Melt Breccia Research Articles

A large spectral survey of small lunar craters: Implications for the composition of the lunar mantle

A global spectral survey of 4506 immature craters with diameters <1 km was carried out using near-IR data from the Kaguya Spectral Profiler to characterize the composition of the lunar megaregolith. On the basis of band minima and radiative transfer mixing models, small crater spectra fall into three groups: (1) mare basalts with strong absorptions at relatively long wavelengths indicating high ratios of high- to low-Ca pyroxene; (2) norites containing about 50% plagioclase and with pyroxene assemblages dominated by low-Ca pyroxene that occur within the South Pole-Aitken Basin (SPA), near Apollo 14 and other locations near Imbrium Basin, and three major cryptomaria deposits; and (3) noritic anorthosites occurring within the Feldspathic Highlands Terrane containing about 20 wt% pyroxene with a pyroxene assemblage containing exclusively very low-Ca pyroxene. Very few pure anorthosites are present in this survey and there are no occurrences of pyroxene-poor olivine-rich assemblages. Models of the composition of basin ejecta incorporate large amounts of mantle material and the spectral results require that that the sampled mantle is orthopyroxenite. Basin depth-diameter ratios used in the models required to match the measured composition are consistent with prior estimates for the largest basins. The composition found in the SPA and Imbrium regions are consistent with mafic impact melt breccias or basaltic impact melts of basin origin. For SPA we model this composition and find it requires an extremely low impact angle. While this is consistent with prior work on an oblique impact for the SPA event, a more robust solution invokes the production of norite in impact melt seas.

Petrological Evidences of Impact-induced Shock Metamorphism in the Basement Granitoids and Rhyolitic Melt Breccia of Mohar Area, Shivpuri District, Madhya Pradesh

Abstract The circular structure at Mohar (Dhala structure) in the western part of Bundelkhand Gneissic Complex, is marked by a prominent outlier of Kaimur sediments surrounded by low lying concentric sequence of sediments of Dhala Formation and basement granite breccia. This has been interpreted as a volcanic eruption related cauldron structure and meteoritic impact crater structure by various authors, on the basis of absence or presence of shock indicators in the clasts of a rhyolite-like rock that crops out scantily in the north western part of the structure. During the course of extensive sub-surface uranium exploration in this structure, the geoscientists of Atomic Minerals Directorate for Exploration and Research observed unequivocal and rampant evidences of shock metamorphic features for the first time in drill core samples of basement granitoids which constitute the bed rock for the rhyolite-like melt breccia, which overlies it. Published data of shock metamorphic features from this area are largely confined to the surface samples of the rhyolite-like melt rock, exposed in sparse outcrops. The shock metamorphic features recorded in the sub-surface granitoid bed rock samples during the present study, comprise planar deformation features (PDF) in quartz, feldspar, apatite and zircon, toasted, diaplectic, ladder-textured feldspars, selectively shock-melted feldspars and melt-veined quartz. The shock metamorphic features recorded in surface and sub-surface samples of the melt rock include ballen quartz, PDF in quartz clasts, toasted and diaplectic feldspar clasts shocked basic rock fragments with isotropised feldspars. Both the shocked bedrock granitoid and the melt rock bear uncharacteristic geochemical signatures with elevated K2O, MgO and depleted CaO. The study also observes that the melt breccia overlying the granitoid bedrock also occurs as pocket-like patches at various depths within the granitoids. Thus, the present findings have helped in understanding the attributes of the basement granitoid and associated melt breccia, thereby linking the genesis of the latter by selective melting of the former, due to the process of impact. It reinforces the already propounded theory of impact as the likely cause for the development of the structure in the basement Bundelkhand granitoid that was later filled by sediments standing out presently as a mesa.

