, and YAX- 1_876.46, which represent both the top and lower portion of the green impact meltrock. The middle sample in the sequence has the least amount of (mineralogical) alteration [10]. The second type of melt breccia under study is a brown altered impact meltrock. It also has a microcrystalline texture and both shocked and unshocked clasts of the target material. Even though this rock type has been altered, remnant schleiren, metaquartzite, and micritic calcite have been identified. Sample YAX-1_841.32 is representative of this type of rock. It was recovered from a polymict breccia in the middle of the impact sequence.

Gelinas, A.↗
NASA NTRS2011

Initial Observations of Lunar Impact Melts and Ejecta Flows with the Mini-RF Radar

The Mini-RF radar on the Lunar Reconnaissance Orbiter's spacecraft has revealed a great variety of crater ejecta flow and impact melt deposits, some of which were not observed in prior radar imaging. The craters Tycho and Glushko have long melt flows that exhibit variations in radar backscatter and circular polarization ratio along the flow. Comparison with optical imaging reveals that these changes are caused by features commonly seen in terrestrial lava flows, such as rafted plates, pressure ridges, and ponding. Small (less than 20 km) sized craters also show a large variety of features, including melt flows and ponds. Two craters have flow features that may be ejecta flows caused by entrained debris flowing across the surface rather than by melted rock. The circular polarization ratios (CPRs) of the impact melt flows are typically very high; even ponded areas have CPR values between 0.7-1.0. This high CPR suggests that deposits that appear smooth in optical imagery may be rough at centimeter- and decimeter- scales. In some places, ponds and flows are visible with no easily discernable source crater. These melt deposits may have come from oblique impacts that are capable of ejecting melted material farther downrange. They may also be associated with older, nearby craters that no longer have a radar-bright proximal ejecta blanket. The observed morphology of the lunar crater flows has implications for similar features observed on Venus. In particular, changes in backscatter along many of the ejecta flows are probably caused by features typical of lava flows.

Carter, Lynn M.↗
NASA NTRS2016

Ar-40-Ar-39 Age of an Impact-Melt Lithology in Lunar Meteorite Dhofar 961

The Dhofar 961 lunar meteorite was found in 2003 in Oman. It is texturally paired with Dhofar 925 and Dhofar 960 (though Dhofar 961 is more mafic and richer in incompatible elements). Several lines of reasoning point to the South Pole-Aitken Basin (SPA) basin as a plausible source (Figure 2): Mafic character of the melt-breccia lithic clasts consistent the interior of SPA, rules out feldspathic highlands. Compositional differences from Apollo impact-melt groups point to a provenance that is separated and perhaps far distant from the Procellarum KREEP Terrane SPA "hot spots" where Th concentrations reach 5 ppm and it has a broad "background" of about 2 ppm, similar to lithic clasts in Dhofar 961 subsamples If true, impact-melt lithologies in this meteorite may be unaffected by the Imbrium-forming event that is pervasively found in our Apollo sample collection, and instead record the early impact history of the Moon.

Impact-Melt Lithology↗
NASA NTRS2018

Age Distribution of Lunar Impact-Melt Rocks in Apollo Drive-Tube 68001/2

Apollo 16 double-drive tube 68001 /68002 provides impact and volcanic materials along a depth of approximately 60 cm in five compositional distinct units. 68001 /2 offers the potential to study distinct populations of impact melts with depth to understand how 'gardening' affects these samples. We will use unbiased major-element chemistry, mineralogy, and age to understand the impact history of Apollo 16 landing site. The study demonstrates the techniques that landed missions require to identify lithologies of interest (e.g., impact melts).

Curran, N. M.↗
NASA NTRS1987

The meteorite component of Apollo 16 noritic impact melt breccias

The compositions of noritic melt breccias from Apollo 16 were compared with those of noritic impact melt breccias from other landing sites (A14 14310, A15 LKFM, and A17 stations 2 and 3). It was found that the Apollo 16 breccias contain greater concentrations of siderophile elements than the other noritic melt breccias, due to a large amount of Fe-Ni metal and Ni (1150 micrograms/g in the A16 Type-2 DB). The Fe-Ni metal is the carrier of the chemical signature of the ancient meteorite group 1H of Hertogen et al. (1977), with the bulk composition dissimilar to metal found in ordinary chondrites but similar to that found in some iron meteorites. It is argued that the meteorites that produced the impact melts were irons.