Petrography, geochronology and source terrain characteristics of lunar meteorites Dhofar 925, 961 and Sayh al Uhaymir 449

Dhofar (Dho) 925, 961 and Sayh al Uhaymir (SaU) 449 are brecciated lunar meteorites consisting of mineral fragments and clasts from a range of precursor lithologies including magnesian anorthositic gabbronorite granulites; crystalline impact melt breccias; clast-bearing glassy impact melt breccias; lithic (fragmental) breccias; mare basalts; and evolved (silica-rich) rocks. On the similarity of clast type and mineral chemistry the samples are likely grouped, and were part of the same parent meteorite. Phosphate Pb–Pb ages in impact melt breccias and matrix grains demonstrate that Dho 961 records geological events spanning ∼500Ma between 4.35 and 3.89Ga. These Pb–Pb ages are similar to the ages of ‘ancient’ intrusive magmatic samples and impact basin melt products collected on the lunar nearside by the Apollo missions. However, the samples’ bulk rock composition is chemically distinct from these types of samples, and it has been suggested that they may have originated from the farside South Pole-Aitken impact basin (i.e., Jolliff et al., 2008). We test this hypothesis, and conclude that although it is possible that the samples may be from the South Pole-Aitken basin, there are other regions on the Moon that may have also sourced these complex breccias.

H2O and O(3PJ) photodesorption from amorphous solid water deposited on a lunar mare basalt

H2O (ν=0) and O(3PJ=2,1,0) desorbates were measured with resonance-enhanced multiphoton ionization following 157-nm irradiation of amorphous solid water (ASW) deposited on a lunar mare basalt. Both H2O photodesorption and O(3PJ) photodissociation products of ASW were studied in the attempt to better understand the competition between photodesorption and photodissociation of water in the condensed phase on a lunar surface. The oxygen atom time-of-flight (TOF) spectrum was measured as a function of spin–orbit state, H2O exposure, and 157-nm irradiation time. Maxwell–Boltzmann distributions with translational temperatures of 10,000K, 1800K, 400K, and 89K fit the four TOF components. For high H2O exposures, diffusion out of pores in the lunar substrate made a large portion of the O(3PJ) signal appear to be sub-thermal. Water depletion cross sections were measured at exposures between 0.1 and 10Langmuir (1L=10−6Torrs). These cross sections decreased with increasing coverage and matched previously measured cross sections from a lunar impact melt breccia. Additionally, non-resonant ionization was employed to detect vibrationally excited water indirectly through its fragments. The OH+ fragment of H2O (ν*) and the O(3PJ) photodissociation product increased in intensity during prolonged irradiation as hydroxyl groups accumulated on the surface and then recombined. For an initial exposure of 5L H2O, after reaching maximum signal, the cross sections for H2O (ν*) and O(3P2) depletion were measured to be 1.2×10−19cm2 and 6.7×10−20cm2, respectively.

Preserved flora and organics in impact melt breccias

At least seven glass-bearing strata of varying ages occur at different horizons in the Pampean sediments of Argentina dating back to the Miocene. In a strict sense, these impact glasses are melt-matrix breccias composed of partially digested minerals clasts and basement fragments indicative of crater excavation. Ar-40/Ar-39 dating yield ages (+/- 2 sigma) of 6 +/- 2 Ka, 114 +/- 26 Ka, 230 +/- 30 Ka, 445 +/- 21 Ka, 3.27 +/- 0.08 Ma (near Mar del Plata = MdP), 5.28 +/- 0.04 Ma, and 9.21 +/- 0.08 Ma (near Chasico = CH) Where found in place (not reworked), these ages are consistent with the local stratigraphy and faunal assemblages. A striking property of some of these impact glasses is the encapsulation of preserved fragments of floral (and even soft-tissue faunal remains). Here we identify retained organics and describe a likely process of encapsulation and preservation.

Chlorine distribution and its isotopic composition in “rusty rock” 66095. Implications for volatile element enrichments of “rusty rock” and lunar soils, origin of “rusty” alteration, and volatile element behavior on the Moon

An interesting characteristic of the pyroclastic glass bead deposits, select impact produced lithologies such as the “rusty rock” 66095, and unique lunar soils from the Apollo 16 landing site, is their unusual enrichments in 204Pb, Cd, Bi, Br, I, Ge, Sb, Tl, Zn, and Cl which indicates that portions of these sample contain a substantial volatile component. Sample 66095, a fine-grained, subophitic to ophitic polymict melt breccia, also hosts a pervasive low-temperature, volatile-rich, oxyhydrated mineral assemblage. The volatile element enrichments in these assorted lunar lithologies have been attributed to a variety of extra-lunar and lunar processes, whereas the oxyhydration in 66095 has long been thought to represent either terrestrial alteration of lunar chlorides and Fe–Ni metal to βFeO(OH,Cl) or indigenous lunar processes. In 66095, Cl is accommodated in FeO(OH,Cl), phosphates, and chlorides and is heterogeneously distributed. The low-temperature alteration occurs as rims around Fe–Ni metal and sulfide grains, and as dispersed grains in the adjacent matrix. Micro-Raman and transmission electron microscope (TEM) imaging indicate that akaganéite (βFeO(OH,Cl)) is the dominant FeO(OH) polymorph and is intergrown with goethite (αFeO(OH)) and hematite (αFe2O3). TEM observations indicate a well-defined “nanometer-scale” stratigraphy” to the alteration. For example, kamacite (body centered cubic)→face-centered cubic (fcc) Fe–Ni alloy→lawrencite (FeCl2)→akaganéite. The lunar lawrencite (Fe,Ni)Cl2 in 66095 does not react directly to akaganéite on Earth. Rather, lawrencite exposed to terrestrial conditions reacts to form an amorphous Fe- and Cl-bearing phase, nano-crystalline goethite, and hematite. The morphology of these terrestrial alteration products is significantly different than that of the akaganéite occurring in 66095. The chlorine isotopic compositions of these volatile-rich samples are enriched in heavy Cl. For 66095, the δ37Cl ranges from +14.0‰ to +15.6‰, whereas the δ37Cl for the volatile-rich A16 soils ranges from +5.6‰ to +15.7‰. Based on these data it appears likely that the volatile element enrichments and the Cl isotopic fractionation observed in 66095 and the Apollo 16 soils did not result from extra-lunar additions, but are most likely indigenous to the Moon. Lawrencite was deposited on mineral surfaces at approximately 650°C to 570°C from a metal-chloride-bearing, H-poor gas phase. This gas phase was also responsible for the transport of other metals (e.g. Zn, Cu, Pb, Fe). The fractionation of Cl isotopes in the rusty rock can be attributed to fumarole processes in a low-H system. The origin and formation of the akaganéite is more enigmatic. The Cl-isotopes are consistent with it replacing lawrencite. However, numerous nanometer-scale observations are not consistent with a terrestrial origin and indicate multiple episodes of oxyhydration.

Photodissociation of water and O(3PJ) formation on a lunar impact melt breccia

AbstractPhotodissociation of water deposited on an impact melt breccia collected during Apollo 16 was studied by measuring O(3PJ = 2,1,0) photoproducts detected with resonance‐enhanced multiphoton ionization. For each spin‐orbit state, the oxygen atom time‐of‐flight (TOF) spectrum was measured as a function of H2O exposure and 157 nm irradiation time. Four Maxwell‐Boltzmann distributions with translational temperatures of 10,000 K, 1800 K, 400 K, and 102 K were required to fit the data. The most likely formation mechanisms are molecular hydrogen elimination following ion‐electron recombination, secondary recombination of hydroxyl radicals, and photodissociation of adsorbed hydroxyls. The irradiation time required to reach maximum oxygen signal suggests that water clusters into islands when adsorbing on the lunar impact melt breccia. After enough irradiation for the oxygen atom yield to reach its maximum, the slowly decreasing signal was fit with an exponential curve to obtain a cross section that represents the rate of surface hydroxyl depletion. For 0.1, 1, and 5 Langmuir (1 L = 10−6 Torr s) H2O exposure, respectively, the measured O(3P) depletion cross sections were 4.9 × 10−20, 6.6 × 10−20, and 4.6 × 10−20 cm2. These results imply that photodissociation of water on the lunar surface cannot account for the large mass‐16 (±1 amu) signal observed in the lunar atmosphere. Unless another significant source of oxygen atoms is present, this unexpectedly large signal is likely due to CH4 or OH.

Mechanisms and cross sections for water desorption from a lunar impact melt breccia

AbstractDesorption of H2O (ν = 0) following 157‐nm irradiation of amorphous solid water on a lunar impact melt breccia was measured with resonance‐enhanced multiphoton ionization. Photofragments of vibrationally excited water were detected with nonresonant ionization. The average cross section for H2O (ν = 0) removal and destruction at 0.1 Langmuir (1 L = 10−6 Torr∙s) H2O exposure was measured to be (7.1 ± 1.9) × 10−19 cm2. Cross sections were also measured at 0.3, 1, 5, and 10 L exposures. Because these cross sections increase drastically with decreasing water coverage, water is not expected to remain intact as H2O on the sunlit lunar surface. Instead, photons are likely to cause H2O to desorb or dissociate. The OH+ fragment of H2O (ν*) increased in intensity with increasing irradiation as hydroxyl groups built up on the surface and then recombined. The OH+ signal eventually began to decrease after a dose of 5 × 1018 photons cm−2. Under these conditions, the cross section for H2O (ν*) photodesorption was measured to be 6.4 × 10−20 cm2 for an initial exposure of 5 L H2O.