Korotev, Randy L.↗
NASA NTRS2015

Stratigraphy of the Caloris Basin, Mercury: Implications for Volcanic History and Basin Impact Melt

Caloris basin, Mercury's youngest large impact basin, is filled by volcanic plains that are spectrally distinct from surrounding material. Post-plains impact craters of a variety of sizes populate the basin interior, and the spectra of the material they have excavated enable the thickness of the volcanic fill to be estimated and reveal the nature of the subsurface. The thickness of the interior volcanic plains is consistently at least 2.5 km, reaching 3.5 km in places, with thinner fill toward the edge of the basin. No systematic variations in fill thickness are observed with long-wavelength topography or azimuth. The lack of correlation between plains thickness and variations in elevation at large horizontal scales within the basin indicates that plains emplacement must have predated most, if not all, of the changes in long-wavelength topography that affected the basin. There are no embayed or unambiguously buried (ghost) craters with diameters greater than 10 km in the Caloris interior plains. The absence of such ghost craters indicates that one or more of the following scenarios must hold: the plains are sufficiently thick to have buried all evidence of craters that formed between the Caloris impact event and the emplacement of the plains; the plains were emplaced soon after basin formation; or the complex tectonic deformation of the basin interior has disguised wrinkle-ridge rings localized by buried craters. That low-reflectance material (LRM) was exposed by every impact that penetrated through the surface volcanic plains provides a means to explore near-surface stratigraphy. If all occurrences of LRM are derived from a single layer, the subsurface LRM deposit is at least 7.5-8.5 km thick and its top likely once made up the Caloris basin floor. The Caloris-forming impact would have generated a layer of impact melt 3-15 km thick; such a layer could account for the entire thickness of LRM. This material would have been derived from a combination of lower crust and upper mantle.

Cratering↗
NASA NTRS1994

Compositional Variation in Apollo 16 Impact-Melt Breccias and Inferences for the Geology and Bombardment History of the Central Highlands of the Moon

High-precision data for the concentrations of a number of lithophile and siderophile elements were obtained on multiple subsamples from 109 impact-melt rocks and breccias (mostly crystalline) from the Apollo 16 site. Compositions of nearly all Apollo 16 melt rocks fall on one of two trends of increasing Sm concentration with increasing Sc concentration. The Eastern trend (lower Sm/Sc, Mg/Fe, and Sm/Yb ratios) consists of compositional groups 3 and 4 of previous classification schemes. These melt rocks are feldspathic, poor in incompatible and siderophile elements, and appear to have provenance in the Descartes formation to the east of the site. The Western trend (higher Sm/Sc. Mg/Fe, and Sm/ Yb ratios) consists of compositional groups 1 and 2. These relatively mafic, KREEP-bearing breccias are a major component (approx.35%) of the Cayley plains west of the site and are unusual, compared to otherwise similar melt breccias from other sites, in having high concentrations of Fe-Ni metal ( 1-2 %). The metal is the carrier of the low-Ir/Au (approx. 0.3 x chondritic) siderophile-element signature that is characteristic of the Apollo 16 site. Four compositionally distinct groups (1M, 1F, 2DB, and 2NR) of Western-trend melt breccias occur that are each represented by at least six samples. Compositional group 1 or previous classification schemes (the 'poikilitic' or 'LKFM' melt breccias) can be subdivided into two groups. Group 1M (represented by six samples, including 60315) is characterized by lower Al2O3 concentrations, higher MgO and alkali concentrations, and higher Mg/Fe and Cr/Sc ratios than group 1F (represented by fifteen samples, including 65015). Group 1M also has siderophile-element concentrations averaging about twice those of group lF and Ir/Au and Ir/Ni ratios that are even lower than those of other Western-trend melt rocks (Ir/Au = 0.24 +/- 0.03. CI-normalized). At the mafic extreme of group 2 ('VHA' melt breccias), the melt lithology occurring as clasts in feldspathic fragmental breccias from North Ray crater (group 2NR) is compositionally distinct from the melt lithology ofdimict breccias from the Cayley plains (group 2DB) in having higher concentrations of Sc, Cr, and heavy rare earth elements and lower concentrations of siderophile elements. The distinct siderophile-element signature (high absolute abundances, low Ir/Au ratio) suggest that the four groups ofmafic melt breccia are all somehow related. Ratios ofsome lithophile elements also suggest that they are more closely related to each other than then, are to melt breccias from other Apoll sites. However, none of the breccia compositions can be related to any of the others by any simple process of igneous fractionation or mixing involving common lunar materials. Thus, the origin of the four groups of mafic melt breccia is enigmatic. If they were produced in only one or two impacts, then a mechanism exists for generating regimes of impact-melt breccia in a single impact that are substantially different from each other in composition. For various reasons, including the problem of delivering large volumes of four different types of melt to the Apollo 16 site, it is unlikely that any of these breccias were produced in basin-forming impacts. If they were produced in as many as four crater-forming impacts, then the unusual siderophile-element signature is difficult to explain. Possible explanations are (1) the four groups of melt breccia all contain metal from a single, earlier impact, (2) they were each formed by related metal-rich meteoroids, or (3) some common postimpact process has resulted in metal of similar composition in each of four melt pools. Within a compositional group, most intrasample and intersample variation in lithophile element concentrations is caused by differences among samples in the proportion of a component of normative anorthosite or noritic anorthosite. In most cases, this compositional variation probably reflects variation in clast abundance. For group 2DB (and probably 2NR), differences in abundance of a component of ferroan anorthosite (estimated Al2O3 approx. 32%) accounts for the compositional variation. For groups 1M and 1F, the anorthositic component is more mafic (estimated Al203 approx. 26%). Some group-2 samples may be related by a troctolitic component of varying abundance.