Rochechouart impact crater melt breccias record no geomagnetic field reversal

Impact melt breccias from the Rochechouart (France) meteorite crater possess paleomagnetic directions with both normal and reverse polarities, raising the question whether shock from the collision initiated a geomagnetic field reversal. Stepwise thermal demagnetization together with a suite of rock magnetic experiments, optical microscopy, Raman spectroscopy and electron microprobe analyses identify adjacent, multiphase, titanohematite in the samples containing normal and mixed polarities—typical of lithologies bearing self-reversal behavior stemming from magnetic exchange interaction. Melt breccias possessing mostly titanium-free hematite as the magnetic remanence carrier have solely reverse-polarity directions, leading to the conclusion that the Rochechouart meteorite impact did not spawn a geomagnetic event. Samples displaying paleomagnetic directions with normal or mixed polarity yield unreliable paleointensity data. 30 samples with reverse polarity that pass stringent data selection criteria from Thellier–Thellier paleointensity experiments with alteration, tail and additivity checks define an average field value of 12.8±3.7 μT. This translates into a virtual dipole moment of 2.7±0.8×1022 Am2, which is relatively low, but within uncertainty of other Mesozoic paleointensity data.

The effect of 1.9 and 1.4 Ga impact events on 4.3 Ga zircon and phosphate from an Apollo 15 melt breccia

Zircon and phosphate grains from matrix and quartz‐monzodiorite (QMD) clasts in two thin sections of Apollo 15 impact melt breccia 15405 were investigated using optical microscopy, scanning electron microscopy, Raman spectroscopy, and ion microprobe U‐Pb analyses. U‐Pb results for zircon grains with well‐defined cathodoluminescence zoning define the primary (i.e., magmatic) crystallization age as 4330 ± 6 Ma (2σ). One zircon consists of a preserved inner part surrounded by a porous polycrystalline (“granular”) mixture of zircon and baddeleyite, indicating incomplete reaction of the zircon with melt. Previous work showed that this microstructure could form at pressures above 60 GPa and a temperature close to ~1700°C and is evidence of an impact‐related melting event. The U‐Pb system of this grain indicates a resetting event at 1940 ± 10 Ma, interpreted as the age of this impact (impact #1). Other zircon and phosphate grains also have disturbed U‐Pb systems, showing an even younger reset event (impact #2) at 1407 ± 57 Ma. Evidence of impact is supported by microstructures of zircon and baddeleyite such as secondary rims. These impacts are tentatively identified as those having formed Autolycus and Aristillius craters.

Geochemical studies of the SUBO 18 (Enkingen) drill core and other impact breccias from the Ries crater, Germany

AbstractSuevite and melt breccia compositions in the boreholes Enkingen and Polsingen are compared with compositions of suevites from other Ries boreholes and surface locations and discussed in terms of implications for impact breccia genesis. No significant differences in average chemical compositions for the various drill cores or surface samples are noted. Compositions of suevite and melt breccia from southern and northeastern sectors of the Ries crater do not significantly differ. This is in stark contrast to the published variations between within‐crater and out‐of‐crater suevites from northern and southern sectors of the Bosumtwi impact structure, Ghana. Locally occurring alteration overprint on drill cores—especially strong on the carbonate‐impregnated suevite specimens of the Enkingen borehole—does affect the average compositions. Overall, the composition of the analyzed impact breccias from Ries are characterized by very little macroscopically or microscopically recognized sediment‐clast component; the clast populations of suevite and impact melt breccia are dominated consistently by granitic and intermediate granitoid components. The Polsingen breccia is significantly enriched in a dioritic clast component. Overall, chemical compositions are of intermediate composition as well, with dioritic‐granodioritic silica contents, and relatively small contributions from mafic target components. Selected suevite samples from the Enkingen core have elevated Ni, Co, Cr, and Ir contents compared with previously analyzed suevites from the Ries crater, which suggest a small meteoritic component. Platinum‐group element (PGE) concentrations for some of the enriched samples indicate somewhat elevated concentrations and near‐chondritic ratios of the most immobile PGE, consistent with an extraterrestrial contribution of 0.1–0.2% chondrite‐equivalent.

Geochemistry and mineralogy of a feldspathic lunar meteorite (regolith breccia), Northwest Africa 2200

The lunar meteorite Northwest Africa (NWA) 2200 is a regolith breccia with a ferroan feldspathic bulk composition (Al2O3 = 30.1 wt.%; Mg# = molar 100 × Mg/(Mg + Fe) = 59.2) and low Th content (0.42 μg/g). Lithologically, NWA 2200 is a diverse mixture of lithic and glassy clasts, mineral fragments, and impact glass spherules, all embedded in a dark glassy matrix. NWA 2200 contains some feldspathic brecciated rock components (ferroan anorthositic granulitic breccia, poikiloblastic granulitic breccia, and glassy melt breccia with an intersertal texture). The bulk compositions of these brecciated components indicate they are derived from ferroan troctolitic or noritic anorthosite lithologies (bulk Al2O3 = 26–30 wt.%; bulk FeO/MgO > 1.0). The bulk composition of NWA 2200 is more ferroan and feldspathic than the Apollo feldspathic regolith samples and feldspathic lunar regolith meteorites, and is also more depleted in incompatible elements (e.g., rare earth elements) than Apollo 16 feldspathic regolith samples. We conclude that NWA 2200 originated from a location different to the Apollo landing sites, and may have been sourced from the ferroan KREEP-poor highlands, “KREEP” materials are enriched in such elements as potassium (K), rare earth elements (REE), phosphorus (P).

Drilling the Waqf as Suwwan impact structure

The about 6-km diameter, near-circular Waqf as Suwwan structure located at E36°48′/N31°03′ in eastern Jordan was only recently recognized as a somewhat eroded, complex impact structure. Surface geological mapping, geophysical interpretation, remote sensing, and petrographic and mineralogical analyses have been carried out to understand the structure. In particular, the complex geology of the remnant of the central uplift has been scrutinized. A recent drilling project afforded an opportunity to expand the investigation of the structure to previously inaccessible strata of the ring syncline in the environs of the central uplift. Three boreholes were drilled, two to 140 and 110 m depth to the north and outside of the central uplift, and a further short hole to 5 m depth into the innermost part of the central uplift. Preliminary assessment of these cores has revealed the presence of around 11 m of fluvial breccias (wadi deposit) that are dominated by chert fragments at the top of the syncline fill. This is underlain by a normal succession of late Maastrichtian to Campanian strata. A variety of microstructures such as fracturing with vertical, as well as inclined at 45° and 30° fractures occurs throughout the cores. Some joints have slickensides along their walls. Limestone and marly limestone constitute the most abundant rocks in the boreholes. Distinct shock deformation effects are entirely lacking in the cores from the syncline. These observations are interpreted as a result of substantial erosion of the impact structure down to a level within the crater floor. The microstructures and the preliminary results of the analyses of sediment ages, textures, and compositions (nanofossils and sediment mineralogy) show that sediments as old as Campanian and as young as late Maastrichtian were affected by the impact. Unfortunately, the drilling did not expose any lithologies such as impact melt breccias that could lend themselves to absolute chronological analysis for a better constraint of the impact age.

Petrography of impact glasses and melt breccias from the El'gygytgyn impact structure, Russia

AbstractThe El'gygytgyn impact structure, 18 km in diameter and 3.6 Ma old, in Arctic Siberia, Russia, is the only impact structure on Earth mostly excavated in acidic volcanic rocks. The Late Cretaceous volcanic target includes lavas, tuffs, and ignimbrites of rhyolitic, dacitic, and andesitic composition, and local occurrence of basalt. Although the ejecta blanket around the crater is nearly completely eroded, bomb‐shaped impact glasses, redeposited after the impact event, occur in lacustrine terraces within the crater. Here we present detailed petrographic descriptions of newly collected impact glass‐bearing samples. The observed features contribute to constrain the formation of the melt and its cooling history within the framework of the impact process. The collected samples can be grouped into two types, characterized by specific features: (1) “pure” glasses, containing very few clasts or new crystals and which were likely formed during the early stages of cratering and (2) a second type, which represents composite samples with impact melt breccia lenses embedded in silicate glass. These mixed samples probably resulted from inclusion of unmelted impact debris during ejection and deposition. After deposition the glassy portions continued to deform, whereas the impact melt breccia inclusions that probably had already cooled down behaved as rigid bodies in the flow.