Korotev, Randy L.↗
NASA NTRS2016

Identifying and Characterizing Impact Melt Outcrops in the Nectaris Basin

The Nectaris Basin is an 820-km diameter, multi-ring impact basin located on the near side of the Moon. Nectaris is a defining stratigraphic horizon based on relationships between ejecta units, giving its name to the Nectarian epoch of lunar history. Lunar basin chronology based on higher resolution LRO imagery and topography, while assigning some important basins like Serenitatis to pre-Nectarian time, were generally consistent with those previously derived. Based on this stratigraphy, at least 11 large basins formed in the time between Nectaris and Imbrium. The absolute age of Nectaris, therefore, is a crucial marker in the lunar time-stratigraphic sequence for understanding the impact flux on the Moon, and by extension, the entire inner solar system. For several decades, workers have attempted to constrain the age of the Nectaris basin through radiometric dating of lunar samples. However, there is little agreement on which samples in our collection represent Nectaris, if any, and what the correct radiometric age of such samples is. The importance of the age of Nectaris goes far beyond assigning a stratigraphic marker to lunar chronology. Several dynamical models use Nectaris as their pin date, so that this date becomes crucial in understanding the time-correlated effects in the rest of the solar system. The importance of the Nectaris basin age, coupled with its nearside, mid-latitude location, make remnants of the impact-melt sheet an attractive target for a future mission, either for in-situ dating or for sample return. We have started exploring this possibility. We have begun a consortium data-analysis effort bringing multiple datasets and analysis methods to bear on these putative impact-melt deposits to characterize their extent, elemental composition and mineralogy, maturity and geologic setting, and to identify potential landing sites that meet both operational safety and science requirements.

Nectaris Basin↗
NASA NTRS2018

Aubrite and Enstatite Chondrite Impact Melt Meteorites: Analogs to Mercury?

New data obtained during the MESSENGER mission has allowed us to better contrain the composition and mineralogy of the mercurian surface. One unique feature of Mercury is its extremely low oxygen fugacity (ƒO2) (Iron Wustite (IW) -7.3 to IW-2.6). At such extreme conditions, elements that exhibit lithophile behavior on Earth can exhibit chalcophile or siderophile behavior, leading to the formation of exotic sulfides and metals. As no samples have been returned from Mercury, it is critical to study meteorite analogs to better under-stand the formation conditions of the minerals present at the mercurian surface, as well as mercurian magmatic processes. Given the low fO2 on Mercury, we have selected to investigate potential meteoritic analogs for Mercury among the most reduced meteorite types, including the aubrites and enstatite chondrite impact melts. The aubrites are differentiated meteorites that show varying degrees of brecciation, have a similar ƒO2 to the mercurian surface and interior, and contain exotic sulfides that have been inferred to be present on the mercurian surface. The enstatite chondrite impact melts are from undifferentiated parent bodies, have a similar ƒO2 to the mercurian surface and interior, and contain exotic sulfides that have been inferred to be present on the mercurian surface. In this study, we present a comprehensive analysis of a representative suite of aubrites and enstatite chondrite impact melts and assess their relevance to under-standing magmatic processes on Mercury.

Wilbur, Z. E.↗
NASA NTRS1975

Rocks 60618 and 65785 - Evidence for admixture of KREEP in lunar impact melts

We present evidence to support the hypothesis that the fine-grained, basaltic-textured portions of rocks 60618 and 65785 were produced by impact melting of the coarse-grained spinel-olivine anorthosite (60618) and spinel troctolite (65785) portions, coupled with admixture of approximately 33% and 63%, respectively, material of alkalic high-alumina basalt (KREEP) composition. The abundance of impact-modified rocks at the Apollo 16 site with compositional similarities to the impact melts reported here, suggests that the relations observed in these rocks are indicative of widespread impact mixing with KREEP in rocks of the lunar highlands.

Keil, K.↗
NASA NTRS2017

The Onset of the Cataclysm: In Situ Dating of a Nearside Basin Impact-Melt Sheet Or, There and Not Back Again

Impact-melt samples from Apollo Luna are 3.85-4.1 Ga, tied to Imbrium, Serenitatis, Crisium, Nectaris, plus other craters? May have been caused by destabilization of material in early solar system by dynamic forces such as gas drag and gravitational interactions Coincident with the oldest rocks on the Earth and later than the earliest isotopic signs of life on Earth. Earth was already a planet with oceans, plate tectonics, and single celled life What was happening on the Moon before 3.9 Ga affected the course of life on Earth, the structure of our Solar System, and the dynamics of extra solar planetary systems.

Cohen, Barbara A.↗
NASA NTRS2015

What Do Nectaris Basin Impact Melt Rocks Look like and Where Can We Find Them?

The formation of the Nectaris basin is a key event defining the stratigraphy of the Moon. Its absolute age, therefore, is a linchpin for lunar bombardment history. Fernandes et al. gave a thorough account of the history of different samples thought to originate in Nectaris, with the upshot being there is little agreement on what samples represent Nectaris, if any. We are revisiting the effort to identify Nectaris basin impact-melt rocks at the Apollo 16 site, to model their emplacement, and to use these parameters to examine other sites where Nectaris impact melt is more abundant and/or more recognizable for potential further study.

nectors basin impact melt rocks↗