The chlorine isotope composition of chondrites and Earth

The chlorine isotope composition of chondrites provides information about isotopic reservoirs that were present in the solar nebula and the effects of secondary processes on their isotopic composition. We analyzed chlorine isotope ratios of 19 chondrites that included both unaltered (type 3) and altered (types 1,2 and 4–6) chondrites from the carbonaceous, ordinary, and enstatite classes as well as an enstatite chondrite impact melt breccia and a partial melt residue. Chlorine isotope compositions (δ37Cl, ‰ vs. SMOC) and 1σ uncertainty () are as follows (excluding melt residue and anomalous Parnallee):MaterialCarbonaceous chondritesOrdinary chondritesEnstatite chondritesStructurally-bound Cl−0.3(0.6)−0.4(0.6)0.4(0.4)Water-soluble Cl−0.9(0.9)0.3(2.1)0.4(0.6)Bulk (total) Cl−0.2(0.6)−0.4(0.7)0.4(0.3)Petrologic type 3 chondrites are the least equilibrated and are those most likely to retain primary nebular signatures. Type 3 carbonaceous chondrites have bulk δ37Cl values that average −0.3±0.5‰ and structurally-bound Cl values of −0.3±0.3‰. These values are indistinguishable from the δ37Cl values of the bulk Earth (−0.2‰) and close to the lowest measured value of the Moon (−0.7‰). From this similarity, we conclude that the inner regions of the early solar nebula had a homogeneous chlorine isotope reservoir. For samples that have undergone secondary processing, δ37Clbulk values range from −1.2‰ to 0.8‰ for metamorphosed samples (average 0.0‰) and −0.3‰ to 0.3‰ (average 0.1‰) for aqueously-altered samples (Orgueil, Ivuna, Murray, and Murchison). Most of the data can be explained in terms of fractionation processes similar to those that occur on the Earth, such as interaction with pore waters. No apparent correlations exist with Cl concentration data. Ordinary chondrites have the lowest and most restricted range of Cl concentration, but the largest range in δ37Cl values. Carbonaceous and enstatite chondrites have wide ranges of Cl concentration and more restricted ranges of δ37Cl values.We have analyzed additional terrestrial mantle-derived materials of the Earth in order to better constrain the δ37Cl value of this reservoir and compare with the chondrite data. The mantle data cluster at −0.3‰ to −0.1‰, and support the idea that the most pristine type C chondrites and the bulk Earth have the same δ37Cl value, indicating that no fractionation by evaporative processes occurred during the formation of the Earth. The discrepancy between our data and previously published results is discussed.

Jiddat al Harasis 556: A howardite impact melt breccia with an H chondrite component

Abstract– A petrographic and geochemical study was undertaken to characterize Jiddat al Harasis (JaH) 556, a howardite find from the Sultanate of Oman. JaH 556 is a polymict impact melt breccia containing highly shocked clasts, including mosaicized olivine and recrystallized plagioclase, set in a finely recrystallized vesicular matrix (grain diameter &lt;5–10 μm). Plagioclase (An76–92) and clinopyroxene (En48–62Wo7–15) are associated with orthopyroxene and olivine clasts like in a howardite. JaH 556 oxygen isotope data indicate that it has an anomalous bulk‐rock composition as howardite, resulting from a mixture between HED material and at least one second reservoir characterized by a higher Δ17O. The bulk meteorite has a composition consistent with howardites, but it is enriched in siderophile elements (Ni = 3940 and Co = 159 ppm) arguing for a chondritic material as second reservoir. This is independently confirmed by the occurrence of chondrule relics composed of olivine (Fo56–80), orthopyroxene (En79Wo2), and plagioclase (An61–66). Based on oxygen isotopic signature, siderophile composition, and chondrule core Mg number (Fo80 and En79Wo2), it is proposed that JaH 556 is a howardite containing approximately 20% H chondrite material. This percentage is high compared with that observed petrographically, likely because chondritic material dissolved in the impact melt. This conclusion is supported by the observed reaction of orthopyroxene to olivine, which is consistent with a re‐equilibration in a Si‐undersaturated melt. JaH 556’s unique composition enlarges the spectrum of howardite‐analogs to be expected on the surface of 4 Vesta. Our data demonstrate that oxygen isotopic anomalies can be produced by a mixture of indigenous and impactor materials and must be interpreted with extreme caution within the HED group.

REPLY TO COMMENT BY W.U. REIMOLD ON "PSEUDOTACHYLITE IN THE SOUTH BOUNDARY FAULT AT THE COOKE SHAFT, WITWATERSRAND BASIN, SOUTH AFRICA BY P.W. MAMBANE ET AL., SOUTH AFRICAN JOURNAL OF GEOLOGY 114.2, 109-120

In reply to comments by W.U. Reimold on ”Pseudotachylite in the South Boundary Fault at the Cooke Shaft, Witwatersrand Basin, South Africa by P.W. Mambane et al., S. Afr. J. Geol. 114.2, 109–120. 1. The Introduction statement in Mambane et al. (2011) directs the reader to consider that authors such as Schwarzman et al. (1983), Gibson et al. (1997), Spray (1998), Eiko (2004), Riller et al. (2010) and others, have in the past, considered pseudotachylites as a diagnostic feature of shock tectonics. Mambane et al. make clear in the Introduction that pseudotachylites are formed in a number of settings and under a number of P-T conditions. 2. The perseverant objections to the use by Mambane et al. of the term “pseudotachylite breccia” are not tenable. It is agreed that terminologies used by Shand (1916), Bisschoff (1962), Reimold and Colliston (1994), and Reimold and Gibson (2006), and others, are not well-defined. A better classification scheme is needed to assist in the description of the different types of pseudotachylite, and the meaning of pseudotachylite textures including breccia, “melt breccia”, and “tectonic pseudotachylite”. It seems that Mohr-Westheide and Reimold (2011) and Reimold (2011) are occupied with this task, but that being mostly after review and publication of Mambane et al., which raises the question of how Mambane et al. could consider articles that appeared after the March 2011 acceptation date of their article, or a June 2011 publishing date. For example, Mohr-Westheide and Reimold (2011) was first published on the 17 March 2011, …

The 40Ar/39Ar dating technique applied to planetary sciences and terrestrial impacts

The 40Ar/39Ar technique is a powerful geochronological method derived from the K/Ar technique that can help to unravel the evolution of the solar system. The 40Ar/39Ar system can not only record the timing of volcanic and metamorphic processes on asteroids and planets, it finds domain of predilection in dating impact events throughout the solar system. The 40Ar/39Ar method is a robust analytical technique when the events to be dated are well understood and data are not over interpreted. The power of the 40Ar/39Ar technique resides in the ability to check the validity of age data internally by statistical means and multiple lines of evidence, and hence to evaluate when Ar age data are unreliable. Yet, too many ‘ages’ reported in the literature are still based on over-interpretation of perturbed age spectra. This review is by no means exhaustive and is centred on the most recent applications of the 40Ar/39Ar technique applied to planetary material, not the history of the planetary bodies themselves, or a historical review of the development of the argon dating technique. This paper presents selected case-study examples on terrestrial impact structures, the Moon and meteorites with ages recalculated using the latest decay constants. Currently, only 21 terrestrial impact structures are precisely and accurately dated, and the only proven age concordance is between the Chixculub impact and the Cretaceous/Paleogene mass extinction. 40Ar/39Ar dating of volcanic events on the Moon suggests that volcanism was concentrated between 3.8 and 3.1 Ga. The study of lunar volcanism would also benefit from dating of volcanic spherules for which only few data are available. Rigorous filtering of the 40Ar/39Ar age database of lunar melt breccias yielded concordant ages with high precision for two major basins of the Moon, but more precise age data would be needed to further test and validate the Late Heavy Bombardment (LHB) hypothesis. 40Ar/39Ar dating of lunar impact spherules shows an increase of ages <400 Ma suggesting a recent increase in the impact flux. The impact history of the LL parent body (bodies?) has yet to be well constrained but may mimic the LHB observed on the Moon, which would indicate that the LL parent body was quite large. Basaltic meteorites (HEDs) show an 40Ar/39Ar age range between 4.1 and 3.4 Ga, suggesting a diffuse LHB event; however, the spread of apparent ages may be a data-interpretation artefact, and the LHB parent body (bodies?) might have experienced a bombardment closer to the duration of the LHB age range than expected. Martian meteorites contain clues on Mars atmospheric and mantle argon compositions.

Very high-K KREEP-rich clasts in the impact melt breccia of the lunar meteorite SaU 169: New constraints on the last residue of the Lunar Magma Ocean

In the impact melt breccia (IMB) of Sayh al Uhaymir (SaU) 169, the most KREEP-rich lunar meteorite to date (Gnos et al., 2004), clasts of a new type of lithologies were discovered, consisting of Ca-poor and Ca-rich pyroxenes (60.8vol.%), Ba-rich K-feldspar (27.9vol.%), phosphates (5.6vol.%), Nb-rich ilmenite (4.0vol.%), zircon (1.2vol.%) and minor sulfide (0.6vol.%). These mafic lithic clasts are more enriched in KREEP component (∼1500×CI) than the host meteorite and are highly enriched in potassium. They are referred to as very high-K (VHK) KREEP lithology, and probably most close to the last residual liquid of the Lunar Magma Ocean without significant dilution by other Mg-rich magmas. The fine-grained matrix of the SaU 169 IMB has very similar mineral chemistry to the VHK KREEP lithology, but contains abundant plagioclase with trace K-feldspar. The matrix shows decoupling of K from the REEP-like component; however, it cannot be simply interpreted by mixing the VHK KREEP lithology with anorthosites, which should have diluted the REEP-like component with the same proportion.SIMS Pb–Pb dating was conducted on zircons in various petrographic settings and with different crystal habits. All analyses show a main age peak at 3921±3Ma and a smaller one at 4016±6Ma. The main age peak is identical to the previous Pb–Pb age by Gnos et al. (2006) and U–Pb age by Liu et al. (2009), dating the catastrophic shock event contributed to the formation of SaU 169 IMB. The older ages are consistent with the previous report of an older bulk U–Pb age by Kramers et al. (2007), suggestive of presence of relict crystals in a few large zircon grains. The VHK KREEP clasts predated the fine-grained matrix, but have the same zircon Pb–Pb ages as the latter within the analytical uncertainties. Plagioclase was converted to maskelynite whereas zircon was shocked to diaplectic glass, probably by a second event at ∼2.8Ga. However, the identical zircon Pb–Pb ages of the amorphous parts and the remained crystalline areas indicate no resetting of Pb–Pb isotopes by the later shock metamorphism, or there was another severe impact event postdated solidification of the fine-grained matrix within a few million years.

Comparative zircon U–Pb geochronology of impact melt breccias from Apollo 12 and lunar meteorite SaU 169, and implications for the age of the Imbrium impact

The ages of zircons from high-Th impact-melt breccias (IMBs) from meteorite Sayh al Uhaymir (SaU) 169 and from rock fragments in soil samples from Apollo 12 have been determined using the SHRIMP-II ion microprobe. The IMBs are very similar to each other in chemistry, mineralogy and texture, and the zircons from the KREEP-rich (high-Th) crystalline impact melt have similar U and Th contents and identical ages, within uncertainties, of 3920±13 (2σ) Ma (SaU 169) and 3914±7 (2σ) Ma (Apollo 12). The age results support the idea that the high-Th IMBs (Apollo 12 and SaU 169) formed in the same impact event. The similarity of composition and age suggest that SaU 169 and the high-Th IMB fragments of Apollo 12 originated from the same area of the Procellarum KREEP Terrane. We interpret the age of zircon grains in the Apollo 12 high-Th IMB as a precise and direct determination of the age of the Imbrium impact. This age is significantly older than the commonly cited age of 3.85Ga but is similar to recent determinations from SIMS U–Pb dating of Apollo 14 apatite grains and with anticipated revision of ages by 40Ar–39Ar and 87Rb–86Sr. The present zircon 207Pb–206Pb age is the first direct zircon age determination of the Imbrium impact event from an Apollo sample. Previous measurements of zircon ages of Apollo IMBs have recorded events pre-dating the Imbrium basin-forming event.